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JavaScript check if undefined | Example code

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In JavaScript, you can use the typeof operator to check if a variable is undefined. If a variable is not initialized, its value will also be undefined, so you can also use typeof to check if a variable has been declared but not assigned a value.

In JavaScript, you can check if a variable is undefined using the typeof operator:

if (typeof something != "undefined") {
    // ...

Alternatively, you can also use the === operator to directly compare the variable to undefined:

if (variableName === undefined) {
  // variable is undefined

JavaScript checks if undefined

A simple example code check is if a variable is undefined using the typeof operator.

// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"

If you are interested in knowing whether the variable hasn’t been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string:

<!DOCTYPE html>

   if (typeof myVar == 'undefined')
    alert("Hello variable")




JavaScript check if undefined


Do comment if you have any doubts or suggestions on this JS if the topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version

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