assignment property in javascript

11 Properties: assignment vs. definition

• 11.1.1  Assignment
• 11.1.2  Definition
• 11.2.1  Assigning to a property
• 11.2.2  Defining a property
• 11.3.1  Only definition allows us to create a property with arbitrary attributes
• 11.3.2  The assignment operator does not change properties in prototypes
• 11.3.3  Assignment calls setters, definition doesn’t
• 11.3.4  Inherited read-only properties prevent creating own properties via assignment
• 11.4.1  The properties of an object literal are added via definition
• 11.4.2  The assignment operator = always uses assignment
• 11.4.3  Public class fields are added via definition
• 11.5  Further reading and sources of this chapter

There are two ways of creating or changing a property prop of an object obj :

• Assigning : obj.prop = true
• Defining : Object.defineProperty(obj, '', {value: true})

This chapter explains how they work.

For this chapter, you should be familiar with property attributes and property descriptors. If you aren’t, check out §9 “Property attributes: an introduction” .

11.1 Assignment vs. definition

11.1.1 assignment.

We use the assignment operator = to assign a value value to a property .prop of an object obj :

This operator works differently depending on what .prop looks like:

Changing properties: If there is an own data property .prop , assignment changes its value to value .

Invoking setters: If there is an own or inherited setter for .prop , assignment invokes that setter.

Creating properties: If there is no own data property .prop and no own or inherited setter for it, assignment creates a new own data property.

That is, the main purpose of assignment is making changes. That’s why it supports setters.

11.1.2 Definition

To define a property with the key propKey of an object obj , we use an operation such as the following method:

This method works differently depending on what the property looks like:

• Changing properties: If an own property with key propKey exists, defining changes its property attributes as specified by the property descriptor propDesc (if possible).
• Creating properties: Otherwise, defining creates an own property with the attributes specified by propDesc (if possible).

That is, the main purpose of definition is to create an own property (even if there is an inherited setter, which it ignores) and to change property attributes.

11.2 Assignment and definition in theory (optional)

In specification operations, property descriptors are not JavaScript objects but Records , a spec-internal data structure that has fields . The keys of fields are written in double brackets. For example, Desc.[[Configurable]] accesses the field .[[Configurable]] of Desc . These records are translated to and from JavaScript objects when interacting with the outside world.

11.2.1 Assigning to a property

The actual work of assigning to a property is handled via the following operation in the ECMAScript specification:

These are the parameters:

• O is the object that is currently being visited.
• P is the key of the property that we are assigning to.
• V is the value we are assigning.
• Receiver is the object where the assignment started.
• ownDesc is the descriptor of O[P] or null if that property doesn’t exist.

The return value is a boolean that indicates whether or not the operation succeeded. As explained later in this chapter , strict-mode assignment throws a TypeError if OrdinarySetWithOwnDescriptor() fails.

This is a high-level summary of the algorithm:

• It traverses the prototype chain of Receiver until it finds a property whose key is P . The traversal is done by calling OrdinarySetWithOwnDescriptor() recursively. During recursion, O changes and points to the object that is currently being visited, but Receiver stays the same.
• Depending on what the traversal finds, an own property is created in Receiver (where recursion started) or something else happens.

In more detail, this algorithm works as follows:

If O has a prototype parent , then we return parent.[[Set]](P, V, Receiver) . This continues our search. The method call usually ends up invoking OrdinarySetWithOwnDescriptor() recursively.

Otherwise, our search for P has failed and we set ownDesc as follows:

With this ownDesc , the next if statement will create an own property in Receiver .

