typeof Compared Against Non-Standard String

ID

javascript.typeof_nonstandard

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

JavaScript

Tags

reliability, suspicious-comparison

Description

Reports comparisons of the form typeof X === "literal" (or !==) where the right-hand string literal is not one of the eight values that typeof can ever return:

"undefined", "object", "boolean", "number", "bigint", "string", "symbol", "function".

Any other string makes the comparison statically false (or true for !==), so the branch behind the test is dead code.

Rationale

The rule catches three distinct mistakes:

  • Typos: typeof fn === "fucntion" never matches.

  • Confused with instanceof: arrays and dates appear as "object" to typeof, never as "array" or "date".

  • Confused with === null: typeof x === "null" never matches; typeof null is "object".

// Bad
if (typeof xs === "array")    handle(xs);   // never fires
if (typeof fn === "fucntion") fn();         // typo, never fires
if (typeof v === "null")      handleNull(); // typeof null === "object"

Remediation

Use Array.isArray, instanceof, or a direct === null check:

// Good
if (Array.isArray(xs))           handle(xs);
if (typeof fn === "function")    fn();
if (v === null)                  handleNull();

References