Компилятор android studio object assign undefined

Understanding Object.assign() Method in JavaScript

The Object.assign() method was introduced in ES6 that copies all enumerable own properties from one or more source objects to a target object, and returns the target object.

The Object.assign() method invokes the getters on the source objects and setters on the target object. It assigns properties only, not copying or defining new properties.

The properties in the target object are overwritten by the properties in source objects if they have the same key. Similarly, the later source objects’ properties are overwritten by the earlier ones.

The Object.assign() method handles null and undefined source values gracefully, and doesn’t throw any exception.

Syntax

Here is how the syntax of Object.assign() looks like:

• target — The target object that is modified and returned after applying the sources’ properties.
• sources — The source object(s) containing the properties you want to apply to the target object.

Cloning an object

The Object.assign() method is one of the four ways, I explained earlier, to clone an object in JavaScript.

The following example demonstrates how you can use Object.assign() to clone an object:

Remember that Object.assign() only creates a shallow clone of the object and not a deep clone.

To create a deep clone, you can either use JSON methods or a 3rd-party library like Lodash.

Merging objects

The Object.assign() method can also merge multiple source objects into a target object. If you do not want to modify the target object, just pass an empty ( <> ) object as target as shown below:

If the source objects have same properties, they are overwritten by the properties of the objects later in the parameters order:

Converting an array to an object

Lastly, you could also use the Object.assign() method to convert an array to an object in JavaScript. The array indexes are converted to object keys, and array values are converted to object values:

Browser compatibility

As Object.assign() is part of ES6, it only works in modern browsers. To support older browsers like IE, you can use a polyfill available on MDN.

To learn more about JavaScript objects, prototypes, and classes, take a look at this guide.

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Object.assign()

The Object.assign() method copies all enumerable own properties from one or more source objects to a target object. It returns the modified target object.

Syntax

Parameters

The target object — what to apply the sources’ properties to, which is returned after it is modified.

The source object(s) — objects containing the properties you want to apply.

Return value

The target object.

Description

Properties in the target object are overwritten by properties in the sources if they have the same key. Later sources’ properties overwrite earlier ones.

The Object.assign() method only copies enumerable and own properties from a source object to a target object. It uses [[Get]] on the source and [[Set]] on the target, so it will invoke getters and setters. Therefore it assigns properties, versus copying or defining new properties. This may make it unsuitable for merging new properties into a prototype if the merge sources contain getters.

For copying property definitions (including their enumerability) into prototypes, use Object.getOwnPropertyDescriptor() and Object.defineProperty() instead.

Both String and Symbol properties are copied.

In case of an error, for example if a property is non-writable, a TypeError is raised, and the target object is changed if any properties are added before the error is raised.

Note: Object.assign() does not throw on null or undefined sources.

Examples

Cloning an object

Warning for Deep Clone

For deep cloning, we need to use alternatives, because Object.assign() copies property values.

If the source value is a reference to an object, it only copies the reference value.

Merging objects

Merging objects with same properties

The properties are overwritten by other objects that have the same properties later in the parameters order.

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Javascript: Object.assign to assign property values for classes is not respected #26792

Comments

ycmjason commented Aug 30, 2018 •

Perhaps TS should be able to identify Object.assign(this, . ) then infer the properties that are assigned to this ? Or is this not possible to do? I am not sure.

TypeScript Version:
Version 3.0.3

Search Terms:

• object assign
• class
• does not exist on type

Code
A.js

Expected behavior:
TS should be able to identify x as a property of this .

Actual behavior:
Throwing:

Related Issues:
no

Side Note:
I am using TS with Javascript because YouCompleteMe switched to using TSServer for semantic completion. I absolutely love TS so was really happy about the switch! Thanks for the great work!

The text was updated successfully, but these errors were encountered:

This comment has been hidden.

GongT commented Sep 3, 2018

Why not support return different thing from constructor()?

It’s more common than only Object.assign.

eritbh commented Feb 20, 2019 •

Came here from #28883. I understand that in the general case, the type of the second argument to Object.assign could be of type any , but in my specific case I define that type via an interface beforehand:

It seems to me that comparison between the types of this and config could be done, and if config is some type that’s not assignable to this , then that would be the condition where you couldn’t determine the type.

The alternative right now is to manually assign every property of the config interface to the corresponding class property, which works but is a pain:

(This whole thing gets even more ugly when dealing when using optional properties on the config object to allow the use of default values:

and as you can see, with lots of properties on the config object, you quickly wind up with a large chunk of code that just does the same job Object.assign normally does, as it automatically handles optional properties. However, because Object.assign interprets a key with value undefined as present, this would be blocked on #13195 for full compatibility.)

Is there any way to achieve this right now? If not, is it viable to implement? I could look into putting a PR together, but no promises as I’ve never worked with TS’s source before.

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Object.assign: How to Copy Object Values in JavaScript

In the object-oriented programming world, we already know the importance of classes and objects. However, unlike all the other programming languages, JavaScript does not have any traditional classes seen in different programming languages. Still, JavaScript has objects and constructors which work mostly in the same way to perform the same kind of operations.

