What Values Does Lodash isNil Check For?

The _.isNil method in the Lodash JavaScript library is a utility function designed specifically to determine whether a target value is either null or undefined. This article explains how the function evaluates these two specific values, illustrates its behavior with practical examples, and clarifies how it differs from other type-checking alternatives in JavaScript.

The Two Values Checked by _.isNil

In the Lodash library, _.isNil checks exclusively for the following two values:

null
undefined

If the argument passed to _.isNil is either null or undefined, the function returns true. For any other value—including other falsy values such as false, 0, "" (empty string), or NaN—it returns false.

How It Works

Under the hood, _.isNil functions equivalently to the loose equality check value == null. In standard JavaScript, using the loose equality operator (==) against null returns true only if the operand is null or undefined.

Lodash wraps this check into a descriptive, readable utility function.

Code Examples

const _ = require('lodash');

// Returns true
_.isNil(null);       // => true
_.isNil(undefined);  // => true
_.isNil(void 0);     // => true

// Returns false
_.isNil(false);      // => false
_.isNil(0);          // => false
_.isNil('');         // => false
_.isNil(NaN);        // => false
_.isNil({});         // => false
_.isNil([]);         // => false

Difference Between _.isNil, _.isNull, and _.isUndefined

Lodash provides individual checks for developers who need more strictness:

_.isNull(value): Returns true only if the value is strictly null.
_.isUndefined(value): Returns true only if the value is strictly undefined.
_.isNil(value): Combines the checks above, returning true for both null and undefined.

Summary

_.isNil is a nullish-checking utility that returns true strictly for null and undefined. It ensures that default values or guards are applied appropriately without mistakenly catching valid falsy primitives like zero or empty strings.