How to determine if variable is undefined or null in JavaScript
source link: https://www.laravelcode.com/post/how-to-determine-if-variable-is-undefined-or-null-in-javascript
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How to determine if variable is undefined or null in JavaScript
Use the equality operator (==
)
In JavaScript if a variable has been declared, but has not been assigned a value, is automatically assigned the value undefined
. Therefore, if you try to display the value of such variable, the word "undefined" will be displayed. Whereas, the null
is a special assignment value, which can be assigned to a variable as a representation of no value.
In simple words you can say a null
value means no value or absence of a value, and undefined
means a variable that has been declared but no yet assigned a value.
To check if a variable is undefined or null you can use the equality operator ==
or strict equality operator ===
(also called identity operator). Let's take a look at the following example:
<script>
var firstName;
var lastName = null;
// Try to get non existing DOM element
var comment = document.getElementById('comment');
console.log(firstName); // Print: undefined
console.log(lastName); // Print: null
console.log(comment); // Print: null
console.log(typeof firstName); // Print: undefined
console.log(typeof lastName); // Print: object
console.log(typeof comment); // Print: object
console.log(null == undefined) // Print: true
console.log(null === undefined) // Print: false
/* Since null == undefined is true, the following statements will catch both null and undefined */
if(firstName == null){
alert('Variable "firstName" is undefined.');
}
if(lastName == null){
alert('Variable "lastName" is null.');
}
/* Since null === undefined is false, the following statements will catch only null or undefined */
if(typeof comment === 'undefined') {
alert('Variable "comment" is undefined.');
} else if(comment === null){
alert('Variable "comment" is null.');
}
</script>
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