There are two types of constructors in JavaScript,

1. Built-in constructors(array and Object)
2. Custom constructors(define properties and methods for specific objects).

The Object constructor creates the object wrapper for the given value.

If the value is null or undefined, it will create and return the empty Object; Otherwise, it will return the object of a type corresponding to a given value.

If the value is the Object already, it will return the value.

When called in the non-constructor context, Object behaves identically to new Object ().

Properties of the Object constructor

Object.length

It has a value of 1.

Object.prototype

It allows the addition of properties to all objects of the type Object. Methods of the Object constructor.

Object.assign in JavaScript

The Object. assign() is a built-in JavaScript function used to copy the values of all own enumerable properties from one or more source objects to the target object. The Object.assign() method takes target and sources as parameters and returns the target object.

The Object.assign() method is used for cloning an object or merging the objects shallowly. It merges objects with the same properties. The properties in the sources will overwrite properties in a target object if they have the same key.

From the many Object constructor methods, Object.assign() is one method that is used to copy the values and properties from one or more source objects to a target object.

Syntax

The syntax for a Javascript Object assign() method is the following.

Parameters

A target parameter is a target object.

The sources parameter are the objects that need to be cloned.

Return Value

The Object.assign() method returns a the target object.

Example

Let us take a simple example by cloning the Object.

In the above example, we have defined one Object, and then we have created a clone of that object with a new object. The output will be the following.

So, it has copied all the properties from a source object.

Let us merge three objects with Object.assign() method.

Merging objects with the same properties

If the properties get repeated inside the source objects, then in the clone object, the repeated properties will be skipped, and we only get the unique properties on the target object.

Immutability of Object.assign() method

Let’s see the following example.

In the code above, we changed the value of a property ‘mike’ in objCopy Object to ‘Noah Schnapp‘, and when we log a modified objCopy object in the console, the changes only apply objCopy.

The last line of the code checks that the obj object is still intact and hasn’t changed.

This means that we have successfully created a copy of a source object without any references to it.

Pitfall of Object.assign() In Javascript

Let’s discuss a little bit about shallow copying. See the following code example.

So, here Properties on the prototype chain and non-enumerable properties cannot be copied.

Now, let’s understand. Shallow copy and deep copy in Javascript.

What is a shallow copy in Javascript?

When we say we are making a shallow copy of an Array or Object, what that means is that we are creating new references to the primitive values inside the Object, copying them. That means changes to an original array will not affect the copied array, which would happen if only the reference to an array had been copied (that would occur with the assignment operator =).

A shallow copy cites that only one level is copied, which will work fine for the array or Object containing only primitive values.

For objects and arrays containing other objects or arrays, copying these objects requires a deep copy. Otherwise, the nested references’ modification will modify the data nested in an original Object or array.

What is a deep copy in Javascript?

For Javascript objects and arrays containing other objects or arrays, copying these objects needs a deep copy. Otherwise, modifications made to the nested references will change the data nested in the original Object or array.

Compared to the shallow copy, which works fine for the object or array containing only primitive data type values, but will fail for any object or array with nested references to other objects or arrays.

Understanding the difference between == and === can help us visually see the difference between shallow and deep copy, as the strict equality operator (===) shows that the nested references are the same.

A deep copy of an Object in Javascript

Object.assign() is the standard way of copying an object in JavaScript. And also spread operator does the same. Both methodologies can be equivalently used to copy the enumerable properties of an object to another object, with the spread operator being the shorter of the two. They are also handy to merge objects since both methods automatically overwrite the properties in the target object that have the same keys as those in the source object.

A deep copy will duplicate every Object it encounters. The copy and the original Object will not share anything to be a copy of the original.

The spread operator and the Object.assign() method can only make shallow copies of objects. This means that the deeply nested values inside the copied Object are put there to reference the source object. If we modify the copied Object’s deeply nested value, we will, therefore, modify the value in the source object.

JSON.parse(JSON.stringify(object));

I have already written one article on how we can parse JSON in Javascript. Suppose you don’t know what json.parse() does, then please check out that article. Now, this fixes the issue of the above problem.

This fixes the issue we had earlier. Now, newObj.b has a copy and not a reference! This is a way to deep copy objects.

See the following code example.

Benchmarking of Object.assign() and Spread Operator

The Object spread operator <…obj>is similar to Object.assign(). So the question is, which one should you use? And it turns out the answer is a bit more nuanced than you might expect.

The foundation idea of the object spread operator and Object.assign() the same, to create a new plain object using the own properties of the existing object.

For performance checking, we will use the benchmark library.

You can install the benchmark library using the following command.

Now, write the following code inside the app.js file.

Output

You can see that the Object.assign() and Spead operator are interchangeable, and both are faster. But if you provide the empty object, then the Spread operator is even faster than Object.assign().

Benchmarking aims at evaluating something by comparison with standard procedure. However, the question here is that why we need benchmarking in the case of software programming. Benchmarking the code means how fast the code is executing and where the bottleneck is.

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