• If ownDesc.[[Writable]] is false , return false . This means that any non-writable property P (own or inherited!) prevents assignment.
• The current object O and the current property descriptor ownDesc on one hand.
• The original object Receiver and the original property descriptor existingDescriptor on the other hand.
• (If we get here, then we are still at the beginning of the prototype chain – we only recurse if Receiver does not have a property P .)
• If existingDescriptor specifies an accessor, return false .
• If existingDescriptor.[[Writable]] is false , return false .
• Return Receiver.[[DefineOwnProperty]](P, { [[Value]]: V }) . This internal method performs definition, which we use to change the value of property Receiver[P] . The definition algorithm is described in the next subsection.
• (If we get here, then Receiver does not have an own property with key P .)
• Return CreateDataProperty(Receiver, P, V) . ( This operation creates an own data property in its first argument.)
• (If we get here, then ownDesc describes an accessor property that is own or inherited.)
• Let setter be ownDesc.[[Set]] .
• If setter is undefined , return false .
• Perform Call(setter, Receiver, «V») . Call() invokes the function object setter with this set to Receiver and the single parameter V (French quotes «» are used for lists in the specification).

Return true .

11.2.1.1 How do we get from an assignment to OrdinarySetWithOwnDescriptor() ?

Evaluating an assignment without destructuring involves the following steps:

• In the spec, evaluation starts in the section on the runtime semantics of AssignmentExpression . This section handles providing names for anonymous functions, destructuring, and more.
• If there is no destructuring pattern, then PutValue() is used to make the assignment.
• For property assignments, PutValue() invokes the internal method .[[Set]]() .
• For ordinary objects, .[[Set]]() calls OrdinarySet() (which calls OrdinarySetWithOwnDescriptor() ) and returns the result.

Notably, PutValue() throws a TypeError in strict mode if the result of .[[Set]]() is false .

11.2.2 Defining a property

The actual work of defining a property is handled via the following operation in the ECMAScript specification:

The parameters are:

• The object O where we want to define a property. There is a special validation-only mode where O is undefined . We are ignoring this mode here.
• The property key P of the property we want to define.
• extensible indicates if O is extensible.
• Desc is a property descriptor specifying the attributes we want the property to have.
• current contains the property descriptor of an own property O[P] if it exists. Otherwise, current is undefined .

The result of the operation is a boolean that indicates if it succeeded. Failure can have different consequences. Some callers ignore the result. Others, such as Object.defineProperty() , throw an exception if the result is false .

This is a summary of the algorithm:

If current is undefined , then property P does not currently exist and must be created.

• If extensible is false , return false indicating that the property could not be added.
• Otherwise, check Desc and create either a data property or an accessor property.

If Desc doesn’t have any fields, return true indicating that the operation succeeded (because no changes had to be made).

If current.[[Configurable]] is false :

• ( Desc is not allowed to change attributes other than value .)
• If Desc.[[Configurable]] exists, it must have the same value as current.[[Configurable]] . If not, return false .
• Same check: Desc.[[Enumerable]]

Next, we validate the property descriptor Desc : Can the attributes described by current be changed to the values specified by Desc ? If not, return false . If yes, go on.

• If the descriptor is generic (with no attributes specific to data properties or accessor properties), then validation is successful and we can move on.
• The current property must be configurable (otherwise its attributes can’t be changed as necessary). If not, false is returned.
• Change the current property from a data property to an accessor property or vice versa. When doing so, the values of .[[Configurable]] and .[[Enumerable]] are preserved, all other attributes get default values ( undefined for object-valued attributes, false for boolean-valued attributes).
• (Due to current.[[Configurable]] being false , Desc.[[Configurable]] and Desc.[[Enumerable]] were already checked previously and have the correct values.)
• If Desc.[[Writable]] exists and is true , then return false .
• If Desc.[[Value]] exists and does not have the same value as current.[[Value]] , then return false .
• There is nothing more to do. Return true indicating that the algorithm succeeded.
• (Note that normally, we can’t change any attributes of a non-configurable property other than its value. The one exception to this rule is that we can always go from writable to non-writable. This algorithm handles this exception correctly.)
• If Desc.[[Set]] exists, it must have the same value as current.[[Set]] . If not, return false .
• Same check: Desc.[[Get]]

Set the attributes of the property with key P to the values specified by Desc . Due to validation, we can be sure that all of the changes are allowed.

11.3 Definition and assignment in practice

This section describes some consequences of how property definition and assignment work.

11.3.1 Only definition allows us to create a property with arbitrary attributes

If we create an own property via assignment, it always creates properties whose attributes writable , enumerable , and configurable are all true .

