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C # tutorial
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.
function myFunction(a, b) {
return a * b;
}
myFunction(10, 2); //
Will return 20
The function above 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.
This is a common way to invoke a JavaScript function, but not a very good practice.
Global variables, methods, or functions can easily create name conflicts and bugs in the global object.
myFunction() and window.myFunction() is the same function:
function myFunction(a, b) {
return a * b;
}
window.myFunction(10, 2); // Will also return 20
In JavaScript, the this
keyword refers to an object.
this
is being invoked (used or called).
The this
keyword refers to different objects depending on how it is used:
In an object method, this refers to the object. |
Alone, this refers to the global object. |
In a function, this refers to the global object. |
In a function, in strict mode, this is undefined . |
In an event, this refers to the element that received the event. |
Methods like call() , apply() ,
and bind() can refer this to any object. |
this
is not a variable. It is a keyword. You cannot change the value of this
.
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.
This example returns the window object as the value of this
:
let x = myFunction();
// x will be the window object
function myFunction() {
return this;
}
Invoking a function as a global function, causes the value of this to be the global object.
Using the window object as a variable can easily crash your program.
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.
Test it! Change the fullName method to return the value of this
:
const myObject = {
firstName:"John",
lastName: "Doe",
fullName: function () {
return this;
}
}
// This will return [object Object] (the owner object)
myObject.fullName();
Invoking a function as an object method, causes the value of this
to be the object itself.
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:
// 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");
// This will return "John"
myObj.firstName;
A constructor invocation creates a new object. The new object inherits the properties and methods from its constructor.
The this
keyword in the constructor does not have a value.
The value of this
will be the new object created when the function is invoked.