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Severity: warning
Category: lint
Stage: Stage 3

Description

A let binding that is declared and initialized but never reassigned should be declared with const instead. Using const communicates intent (the binding will not change), prevents accidental reassignment, and allows the compiler to apply stronger optimizations.

The lint pass tracks every let binding in a scope and checks whether any subsequent assignment or compound-assignment operator (=, +=, -=, ++, --, etc.) targets it. If none does, the binding is flagged.

Example

// ✗ lint warning
let x = 5          // SJS-L001 — x is never reassigned
let name = "Alice" // SJS-L001 — name is never reassigned

function greet() {
  let prefix = "Hello" // SJS-L001
  return `${prefix}, world`
}

Fix

Replace let with const for bindings that are never reassigned:

// ✓ correct
const x = 5
const name = "Alice"

function greet() {
  const prefix = "Hello"
  return `${prefix}, world`
}

Keep let only when reassignment is genuinely needed:

// ✓ correct — reassignment occurs
let count = 0
count += 1

Auto-fix

This diagnostic is auto-fixable. Running superjs fix replaces let with const for every flagged binding.

Configuration

Configurable in superjs.config.json:

{
  "lint": {
    "L001": "error" | "warn" | "off"
  }
}

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

Related codes

  • SJS-L002 — prefer let or const over var
Documentation