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).Example
function myFunction(a, b) {
return a * b;
}
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.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
Function Expressions
A JavaScript function can also be defined using an expression.A function expression can be stored in a variable:
Example
var x = function (a, b) {return a * b};
Example
var x = function (a, b) {return a * b};
var z = x(4, 3);
Functions stored in variables do not need function names. They are always invoked (called) using the variable name.
The function above ends with a semicolon because it is a part of an executable statement.
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
var myFunction = new Function("a", "b", "return a * b");
var x = myFunction(4, 3);
Example
var myFunction = function (a, b) {return a * b};
var x = myFunction(4, 3);
Most of the time, you can avoid using the new keyword in JavaScript.
Function Hoisting
Earlier in this tutorial, you learned about "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.Self-Invoking Functions
Function expressions can be made "self-invoking".A self-invoking expression is invoked (started) automatically, without being called.
Function expressions will execute automatically if the expression is followed by ().
You cannot self-invoke a function declaration.
You have to add parentheses around the function to indicate that it is a function expression:
Example
(function () {
var x = "Hello!!"; // I will invoke myself})();
Functions Can Be Used as Values
JavaScript functions can be used as values:Example
function myFunction(a, b) {
return a * b;
}
var x = myFunction(4, 3);
Example
function myFunction(a, b) {
return a * b;
}
var x = myFunction(4, 3) * 2;
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:
Example
function myFunction(a, b) {
return arguments.length;
}
Example
function myFunction(a, b) {
return a * b;
}
var txt = myFunction.toString();