Thursday 14 February 2019

What is the difference between null and undefined in JavaScript?



I want to know what the difference is between null and undefined in JavaScript.



Answer



In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:



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'

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