Therefore, if we want to specify arbitrary attributes, we must use definition.

And while we can create getters and setters inside object literals, we can’t add them later via assignment. Here, too, we need definition.

11.3.2 The assignment operator does not change properties in prototypes

Let us consider the following setup, where obj inherits the property prop from proto .

We can’t (destructively) change proto.prop by assigning to obj.prop . Doing so creates a new own property:

The rationale for this behavior is as follows: Prototypes can have properties whose values are shared by all of their descendants. If we want to change such a property in only one descendant, we must do so non-destructively, via overriding. Then the change does not affect the other descendants.

11.3.3 Assignment calls setters, definition doesn’t

What is the difference between defining the property .prop of obj versus assigning to it?

If we define, then our intention is to either create or change an own (non-inherited) property of obj . Therefore, definition ignores the inherited setter for .prop in the following example:

If, instead, we assign to .prop , then our intention is often to change something that already exists and that change should be handled by the setter:

11.3.4 Inherited read-only properties prevent creating own properties via assignment

What happens if .prop is read-only in a prototype?

In any object that inherits the read-only .prop from proto , we can’t use assignment to create an own property with the same key – for example:

Why can’t we assign? The rationale is that overriding an inherited property by creating an own property can be seen as non-destructively changing the inherited property. Arguably, if a property is non-writable, we shouldn’t be able to do that.

However, defining .prop still works and lets us override:

Accessor properties that don’t have a setter are also considered to be read-only:

The fact that read-only properties prevent assignment earlier in the prototype chain, has been given the name override mistake :

• It was introduced in ECMAScript 5.1.
• On one hand, this behavior is consistent with how prototypal inheritance and setters work. (So, arguably, it is not a mistake.)
• On the other hand, with the behavior, deep-freezing the global object causes unwanted side-effects.
• There was an attempt to change the behavior, but that broke the library Lodash and was abandoned ( pull request on GitHub ).
• Pull request on GitHub
• Wiki page on ECMAScript.org ( archived )

11.4 Which language constructs use definition, which assignment?

In this section, we examine where the language uses definition and where it uses assignment. We detect which operation is used by tracking whether or not inherited setters are called. See §11.3.3 “Assignment calls setters, definition doesn’t” for more information.

11.4.1 The properties of an object literal are added via definition

When we create properties via an object literal, JavaScript always uses definition (and therefore never calls inherited setters):

11.4.2 The assignment operator = always uses assignment

The assignment operator = always uses assignment to create or change properties.

11.4.3 Public class fields are added via definition

Alas, even though public class fields have the same syntax as assignment, they do not use assignment to create properties, they use definition (like properties in object literals):

11.5 Further reading and sources of this chapter

Section “Prototype chains” in “JavaScript for impatient programmers”

Email by Allen Wirfs-Brock to the es-discuss mailing list : “The distinction between assignment and definition […] was not very important when all ES had was data properties and there was no way for ES code to manipulate property attributes.” [That changed with ECMAScript 5.]

HatchJS.com

Cracking the Shell of Mystery

How to Assign to the Property of a Function Parameter in JavaScript

Assignment to Property of Function Parameter

One of the most powerful features of JavaScript is the ability to assign values to the properties of function parameters. This can be used to create complex and dynamic code that can be easily modified.

In this article, we will take a closer look at assignment to property of function parameter. We will discuss what it is, how it works, and how it can be used to improve your code.

We will also provide some examples of how assignment to property of function parameter can be used in practice. By the end of this article, you will have a solid understanding of this important JavaScript concept.

In JavaScript, a function parameter is a variable that is declared inside the function’s parentheses. When a function is called, the value of the argument passed to the function is assigned to the function parameter.

For example, the following function takes a string argument and prints it to the console:

js function greet(name) { console.log(`Hello, ${name}`); }

greet(“world”); // prints “Hello, world”

In this example, the `name` parameter is assigned the value of the `”world”` argument.

