no-use-before-define

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

The rule goes by position in the file. A function that mentions a variable declared below it is reported, even if the function only runs later. The options relax that.

Configure

Off by default. To turn it on:

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

Every option, set to its default:

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

Examples

Reported:

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

start();
function start() {}

const total = total + 1;

function report() {
  return limit;
}

const limit = 10;

Not reported:

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

function start() {}
start();

Options

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

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

With functions: false, you can call a function declaration early. It's already defined when the file starts to run:

start();

function start() {}

With variables: false, a function may mention a variable that's declared below it:

function report() {
  return 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;

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

export { limit };

const limit = 10;

Types don't exist when the code runs, so by default you can name a type before it's declared:

let current: User;

interface User {
  name: string;
}

With ignoreTypeReferences: false, the same code is reported:

let current: User;

interface User {
  name: string;
}