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Severity: warning
Category: lint
Stage: Stage 3

Description

The abstract equality operators == and != perform type coercion before comparison, producing results that are frequently surprising:

  • "" == false → true
  • 0 == false → true
  • null == undefined → true
  • " \t\n" == 0 → true

The strict equality operators === and !== never coerce: two values are strictly equal only if they have the same type and the same value.

Special case — x == null guard: The pattern x == null (or x != null) is a common idiom that tests for both null and undefined in one expression. SJS flags it all the same, because nullable types (T?) should be narrowed explicitly. Replace the idiom with x === null || x === undefined, or leverage the type system to handle nullability directly.

Example

// ✗ lint warning
if (score == 0) { ... }           // SJS-L003
if (input != "") { ... }          // SJS-L003
const isEmpty = arr.length == 0   // SJS-L003

function check(value: string?) {
  if (value == null) { ... }      // SJS-L003 — use explicit null/undefined check
}

Fix

Replace == with === and != with !==. For the null-guard idiom, expand the check:

// ✓ correct
if (score === 0) { ... }
if (input !== "") { ... }
const isEmpty = arr.length === 0

function check(value: string?) {
  if (value === null || value === undefined) { ... }
  // or, using the nullable type directly:
  if (value === null) { ... }
}

Auto-fix

This diagnostic is auto-fixable. Running superjs fix replaces == with === and != with !== at every flagged site. The x == null idiom is expanded to x === null || x === undefined.

Configuration

Configurable in superjs.config.json:

{
  "lint": {
    "L003": "error" | "warn" | "off"
  }
}

Default: "warn" in standard mode, "error" in --strict mode.

Related codes

  • SJS-E001 — null or undefined assigned to non-nullable type
  • SJS-E003 — property access on possibly-null value
Documentation