Lodash isNull vs Undefined Equality Checks

Lodash’s _.isNull method and strict equality comparisons against undefined serve distinct functional, semantic, and structural purposes in JavaScript development. While _.isNull is an encapsulated utility function that checks solely for the literal null primitive, comparisons against undefined evaluate whether an identifier or property lacks an assigned value. Understanding their structural differences requires analyzing how each handles value representation, JavaScript's type system, engine-level execution, and property resolution.

Internal Implementation and Syntax

Structurally, _.isNull is an exported functional wrapper defined in the Lodash source code as:

function isNull(value) {
  return value === null;
}

By contrast, a strict check against undefined typically occurs inline via native language operators:

value === undefined;
// or defensively
value === void 0;

While both rely on the strict identity operator (===), _.isNull introduces a function invocation boundary. Unless optimized away by a modern Just-In-Time (JIT) compiler through inlining, calling _.isNull(val) involves function execution overhead, argument passing, and stack frame allocation that native inline checks do not require.

Keyword Mechanics vs. Global Identifiers

A core structural distinction lies in how the JavaScript runtime handles null versus undefined:

• null is a Reserved Keyword: null is a language literal and cannot be redefined or assigned to in any scope. Lodash's value === null evaluation is structurally immutable and safe across all JavaScript environments.
• undefined is a Global Property: Unlike null, undefined is a property of the global object. Although ECMAScript 5 made the global undefined non-writable and non-configurable, it can still be shadowed by a local variable inside a function scope (e.g., let undefined = true;). Because of this structural vulnerability in the language specification, developers and libraries historically favored typeof x === 'undefined' or x === void 0 rather than direct === undefined checks.

Semantic Meaning and Type System Representation

In JavaScript's type hierarchy, null and undefined represent fundamentally different states of absence:

• Intentional Emptiness (null): _.isNull evaluates whether a value has been explicitly assigned to point to no object. It represents an intentional absence of value. Under typeof, null historically and structurally returns "object" due to a legacy design quirk in the original JavaScript engine, yet _.isNull bypasses this by using direct identity comparison.
• Implicit Absence (undefined): Evaluating strictly against undefined tests whether a variable has been declared but not initialized, whether an argument was omitted from a function call, or whether a function exited without an explicit return.

Because strict equality (===) does not perform type coercion, _.isNull(undefined) evaluates to false, and null === undefined evaluates to false.

Property Resolution and Access Behavior

When traversing objects, accessing a non-existent property returns undefined by default rather than throwing a reference error. Consequently:

const user = {};

user.profile === undefined; // true
_.isNull(user.profile);     // false

Strictly checking against undefined detects missing properties or unassigned keys. Conversely, _.isNull will only return true if the property was explicitly assigned the value null (user.profile = null;).

When developers need to treat both structural states uniformly, Lodash provides _.isNil, which uses loose equality (value == null) to match both null and undefined simultaneously.