How to check if a variable exists or defined in JavaScript
source link: https://www.laravelcode.com/post/how-to-check-if-a-variable-exists-or-defined-in-javascript
Go to the source link to view the article. You can view the picture content, updated content and better typesetting reading experience. If the link is broken, please click the button below to view the snapshot at that time.
How to check if a variable exists or defined in JavaScript
Use the typeof
operator
If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof
operator.
The most important reason of using the typeof
operator is that it does not throw the ReferenceError if the variable has not been declared. Let's take a look at the following example:
<script>
var x;
var y = 10;
if(typeof x !== 'undefined'){
// this statement will not execute
alert("Variable x is defined.");
}
if(typeof y !== 'undefined'){
// this statement will execute
alert("Variable y is defined.");
}
// Attempt to access an undeclared z variable
if(typeof z !== 'undefined'){
// this statement will not execute
alert("Variable z is defined.");
}
/* Throws Uncaught ReferenceError: z is not defined,
and halt the execution of the script */
if(z !== 'undefined'){
// this statement will not execute
alert("Variable z is defined.");
}
/* If the following statement runs, it will also
throw the Uncaught ReferenceError: z is not defined */
if(z){
// this statement will not execute
alert("Variable z is defined.");
}
</script>
Hi, My name is Harsukh Makwana. i have been work with many programming language like php, python, javascript, node, react, anguler, etc.. since last 5 year. if you have any issue or want me hire then contact me on [email protected]
Recommend
About Joyk
Aggregate valuable and interesting links.
Joyk means Joy of geeK