Assignment to property of function parameter

Assignment to property of function parameter is a JavaScript feature that allows you to assign a value to a property of a function parameter. This can be useful for initializing the value of a parameter or for passing a reference to an object.

For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter:

js function greet(name) { name.value = “hello”; }

greet({ value: “world” }); // prints “hello”

In this example, the `name` parameter is a JavaScript object. The `value` property of the `name` object is assigned the value of the `”hello”` argument.

When to use assignment to property of function parameter?

You should use assignment to property of function parameter when you need to:

• Initialize the value of a parameter
• Pass a reference to an object

Avoid creating a new object

Initializing the value of a parameter

You can use assignment to property of function parameter to initialize the value of a parameter. For example, the following code initializes the `name` property of the `greet` function parameter to the value of the `”world”` argument:

js function greet(name) { name.value = “world”; }

Passing a reference to an object

You can use assignment to property of function parameter to pass a reference to an object. For example, the following code passes a reference to the `person` object to the `greet` function:

js function greet(person) { console.log(`Hello, ${person.name}`); }

const person = { name: “John Doe” };

greet(person); // prints “Hello, John Doe”

You can use assignment to property of function parameter to avoid creating a new object. For example, the following code uses assignment to property of function parameter to avoid creating a new object for the `name` parameter:

greet(“John Doe”); // prints “Hello, John Doe”

In this example, the `name` parameter is a string literal. The `name` property of the `name` parameter is assigned the value of the `”John Doe”` string literal. This avoids creating a new object for the `name` parameter.

Assignment to property of function parameter is a JavaScript feature that can be used to initialize the value of a parameter, pass a reference to an object, and avoid creating a new object. It is a powerful feature that can be used to improve the performance and readability of your code.

Additional resources

• [MDN: Assignment to property of function parameter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Assignment_to_property_of_function_parameter)
• [Stack Overflow: When to use assignment to property of function parameter?](https://stackoverflow.com/questions/1435573/when-to-use-assignment-to-property-of-function-parameter)
• [Codecademy: Assignment to property of function parameter](https://www.codecademy.com/learn/javascript/lessons/assignment-to-property-of-function-parameter)

3. How to use assignment to property of function parameter?

To use assignment to property of function parameter, you can simply assign a value to the property of the function parameter. For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter:

In this example, the `greet` function is called with the argument `”world”`. The `name` property of the `greet` function parameter is then assigned the value `”hello”`. When the `greet` function is called, the value of the `name` property is used to print the message `”Hello, world”`.

Assignment to property of function parameter can be used to initialize the value of a parameter, pass a reference to an object, or avoid creating a new object.

You can use assignment to property of function parameter to initialize the value of a parameter. For example, the following code initializes the value of the `name` property of the `greet` function parameter to the value of the `name` variable:

js function greet(name) { name = “world”; console.log(`Hello, ${name}`); }

In this example, the `name` variable is assigned the value `”world”` before the `greet` function is called. The `name` property of the `greet` function parameter is then assigned the value of the `name` variable. When the `greet` function is called, the value of the `name` property is used to print the message `”Hello, world”`.

You can use assignment to property of function parameter to pass a reference to an object. For example, the following code passes a reference to the `user` object to the `greet` function:

js function greet(user) { console.log(`Hello, ${user.name}`); }

const user = { name: “John Doe”, };

greet(user); // prints “Hello, John Doe”

In this example, the `user` object is passed to the `greet` function as a parameter. The `greet` function then uses the `name` property of the `user` object to print the message `”Hello, John Doe”`.

Avoiding creating a new object

You can use assignment to property of function parameter to avoid creating a new object. For example, the following code uses assignment to property of function parameter to avoid creating a new object for the `user` variable:

In this example, the `user` variable is assigned the value of the `user` object. The `greet` function then uses the `name` property of the `user` variable to print the message `”Hello, John Doe”`.

By using assignment to property of function parameter, you can avoid creating a new object for the `user` variable. This can improve the performance of your code and reduce the amount of memory that is used.

