JavaScript Function Definitions
JavaScript functions are defined with the
function keyword.
You can use a function declaration or a function expression.
Function Declarations
Earlier in this tutorial, you learned that functions are declared with the following syntax:
function functionName(parameters) {
// code to be executed
}
Declared functions are not executed immediately. They are "saved for later use", and will be executed later, when they are invoked (called upon).
Note
Semicolons are used to separate executable JavaScript statements.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
Function Expressions
After a function expression has been stored in a variable, the variable can be used as a function:
The Function() Constructor
As you have seen in the previous examples, JavaScript functions are defined
with the function keyword.
Functions can also be defined with a built-in JavaScript function constructor called
Function().
Example
const myFunction = new Function("a", "b", "return a * b");
let x = myFunction(4, 3);
Try it Yourself »
You actually don't have to use the function constructor. The example above is the same as writing:
Example
const myFunction = function (a, b) {return a * b};
let x = myFunction(4, 3);
Try it Yourself »
Most of the time, you can avoid using the new keyword in JavaScript.
Function Hoisting
Earlier in this tutorial, you learned about "hoisting" (JavaScript Hoisting).
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.
Hoisting applies to variable declarations and to function declarations.
Because of this, JavaScript functions can be called before they are declared:
myFunction(5);
function myFunction(y) {
return y * y;
}
Functions defined using an expression are not hoisted.
Functions Can Be Used as Values
JavaScript functions can be used as values:
JavaScript functions can be used in expressions:
Functions are Objects
The typeof operator in JavaScript returns "function" for
functions.
But, JavaScript functions can best be described as objects.
JavaScript functions have both properties and methods.
The arguments.length property returns the number of arguments received when
the function was invoked:
The toString() method returns the function as a string:
Example
function myFunction(a, b) {
return a * b;
}
let text = myFunction.toString();
Try it Yourself »
A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.
