Strict Equality (===) Required

ID

javascript.strict_equals

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

JavaScript

Tags

best-practice, reliability

Description

Reports the loose-equality operators == and !=, which apply implicit type coercion before comparing. The coercion rules produce many surprising true/false outcomes; almost all of them are bugs. Use === and !==.

The lone idiomatic exception is the x == null (or null == x) shape, which is a common shorthand for "x is null or undefined`". This rule allows that exact shape and flags every other use of `== / !=.

Rationale

  • "" == 0, "0" == false, [] == false, [] == ![] are all true. Each one has bitten thousands of developers.

  • null == undefined is true but null === undefined is false — the asymmetry hides bugs.

  • Type-aware comparisons document intent. Readers and tools (TypeScript, refactoring engines) can reason about typed equality far better than coerced equality.

// Bad - all coerce silently
if (count == "0") { ... }
if (status != 0) { ... }

Remediation

Use === / !==. Keep the x == null shorthand only when you specifically want "null or undefined".

// Good
if (count === 0) { ... }
if (status !== 0) { ... }

// Allowed - null-or-undefined check
if (value == null) { ... }

References