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JavaScript Function apply()


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:

Example

const person = {
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }}

const person1 = {
  firstName: "Mary",
  lastName: "Doe"}

// This will return "Mary Doe":
person.fullName.apply(person1);


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The Difference Between call() and apply()
The difference is:
The call() method takes arguments separately.
The apply() method takes arguments as an array.

The apply() method is very handy if you want to use an array instead of an argument list.


The apply() Method with Arguments
The apply() method accepts arguments in an array:

Example

const person = {
  fullName: function(city, country) {
    return this.firstName + " " + this.lastName
+ "," + city + "," + country;
  }}

const person1 = {
  firstName:"John",
  lastName: "Doe"
}

person.fullName.apply(person1, ["Oslo", "Norway"]);


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Compared with the call() method:

Example

const person = {
  fullName: function(city, country) {
    return this.firstName + " " + this.lastName
+ "," + city + "," + country;
  }}

const person1 = {
  firstName:"John",
  lastName: "Doe"
}

person.fullName.call(person1, "Oslo", "Norway");


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Simulate a Max Method on Arrays
You can find the largest number (in a list of numbers) using the Math.max() method:

Example

Math.max(1,2,3);  // Will return 3


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Since JavaScript arrays do not have a max() method, you can apply the
Math.max() method instead.

Example

Math.max.apply(null, [1,2,3]); // Will also return 3


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The first argument (null) does not matter. It is not used in this example.
These examples will give the same result:

Example

Math.max.apply(Math, [1,2,3]); // Will also return 3


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Example

Math.max.apply(" ", [1,2,3]); // Will also return 3


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Example

Math.max.apply(0, [1,2,3]); // Will also return 3


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JavaScript Strict Mode
In JavaScript strict mode, if the first argument of the apply() method is not an object,
it becomes the owner (object) of the invoked function.
In "non-strict" mode, it becomes the global object.












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