JavaScript checks if the variable is defined | Example code

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Use typeof to check (determine) if the variable is defined or not in JavaScript. The typeof operator can evaluate even an undeclared identifier without throwing an error.

if(typeof variable === "undefined"){
  /* It is undefined */
}

To avoid accidental assignment, I make a habit of reversing the order of the conditional expression:

if ('undefined' !== typeof x) {
}

JavaScript checks if the variable is defined

Simple example code. The typeof operator, unlike the other operators, doesn’t throw a ReferenceError exception when used with an undeclared symbol

<!DOCTYPE html>
<html>
<body>

  <script>
    var variable = "Hello";

    if (typeof variable != 'undefined') {
      console.log(variable)
    }
  </script>

</body>
</html> 

Output:

JavaScript checks if the variable is defined

More code

<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>

Do comment if you have any doubts or suggestions on this JS variable 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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