undefined
means a variable has been declared but has not yet been assigned a value:
var testVar;
alert(testVar); //shows undefined
alert(typeof testVar); //shows undefined
null
is an assignment value. It can be assigned to a variable as a representation of no value:
var testVar = null;
alert(testVar); //shows null
alert(typeof testVar); //shows object
From the preceding examples, it is clear that undefined
and null
are two distinct types: undefined
is a type itself (undefined) while null
is an object.
null === undefined // false
null == undefined // true
null === null // true
and
null="value" // ReferenceError
undefined = 'value' // 'value'