https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply

The apply() method calls a function with a given this value, and arguments provided as an array (or an array-like object).

Syntax Section

function.apply(thisArg, [argsArray])

Parameters Section

thisArg

The value of this provided for the call to func. Note that this may not be the actual value seen by the method: if the method is a function in non-strict mode code, null and undefined will be replaced with the global object, and primitive values will be boxed. This argument is not optional

argsArray

Optional. An array-like object, specifying the arguments with which func should be called, or null or undefined if no arguments should be provided to the function. Starting with ECMAScript 5 these arguments can be a generic array-like object instead of an array. See below for browser compatibility information.

Return value Section

The result of calling the function with the specified this value and arguments.

Description Section

You can assign a different this object when calling an existing function. this refers to the current object, the calling object. With apply, you can write a method once and then inherit it in another object, without having to rewrite the method for the new object.

apply is very similar to call(), except for the type of arguments it supports. You use an arguments array instead of a list of arguments (parameters). With apply, you can also use an array literal, for example, func.apply(this, ['eat', 'bananas']), or an Array object, for example, func.apply(this, new Array('eat', 'bananas')).

You can also use arguments for the argsArray parameter. arguments is a local variable of a function. It can be used for all unspecified arguments of the called object. Thus, you do not have to know the arguments of the called object when you use the apply method. You can use arguments to pass all the arguments to the called object. The called object is then responsible for handling the arguments.

Since ECMAScript 5th Edition you can also use any kind of object which is array-like, so in practice, this means it's going to have a property length and integer properties in the range (0..length-1). As an example you can now use a NodeList or a custom object like { 'length': 2, '0': 'eat', '1': 'bananas' }.

Examples Section

Using apply to append an array to another Section

We can use push to append an element to an array. And, because push accepts a variable number of arguments, we can also push multiple elements at once. But, if we pass an array to push, it will actually add that array as a single element, instead of adding the elements individually, so we end up with an array inside an array. What if that is not what we want? concat does have the behaviour we want in this case, but it does not actually append to the existing array but creates and returns a new array. But we wanted to append to our existing array... So what now? Write a loop? Surely not?

apply to the rescue!

var array = ['a', 'b'];

var elements = [0, 1, 2];

array.push.apply(array, elements);

console.info(array); // ["a", "b", 0, 1, 2]

https://www.w3schools.com/js/js_function_apply.asp

Method Reuse

With the apply() method, you can write a method that can be used on different objects.

The JavaScript apply() Method

The apply() method is similar to the call() method (previous chapter).

In this example the fullName method of person is applied on person1:

 var person = {

  fullName: function() {

    return this.firstName + " " + this.lastName;

  }

}

var person1 = {

  firstName: "Mary",

  lastName: "Doe"

}

person.fullName.apply(person1);  // Will return "Mary Doe" 

The Difference Between call() and apply()

The difference is:

The call() method takes arguments separately.

The apply() method takes arguments as an array.

var person = {

  fullName: function(city, country) {

    return this.firstName + " " + this.lastName + "," + city + "," + country;

  }

}

var person1 = {

  firstName:"John",

  lastName: "Doe"

}

person.fullName.apply(person1, ["Oslo", "Norway"]);  //這里是一個(gè)array參數(shù)

var person = {

  fullName: function(city, country) {

    return this.firstName + " " + this.lastName + "," + city + "," + country;

  }

}

var person1 = {

  firstName:"John",

  lastName: "Doe"

}

person.fullName.call(person1, "Oslo", "Norway"); //這里是2個(gè)參數(shù)

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