4. Pitfalls of assignment to property of function parameter

There are a few pitfalls to be aware of when using assignment to property of function parameter:

• The value of the property may be overwritten. If you assign a value to the property of a function parameter, the value of the property may be overwritten by the next time the function is called. For example, the following code assigns the value `”hello”` to the `name` property of the `greet` function parameter. The next time the `greet` function is called, the value of the `name` property will be overwritten by the value of the `name` argument.

js function greet(name) { name = “hello”; console.log(`Hello, ${name}`); }

greet(“world”); // prints “Hello, hello” greet(“hello”); // prints “Hello, hello”

A: Assignment to property of function parameter occurs when you assign a value to a property of a function parameter. This can be done by using the dot operator (.) to access the property, or by using the bracket operator ([]) to index into the property.

For example, the following code assigns the value “10” to the `x` property of the `foo()` function’s parameter `y`:

const foo = (y) => { y.x = 10; };

foo({ x: 5 }); // { x: 10 }

Q: Why is assignment to property of function parameter dangerous?

A: Assignment to property of function parameter can be dangerous because it can change the value of the property in the calling scope. This can lead to unexpected behavior and errors.

For example, the following code changes the value of the `x` property of the global variable `a`:

foo({ x: 5 }); // a.x is now 10

This behavior can be difficult to debug, as it may not be obvious that the change to the `x` property is being caused by the `foo()` function.

Q: How can I avoid assignment to property of function parameter?

There are a few ways to avoid assignment to property of function parameter. One way is to use the `const` keyword to declare the function parameter as a constant. This will prevent the value of the parameter from being changed.

Another way to avoid assignment to property of function parameter is to use the `readonly` keyword to declare the function parameter as read-only. This will prevent the value of the parameter from being changed, even by assignment to a property of the parameter.

Finally, you can also use the `Object.freeze()` method to freeze the object that is passed as the function parameter. This will prevent any changes to the object, including changes to the values of its properties.

Q: What are the best practices for assignment to property of function parameter?

The best practices for assignment to property of function parameter are as follows:

• Use the `const` keyword to declare function parameters as constants.
• Use the `readonly` keyword to declare function parameters as read-only.
• Use the `Object.freeze()` method to freeze objects that are passed as function parameters.

Here are some key takeaways from this article:

• Assigning to the property of a function parameter can change the value of the original variable.
• This can lead to unexpected behavior and security vulnerabilities.
• To avoid this problem, use the `const` keyword or pass arguments by reference.

By following these tips, you can write more secure and reliable JavaScript code.

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Destructuring assignment

The two most used data structures in JavaScript are Object and Array .

• Objects allow us to create a single entity that stores data items by key.
• Arrays allow us to gather data items into an ordered list.

However, when we pass these to a function, we may not need all of it. The function might only require certain elements or properties.

Destructuring assignment is a special syntax that allows us to “unpack” arrays or objects into a bunch of variables, as sometimes that’s more convenient.

Destructuring also works well with complex functions that have a lot of parameters, default values, and so on. Soon we’ll see that.

Array destructuring

Here’s an example of how an array is destructured into variables:

Now we can work with variables instead of array members.

It looks great when combined with split or other array-returning methods:

As you can see, the syntax is simple. There are several peculiar details though. Let’s see more examples to understand it better.

It’s called “destructuring assignment,” because it “destructurizes” by copying items into variables. However, the array itself is not modified.

It’s just a shorter way to write:

Unwanted elements of the array can also be thrown away via an extra comma:

In the code above, the second element of the array is skipped, the third one is assigned to title , and the rest of the array items are also skipped (as there are no variables for them).

…Actually, we can use it with any iterable, not only arrays:

That works, because internally a destructuring assignment works by iterating over the right value. It’s a kind of syntax sugar for calling for..of over the value to the right of = and assigning the values.

We can use any “assignables” on the left side.

For instance, an object property:

In the previous chapter, we saw the Object.entries(obj) method.

We can use it with destructuring to loop over the keys-and-values of an object:

The similar code for a Map is simpler, as it’s iterable:

There’s a well-known trick for swapping values of two variables using a destructuring assignment:

Here we create a temporary array of two variables and immediately destructure it in swapped order.

