JavaScript Function Invocation
The code inside a JavaScriptfunction will execute when "something" invokes it.
Invoking a JavaScript Function
The code inside a function is not executed when the function isdefined.
The code inside a function is executed when the function isinvoked.
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 useinvoke, because a JavaScript function can be invoked without being called.
Invoking a Function as a Function
Example
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.
Note
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:
Example
return a * b;
}
window.myFunction(10, 2); // Will also return 20
What isthis?
In JavaScript, thethis keyword refers to anobject.
Thethis keyword refers todifferent objects depending on how it is used:
Alone,this refers to theglobal object. |
In a function,this refers to theglobal object. |
In a function, in strict mode,this isundefined. |
In an object method,this refers to theobject. |
In an event,this refers to theelement that received the event. |
Methods likecall(),apply(),andbind() can referthis toany object. |
Note
this is not a variable.
this is a keyword.
You cannot change the value ofthis.
The Global Object
When a function is called without an owner object, the value ofthis becomes the global object.
In a web browser the global object is the browser window.
This example returns the window object as the value ofthis:
Example
function myFunction() {
return this;
}
Invoking a function as a global function, causes the value ofthis to be the global object.
Using the window object as a variable can easily crash your program.
Invoking a Function as a Method
In JavaScript you can define functions as object methods.
The following example creates an object (myObject), with two properties (firstName andlastName), and a method (fullName):
Example
firstName:"John",
lastName: "Doe",
fullName: function () {
return this.firstName + " " + this.lastName;
}
}
myObject.fullName(); // Will return "John Doe"
ThefullName method is a function. The function belongs to the object.myObject is the owner of the function.
The thing calledthis, is the object that "owns" the JavaScript code. In this case the value ofthis ismyObject.
Test it! Change thefullName method to return the value ofthis:
Example
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 ofthis to be the object itself.
Invoking a Function with the Constructor
If a function invocation is preceded with thenew 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:
Example
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.
Thethis keyword in the constructor does not have a value.
The value ofthis will be the new object created when the function is invoked.
