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The code inside a JavaScript function will execute when "something" invokes it.
The code inside a function is not executed when the function is defined.
The code inside a function is executed when the function is invoked.
It is common to use the term "call a function" instead of "invoke a function".
It is also common to say "call upon a function", "start a function", or "execute a function".
In this tutorial, we will use invoke, because a JavaScript function can be invoked without being called.
Sometimes when we define a function, it does not belong to any object. But in JavaScript there is always a default global object.
In HTML the default global object is the HTML page itself, so the function above "belongs" to the HTML page.
In a browser the page object is the browser window. The function above automatically becomes a window function.
myFunction() and window.myFunction() is the same function.
function myFunction(a, b) {
return a * b;
}
myFunction(10, 2); // Will return 20
window.myFunction(10, 2); // Will also return 20
When a function is called without an owner object, the value of this becomes the global object.
In a web browser the global object is the browser window.
let x = myFunction(); // x will be the window object
function myFunction() {
return this;
}
In JavaScript you can define functions as object methods.
The following example creates an object (myObject), with two properties (firstName and lastName), and a method (fullName).
const myObject = {
firstName:"John",
lastName: "Doe",
fullName: function () {
return this.firstName + " " + this.lastName;
}
}
myObject.fullName(); // Will return "John Doe"
The fullName method is a function. The function belongs to the object. myObject is the owner of the function.
The thing called this, is the object that "owns" the JavaScript code. In this case the value of this is myObject.
const myObject = {
firstName:"John",
lastName: "Doe",
fullName: function () {
return this;
}
}
myObject.fullName(); // [object Object]
If a function invocation is preceded with the new keyword, it is a constructor invocation.
It looks like you create a new function, but since JavaScript functions are objects you actually create a new object.
A constructor invocation creates a new object. The new object inherits the properties and methods from its constructor.
// This is a function constructor:
function myFunction(arg1, arg2) {
this.firstName = arg1;
this.lastName = arg2;
}
// This creates a new object
const myObj = new myFunction("John", "Doe");
myObj.firstName; // "John"
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