undefined

undefined — A variable which does not have it’s value defined.

Description

undefined

Examples

Example #1 – undefined

Run
var a;
var b = undefined;
console.log(typeof a);          // undefined
console.log(a);                 // undefined
console.log(typeof b);          // undefined
console.log(b);                 // undefined

Example #2 – In some situations you might en up with the sting 'undefined'

When checking typeof an undefined variable, the result is a string.

Run
var a;
console.log(a === undefined);                 // true
console.log(typeof a === undefined);          // false
console.log(a === 'undefined');               // false
console.log(typeof a === 'undefined');        // true

External references

• Mozilla: undefined