Lodash isUndefined vs Strict Equality Performance

This article evaluates the performance characteristics of using Lodash’s _.isUndefined compared to native strict equality (=== undefined). Contrary to common misconceptions, there is no explicit runtime performance benefit to invoking _.isUndefined for standard variable checks; direct equality checks execute significantly faster in modern JavaScript runtimes. The real advantages of Lodash's utility lie in functional programming pipelines and object-allocation reduction when passing predicates, rather than raw execution speed.

Execution Speed and the V8 Engine

At the CPU and JavaScript engine level, a direct strict equality check like value === undefined maps directly to low-level bytecode comparisons (such as the StrictEquals instruction in the V8 engine). This comparison resolves almost instantaneously in registers without creating execution frames.

Invoking _.isUndefined(value), on the other hand, introduces measurable overhead:

  1. Property Lookup: Accessing the method on the _ namespace (if not destructured).
  2. Function Invocation: Pushing and popping a new call stack frame.
  3. Parameter Passing: Passing the argument to the function scope.

While JIT compilers can inline simple functions during hot loops, un-inlined calls to _.isUndefined can perform orders of magnitude slower than a raw native operator.

How Lodash Implements isUndefined

Looking at the source code of Lodash demystifies its behavior:

function isUndefined(value) {
  return value === undefined;
}

Because the function merely wraps a strict equality operator, calling _.isUndefined cannot inherently run faster than the operator it encapsulates. In fact, within Lodash’s own internal library routines, Lodash avoids calling _.isUndefined and instead relies directly on value === undefined or value === void 0 specifically to avoid unnecessary invocation overhead.

The Only Scenario Where Performance Benefits Apply

The only condition under which _.isUndefined yields an explicit performance benefit over an inlined check is when working with higher-order functions.

Consider evaluating an array using functional iteration:

// Creates a new closure every evaluation
const hasUndefined = array.some(item => item === undefined);

// Reuses an existing function reference
const hasUndefined = array.some(_.isUndefined);

In scenarios where higher-order utilities (such as Array.prototype.filter, Array.prototype.some, or Lodash collection methods) are repeatedly called, passing _.isUndefined directly as a predicate avoids creating and garbage-collecting anonymous arrow functions. This slightly reduces memory churn and garbage collection pressure in high-frequency rendering or heavy compute paths.

Historical Safety vs. Modern JavaScript

Historically, in ECMAScript 3, undefined was a mutable global property that could be reassigned (e.g., window.undefined = true). Older libraries frequently used void 0 or dedicated helper functions to guard against hostile or buggy runtime modifications. In ECMAScript 5 and later, global undefined is non-writable and read-only. As a result, the protective benefit of a specialized helper is obsolete in modern runtimes.

Practical Takeaways