How to Set Default Value Only in Case of null or undefined in JavaScript?

To set a default value only in case of a nullish value (i.e. undefined or null), you can do the following:

• Use the Nullish Coalescing Operator;
• Use the Nullish Assignment Operator;
• Explicitly Check for Nullish Value.

Using the Nullish Coalescing Operator

If the left-hand side of an expression is nullish value, then you can simply use the nullish coalescing operator (??) to specify a default value, for example, like so:

// ES11+
console.log(null ?? 'default'); // 'default'
console.log(undefined ?? 'default'); // 'default'

You can also combine the nullish coalescing operator (??) with the optional chaining operator (?.), to return a default value when the optional chaining operator evaluates to undefined. For example:

// ES11+
const obj = { foo: 'bar' };
const value = obj?.nonExistent ?? 'default';

console.log(value); // 'default'

Using the Nullish Assignment Operator

The logical nullish assignment operator (??=) allows you to only assign a value to a variable if it is nullish. For example:

// ES12+
let value;

value ??= 'default';

console.log(value); // 'default'

In the example above, since the variable "value" is undefined, the nullish assignment operator sets its value to 'default'.

Explicitly Checking for Nullish Value

If you are unable to support a minimum ES11, then you can explicitly check for null and undefined values, for example in an if/else or a ternary, like so:

function isNullish(value) {
    return null === value || typeof value === 'undefined';
}

const foo = null;
const bar = undefined;

console.log(isNullish(foo) ? 'default' : foo); // 'default'
console.log(isNullish(bar) ? 'default' : bar); // 'default'