We can swap more than two variables this way.

The rest ‘…’

Usually, if the array is longer than the list at the left, the “extra” items are omitted.

For example, here only two items are taken, and the rest is just ignored:

If we’d like also to gather all that follows – we can add one more parameter that gets “the rest” using three dots "..." :

The value of rest is the array of the remaining array elements.

We can use any other variable name in place of rest , just make sure it has three dots before it and goes last in the destructuring assignment.

Default values

If the array is shorter than the list of variables on the left, there will be no errors. Absent values are considered undefined:

If we want a “default” value to replace the missing one, we can provide it using = :

Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided.

For instance, here we use the prompt function for two defaults:

Please note: the prompt will run only for the missing value ( surname ).

Object destructuring

The destructuring assignment also works with objects.

The basic syntax is:

We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like “pattern” for corresponding properties. In the simplest case, that’s a list of variable names in {...} .

For instance:

Properties options.title , options.width and options.height are assigned to the corresponding variables.

The order does not matter. This works too:

The pattern on the left side may be more complex and specify the mapping between properties and variables.

If we want to assign a property to a variable with another name, for instance, make options.width go into the variable named w , then we can set the variable name using a colon:

The colon shows “what : goes where”. In the example above the property width goes to w , property height goes to h , and title is assigned to the same name.

For potentially missing properties we can set default values using "=" , like this:

Just like with arrays or function parameters, default values can be any expressions or even function calls. They will be evaluated if the value is not provided.

In the code below prompt asks for width , but not for title :

We also can combine both the colon and equality:

If we have a complex object with many properties, we can extract only what we need:

The rest pattern “…”

What if the object has more properties than we have variables? Can we take some and then assign the “rest” somewhere?

We can use the rest pattern, just like we did with arrays. It’s not supported by some older browsers (IE, use Babel to polyfill it), but works in modern ones.

It looks like this:

In the examples above variables were declared right in the assignment: let {…} = {…} . Of course, we could use existing variables too, without let . But there’s a catch.

This won’t work:

The problem is that JavaScript treats {...} in the main code flow (not inside another expression) as a code block. Such code blocks can be used to group statements, like this:

So here JavaScript assumes that we have a code block, that’s why there’s an error. We want destructuring instead.

To show JavaScript that it’s not a code block, we can wrap the expression in parentheses (...) :

Nested destructuring

If an object or an array contains other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions.

In the code below options has another object in the property size and an array in the property items . The pattern on the left side of the assignment has the same structure to extract values from them:

All properties of options object except extra which is absent in the left part, are assigned to corresponding variables:

Finally, we have width , height , item1 , item2 and title from the default value.

Note that there are no variables for size and items , as we take their content instead.

Smart function parameters

There are times when a function has many parameters, most of which are optional. That’s especially true for user interfaces. Imagine a function that creates a menu. It may have a width, a height, a title, an item list and so on.

Here’s a bad way to write such a function:

In real-life, the problem is how to remember the order of arguments. Usually, IDEs try to help us, especially if the code is well-documented, but still… Another problem is how to call a function when most parameters are ok by default.

That’s ugly. And becomes unreadable when we deal with more parameters.

Destructuring comes to the rescue!

We can pass parameters as an object, and the function immediately destructurizes them into variables:

We can also use more complex destructuring with nested objects and colon mappings:

The full syntax is the same as for a destructuring assignment:

Then, for an object of parameters, there will be a variable varName for the property incomingProperty , with defaultValue by default.

Please note that such destructuring assumes that showMenu() does have an argument. If we want all values by default, then we should specify an empty object:

We can fix this by making {} the default value for the whole object of parameters:

In the code above, the whole arguments object is {} by default, so there’s always something to destructurize.

Destructuring assignment allows for instantly mapping an object or array onto many variables.

The full object syntax:

This means that property prop should go into the variable varName and, if no such property exists, then the default value should be used.

Object properties that have no mapping are copied to the rest object.

The full array syntax:

The first item goes to item1 ; the second goes into item2 , and all the rest makes the array rest .

