no-redeclare

A second var with the same name doesn't make a new variable. It writes to the first one, so two parts of a function that each think they own total overwrite each other. The same goes for a var with the same name as a parameter, and for two functions with the same name, where the second replaces the first.

Declaring a let, const or class twice is already a syntax error.

This rule is on by default in JavaScript files, and off by default in TypeScript files.

Configure#

On by default as a warning. To turn it off:

[lint.rules]
"no-redeclare" = "off" # or "error"

Every option, set to its default:

[lint.rules]
"no-redeclare" = ["warn", {
  builtinGlobals = true,
}]

Examples#

function summarize(items, label) {
  var total = items.length;
  var label = label || "items";

  for (const item of items) {
    var total = item.price;
  }
  return `${total} ${label}`;
}

function setup() {
  function log(message) {
    console.log(message);
  }
  function log(message, level) {
    console.log(level, message);
  }
}

function summarize(items, label = "items") {
  let total = items.length;

  for (const item of items) {
    total += item.price;
  }
  return `${total} ${label}`;
}

If you turn the rule on in bunfig.toml, TypeScript files get @typescript-eslint/no-redeclare instead, which understands function overloads:

export function parse(input: string): number;
export function parse(input: Buffer): number;
export function parse(input: string | Buffer): number {
  return input.length;
}

Options#

A /* global */ comment that names a built-in global counts as a second declaration:

/* global process */

console.log(process.platform);

With { builtinGlobals: false }, the same file isn't reported:

/* global process */

console.log(process.platform);

A top-level var only clashes with a global in a classic script. In a module or a CommonJS file it's local to the file, so it isn't reported:

var name = "Ada";

export { name };