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Severity: warning
Category: lint
Stage: Stage 3

Description

var declarations have function scope (not block scope), are hoisted to the top of their enclosing function with an initial value of undefined, and have no temporal dead zone (TDZ). These semantics are a common source of bugs — a variable declared in the body of a loop or if block leaks out of that block, and accessing it before its declaration silently returns undefined instead of a reference error.

let and const are block-scoped, have a TDZ, and are the idiomatic replacements in modern JavaScript and SJS. Every var declaration should be rewritten as let (if the binding needs reassignment) or const (if it does not — see SJS-L001).

Example

// ✗ lint warning
var total = 0          // SJS-L002

function compute() {
  for (var i = 0; i < 10; i++) {  // SJS-L002 — i leaks outside the loop
    var partial = i * 2            // SJS-L002
    total += partial
  }
  console.log(i)       // 10 — var leaks; with let this would be a reference error
}

Fix

Replace var with let or const as appropriate:

// ✓ correct
let total = 0

function compute() {
  for (let i = 0; i < 10; i++) {
    const partial = i * 2
    total += partial
  }
  // i is not accessible here — block-scoped
}

Auto-fix

This diagnostic is auto-fixable. Running superjs fix replaces var with let. A subsequent pass of SJS-L001 will then upgrade any let that is never reassigned to const.

Note: the auto-fix conservatively emits let (not const) because it does not know at the var replacement stage whether the binding will be reassigned.

Configuration

Configurable in superjs.config.json:

{
  "lint": {
    "L002": "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
Documentation