@typescript-eslint/no-use-before-define

Reading a let, const, or class before its declaration runs throws a ReferenceError. Reading a var early gives you undefined without any error. Function declarations do work before their line, but the reader needs to scroll down to find out what's being called.

A variable used in its own initializer, such as const total = total + 1, is reported too. Unlike ESLint's no-use-before-define, this rule also checks enums, type aliases, and interfaces.

Configure

Off by default. To turn it on:

[lint.rules]
"@typescript-eslint/no-use-before-define" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"@typescript-eslint/no-use-before-define" = ["error", {
  functions = true,
  variables = true,
  classes = true,
  enums = true,
  typedefs = true,
  ignoreTypeReferences = true,
  allowNamedExports = false,
}]

Examples

Reported:

console.log(limit);
const limit = 10;

start(Mode.Fast);
function start(mode: Mode) {}

enum Mode {
  Fast,
  Slow,
}

Not reported:

const limit = 10;
console.log(limit);

enum Mode {
  Fast,
  Slow,
}

function start(mode: Mode) {}
start(Mode.Fast);

Options

Option	Default	What it does
functions	true	Report function declarations that are used early. false allows every early use of them.
variables	true	With false, a function can mention a var, let, or const that's declared below it. An early use in the same scope is still reported.
classes	true	The same as variables, for classes.
enums	true	The same as variables, for enums.
typedefs	true	Report type aliases and interfaces that are used early. This only has an effect with ignoreTypeReferences: false.
ignoreTypeReferences	true	Allow early uses in a type position, such as let user: User or typeof config.
allowNamedExports	false	Allow export { name } above the declaration of name.

The string "nofunc" is short for { "functions": false }.

With functions: false, calling a function declaration early is allowed, because it's already defined when the file starts to run:

start();

function start() {}

By default, only the position in the file counts, even in a function that runs later:

function isFull(count: number) {
  return count >= limit;
}

const limit = 10;

function isFull(count: number) {
  return count >= limit;
}

const limit = 10;

An early use in the same scope runs before the declaration, so it's still reported:

console.log(limit);

const limit = 10;

Types don't exist when the code runs, so by default their order doesn't matter where they're used as types:

let current: User;

type User = { name: string };

let current: User;

type User = { name: string };

let current: User;

type User = { name: string };

With allowNamedExports: true, you can keep the export list at the top of the file:

export { limit };

const limit = 10;