It’s possible to extract data from nested arrays/objects, for that the left side must have the same structure as the right one.

We have an object:

Write the destructuring assignment that reads:

• name property into the variable name .
• years property into the variable age .
• isAdmin property into the variable isAdmin (false, if no such property)

Here’s an example of the values after your assignment:

The maximal salary

There is a salaries object:

Create the function topSalary(salaries) that returns the name of the top-paid person.

• If salaries is empty, it should return null .
• If there are multiple top-paid persons, return any of them.

P.S. Use Object.entries and destructuring to iterate over key/value pairs.

Open a sandbox with tests.

Open the solution with tests in a sandbox.

Lesson navigation

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Shorthand Property and Method Names in JavaScript | ES6

ES6 introduced two new features to make objects more concise - Shorthand Properties and Shorthand Method Names.

Shorthand Properties

With Shorthand Properties, whenever you have a variable which is the same name as a property on an object, when constructing the object, you can omit the property name.

What that means is that code that used to look like this,

can now look like this.

Shorthand Method Names

Now, what if one of those properties was a function?

A function that is a property on an object is called a method. With ES6's Shorthand Method Names, you can omit the function keyword completely. What that means is that code that used to look like this,

can now look like this

Both Shorthand Properties and Shorthand Method Names are just syntactic sugar over the previous ways we used to add properties to an object. However, because they're such common tasks, even the smallest improvements eventually add up.

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JS Tutorial

Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript destructuring, destructuring assignment syntax.

The destructuring assignment syntax unpack object properties into variables:

It can also unpack arrays and any other iterables:

Object Destructuring

The order of the properties does not matter:

Destructuring is not destructive.

Destructuring does not change the original object.

Object Default Values

For potentially missing properties we can set default values:

Object Property Alias

String destructuring.

One use for destructuring is unpacking string characters.

Destructuring can be used with any iterables.

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Array Destructuring

We can pick up array variables into our own variables:

Skipping Array Values

We can skip array values using two or more commas:

Array Position Values

We can pick up values from specific index locations of an array:

The Rest Property

You can end a destructuring syntax with a rest property.

This syntax will store all remaining values into a new array:

Destructuring Maps

Swapping javascript variables.

You can swap the values of two variables using a destructuring assignment:

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Property assignment expected when using template literals to create an Object

I am trying to create an object inside a JSON object that will be later populated from FormData:

What I want to obtain is the following JSON:

but I get Property assignment expected.ts(1136) .

If I replace

with let object = {}; then I get a normal JSON object:

How can I created the nested object that I want ?

• We need to know what form contains –  Thallius Commented Sep 23, 2021 at 15:19
• @ClausBönnhoff How can I do that ? The form has 100 lines in HTML. As I said in the question: using let object = {}; gets me the following JSON { "connection": "wifi", "type": "dhcp", "ssid": "Jorj_2.4", "ip_address": "", "gateway": "", "subnet": "", "dns": "" } –  George Sloata Commented Sep 23, 2021 at 15:23

2 Answers 2

For dynamic keys in script you have to enclose your expression inside []

const form = { name: 'network_settings' } let obj = {[form.name]: {}}; console.log(obj);

• @trincot was faster. Thank you. –  George Sloata Commented Sep 23, 2021 at 15:33

Go back to what you had first:

And after you have populated the object, wrap it using computed property name syntax:

• 1 Thank. Works like a charm. I've first tried return JSON.stringify(object[ ${form.name} ]); but it did not work. –  George Sloata Commented Sep 23, 2021 at 15:29

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Addition Assignment (+=) Operator in Javascript

JavaScript Addition assignment operator( += ) adds a value to a variable, The Addition Assignment (+ =) Sums up left and right operand values and then assigns the result to the left operand. The two major operations that can be performed using this operator are the addition of numbers and the concatenation of strings.

Example 1: In this example, we will concatenate two numbers as well as two strings by using the Addition Assignment operator(+=)

Output: When the operator is used on a number addition is performed and concatenation is performed on strings

Example 2: Here with the help of for..in  loop, we use the Additional assignment operator.

