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Severity: warning
Category: lint
Stage: Stage 3

Description

for…in iterates over all enumerable string keys of an object, including inherited ones from the prototype chain. When used on arrays or other iterables the keys are the index strings ("0", "1", …), not the values, and any enumerable properties added to Array.prototype will also appear. This is almost never the intent.

for…of iterates over the values yielded by an object's [Symbol.iterator], which is the correct and safe choice for arrays, strings, Set, Map, generator results, and any other iterable.

Use for…in only when you genuinely need the enumerable property keys of a plain object — and even then, combine it with Object.hasOwn() to skip inherited keys.

The lint pass flags for…in whenever the operand's static type is an array type (T[]) or any other iterable (i.e., implements Iterable<T>). Plain objects ({ [key: string]: T }) are exempt.

Example

// ✗ lint warning
const nums: number[] = [1, 2, 3]
for (const i in nums) {           // SJS-L004 — i is "0", "1", "2", not 1, 2, 3
  console.log(nums[i])
}

for (const ch in "hello") {       // SJS-L004 — string is iterable, use for…of
  process(ch)
}

Fix

Replace for…in with for…of to iterate values directly:

// ✓ correct
const nums: number[] = [1, 2, 3]
for (const n of nums) {
  console.log(n)
}

for (const ch of "hello") {
  process(ch)
}

If you need the index as well, use Array.prototype.entries():

// ✓ correct — index + value
for (const [i, n] of nums.entries()) {
  console.log(i, n)
}

for…in on a plain object remains acceptable:

// ✓ correct — plain object key iteration
const config: { [key: string]: string } = { a: "1", b: "2" }
for (const key in config) {
  if (Object.hasOwn(config, key)) {
    console.log(key, config[key])
  }
}

Configuration

Configurable in superjs.config.json:

{
  "lint": {
    "L004": "error" | "warn" | "off"
  }
}

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

Related codes

  • SJS-L001 — prefer const over let when a binding is never reassigned
  • SJS-L002 — prefer let or const over var
Documentation