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JS Functions
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JavaScript Function Call
Method Reuse
With the call() method, you can write a method that can be
used on different
objects.
All Functions are Methods
In JavaScript all functions are object methods.
If a function is not a method of a JavaScript object, it is a function of the global object (see previous chapter).
The example below creates an object with 3 properties, firstName, lastName, fullName.
Example
firstName:"John",
lastName: "Doe",
fullName: function () {
return this.firstName + " " + this.lastName;
}
}
// This will return "John Doe":
myObject.fullName();
The this Keyword
In a function definition, this refers to the "owner" of the
function.
In the example above, this is the person
object that "owns" the
fullName function.
In other words, this.firstName means the firstName property of this object.
The JavaScript call() Method
The call() method is a predefined
JavaScript method.
It can be used to invoke (call) a method with an owner object as an argument (parameter).
With call(), an object can use a method belonging to
another object.
This example calls the fullName method of person, using it on person1:
Example
fullName: function() {
return this.firstName + " " + this.lastName;
}
}
const person1 = {
firstName:"John",
lastName: "Doe"
}
const person2 = {
firstName:"Mary",
lastName: "Doe"
}
// This will return "John Doe":
person.fullName.call(person1);
This example calls the fullName method of person, using it on person2:
Example
fullName: function() {
return this.firstName + " " + this.lastName;
}
}
const person1 = {
firstName:"John",
lastName: "Doe"
}
const person2 = {
firstName:"Mary",
lastName: "Doe"
}
// This will return "Mary Doe"
person.fullName.call(person2);
The call() Method with Arguments
The call() method can accept arguments:
Example
fullName: function(city, country) {
return this.firstName + " " + this.lastName + "," + city + "," + country;
}
}
const person1 = {
firstName:"John",
lastName: "Doe"
}
person.fullName.call(person1, "Oslo", "Norway");