JavaScript Learning #1 - undefined

Just for my reference!!

Very well explained about 'undefined' usage in JavaScript.  Source is from @Mozilla.

Examples:

Test#1:

function xx() { 
  var x; 
if (x == undefined) {
window.alert("undefined called"); 
} 
else { 
window.alert("Not called");
}
}

xx();

output: "undefined called" window will popup.


Test#2:

function xx() { 
  var x = null; 
if (x == undefined) {
window.alert("undefined called"); 
} 
else { 
window.alert("Not called");
}
}

xx();

output: "undefined called" window will popup.

Test#3:

function xx() { 
  var x = 10; 
if (x == undefined) {
window.alert("undefined called"); 
} 
else { 
window.alert("Not called");
}
}

xx();

output: "Not called" window will popup.



Test#4:

function xx() { 
  var x = null; 
if (typeof x == undefined) {
window.alert("undefined called"); 
} 
else { 
window.alert("Not called");
}
}

xx();

output: "Not called" window will popup.


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Summary

The value undefined.

Core Global Property
Implemented in	JavaScript 1.3
ECMAScript Edition	ECMAScript 1st Edition

Syntax

undefined 

Description

undefined is a property of the global object, i.e. it is a variable in global scope.

The initial value of undefined is the primitive value undefined.

JavaScript 1.8.5 note

Starting in JavaScript 1.8.5 (Firefox 4), undefined is non-writable, as per the ECMAScript 5 specification.

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.

You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not defined, and the if statement evaluates to true.

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var x; if (x === undefined) {    // these statements execute } else {    // these statements do not execute }

Note: The strict equality operator rather than the standard equality operator must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. null is not equivalent to undefined. See comparison operators for details.

Alternatively, typeof() can be used:

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var x; if (typeof x === 'undefined') {    // these statements execute }

One reason to use typeof() is that it does not throw an error if the variable has not been defined.

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// x has not been defined before if (typeof x === 'undefined') { // evaluates to true without errors    // these statements execute } if(x === undefined){ // throws a ReferenceError }

However, this kind of technique should be avoided. JavaScript is a statically scoped language, so knowing if a variable is defined can be read by seeing whether it is defined in an enclosing context. The only exception is the global scope, but the global scope is bound to the global object, so checking the existence of a variable in the global context can be done by checking the existence of a property on the global object (using the in operator, for instance)