JavaScript Program To Check If A Variable Is undefined or null

Example 1: Check undefined or null

// program to check if a variable is undefined or null

function checkVariable(variable) {

    if(variable == null) {
        console.log('The variable is undefined or null');
    }
    else {
       console.log('The variable is neither undefined nor null');
    }
}

let newVariable;

checkVariable(5);
checkVariable('hello');
checkVariable(null);
checkVariable(newVariable);

Output

The variable is neither undefined nor null
The variable is neither undefined nor null
The variable is undefined or null
The variable is undefined or null

In the above program, a variable is checked if it is equivalent to null. The null with == checks for both null and undefined values. This is because null == undefined evaluates to true.

The following code:

if(variable == null) { ... }

is equivalent to

if (variable === undefined || variable === null) { ... }

Example 2: using typeof

// program to check if a variable is undefined or null

function checkVariable(variable) {

    if( typeof variable === 'undefined' || variable === null ) {
        console.log('The variable is undefined or null');
    }
    else {
       console.log('The variable is neither undefined nor null');
    }
}

let newVariable;

checkVariable(5);
checkVariable('hello');
checkVariable(null);
checkVariable(newVariable);

Output

The variable is neither undefined nor null
The variable is neither undefined nor null
The variable is undefined or null
The variable is undefined or null

The typeof operator for undefined value returns undefined. Hence, you can check the undefined value using typeof operator. Also, null values are checked using the === operator.

Note: We cannot use the typeof operator for null as it returns object.