Output: Here we use an operator to add filtered content to a string

Supported browser:

• Google Chrome
• Mozilla Firefox
• Internet Explorer

We have a complete list of Javascript Assignment operators, to check those please go through the JavaScript Assignment operators article.

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Optional chaining (?.)

The optional chaining ( ?. ) operator accesses an object's property or calls a function. If the object accessed or function called using this operator is undefined or null , the expression short circuits and evaluates to undefined instead of throwing an error.

Description

The ?. operator is like the . chaining operator, except that instead of causing an error if a reference is nullish ( null or undefined ), the expression short-circuits with a return value of undefined . When used with function calls, it returns undefined if the given function does not exist.

This results in shorter and simpler expressions when accessing chained properties when the possibility exists that a reference may be missing. It can also be helpful while exploring the content of an object when there's no known guarantee as to which properties are required.

For example, consider an object obj which has a nested structure. Without optional chaining, looking up a deeply-nested subproperty requires validating the references in between, such as:

The value of obj.first is confirmed to be non- null (and non- undefined ) before accessing the value of obj.first.second . This prevents the error that would occur if you accessed obj.first.second directly without testing obj.first .

This is an idiomatic pattern in JavaScript, but it gets verbose when the chain is long, and it's not safe. For example, if obj.first is a Falsy value that's not null or undefined , such as 0 , it would still short-circuit and make nestedProp become 0 , which may not be desirable.

With the optional chaining operator ( ?. ), however, you don't have to explicitly test and short-circuit based on the state of obj.first before trying to access obj.first.second :

By using the ?. operator instead of just . , JavaScript knows to implicitly check to be sure obj.first is not null or undefined before attempting to access obj.first.second . If obj.first is null or undefined , the expression automatically short-circuits, returning undefined .

This is equivalent to the following, except that the temporary variable is in fact not created:

Optional chaining cannot be used on a non-declared root object, but can be used with a root object with value undefined .

Optional chaining with function calls

You can use optional chaining when attempting to call a method which may not exist. This can be helpful, for example, when using an API in which a method might be unavailable, either due to the age of the implementation or because of a feature which isn't available on the user's device.

Using optional chaining with function calls causes the expression to automatically return undefined instead of throwing an exception if the method isn't found:

However, if there is a property with such a name which is not a function, using ?. will still raise a TypeError exception "someInterface.customMethod is not a function".

Note: If someInterface itself is null or undefined , a TypeError exception will still be raised ("someInterface is null"). If you expect that someInterface itself may be null or undefined , you have to use ?. at this position as well: someInterface?.customMethod?.() .

eval?.() is the shortest way to enter indirect eval mode.

Optional chaining with expressions

You can also use the optional chaining operator with bracket notation , which allows passing an expression as the property name:

This is particularly useful for arrays, since array indices must be accessed with square brackets.

Invalid optional chaining

It is invalid to try to assign to the result of an optional chaining expression:

Template literal tags cannot be an optional chain (see SyntaxError: tagged template cannot be used with optional chain ):

The constructor of new expressions cannot be an optional chain (see SyntaxError: new keyword cannot be used with an optional chain ):

Short-circuiting

When using optional chaining with expressions, if the left operand is null or undefined , the expression will not be evaluated. For instance:

Subsequent property accesses will not be evaluated either.

This is equivalent to:

However, this short-circuiting behavior only happens along one continuous "chain" of property accesses. If you group one part of the chain, then subsequent property accesses will still be evaluated.

Except the temp variable isn't created.

Basic example

This example looks for the value of the name property for the member bar in a map when there is no such member. The result is therefore undefined .

Dealing with optional callbacks or event handlers

If you use callbacks or fetch methods from an object with a destructuring assignment , you may have non-existent values that you cannot call as functions unless you have tested their existence. Using ?. , you can avoid this extra test:

Stacking the optional chaining operator

With nested structures, it is possible to use optional chaining multiple times:

Combining with the nullish coalescing operator

The nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:

Specifications

Browser compatibility

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

• Nullish coalescing operator ( ?? )