no-use-before-define - ESLint - Pluggable JavaScript Linter (original) (raw)

Disallow the use of variables before they are defined

Table of Contents

1. Rule Details
2. Options
   1. functions
   2. classes
   3. variables
   4. allowNamedExports
3. Version
4. Resources

In JavaScript, prior to ES6, variable and function declarations are hoisted to the top of a scope, so it’s possible to use identifiers before their formal declarations in code. This can be confusing and some believe it is best to always declare variables and functions before using them.

In ES6, block-level bindings (let and const) introduce a “temporal dead zone” where a ReferenceError will be thrown with any attempt to access the variable before its declaration.

Rule Details

This rule will warn when it encounters a reference to an identifier that has not yet been declared.

Examples of incorrect code for this rule:

Open in Playground

/*eslint no-use-before-define: "error"*/

alert(a);
var a = 10;

f();
function f() {}

function g() {
    return b;
}
var b = 1;

{
    alert(c);
    let c = 1;
}

{
    class C extends C {}
}

{
    class C {
        static x = "foo";
        [C.x]() {}
    }
}

{
    const C = class {
        static x = C;
    }
}

{
    const C = class {
        static {
            C.x = "foo";
        }
    }
}

export { foo };
const foo = 1;

Examples of correct code for this rule:

Open in Playground

/*eslint no-use-before-define: "error"*/

var a;
a = 10;
alert(a);

function f() {}
f(1);

var b = 1;
function g() {
    return b;
}

{
    let c;
    c++;
}

{
    class C {
        static x = C;
    }
}

{
    const C = class C {
        static x = C;
    }
}

{
    const C = class {
        x = C;
    }
}

{
    const C = class C {
        static {
            C.x = "foo";
        }
    }
}

const foo = 1;
export { foo };

Options

{
    "no-use-before-define": ["error", {
        "functions": true,
        "classes": true,
        "variables": true,
        "allowNamedExports": false
    }]
}

• functions (boolean) - This flag determines whether or not the rule checks function declarations. If this is true, the rule warns on every reference to a function before the function declaration. Otherwise, the rule ignores those references. Function declarations are hoisted, so it’s safe to disable this option (note that some idiomatic patterns, such as mutual recursion, are incompatible with enabling this option). Default is true.
• classes (boolean) - This flag determines whether or not the rule checks class declarations of upper scopes. If this is true, the rule warns on every reference to a class before the class declaration. Otherwise, the rule ignores such references, provided the declaration is in an upper function scope. Class declarations are not hoisted, so it might be dangerous to disable this option. Default is true.
• variables (boolean) - This flag determines whether or not the rule checks variable declarations in upper scopes. If this is true, the rule warns on every reference to a variable before the variable declaration. Otherwise, the rule ignores a reference if the declaration is in an upper scope, while still reporting the reference if it’s in the same scope as the declaration. Default is true.
• allowNamedExports (boolean) - If this flag is set to true, the rule always allows references in export {}; declarations. These references are safe even if the variables are declared later in the code. Default is false.

This rule accepts "nofunc" string as an option."nofunc" is the same as { "functions": false, "classes": true, "variables": true, "allowNamedExports": false }.

functions

Examples of correct code for the { "functions": false } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "functions": false }]*/

f();
function f() {}

This option allows references to function declarations. For function expressions and arrow functions, please see the variables option.

classes

Examples of incorrect code for the { "classes": false } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "classes": false }]*/

new A();
class A {
}

{
    class C extends C {}
}

{
    class C extends D {}
    class D {}
}

{
    class C {
        static x = "foo";
        [C.x]() {}
    }
}

{
    class C {
        static {
            new D();
        }
    }
    class D {}
}

Examples of correct code for the { "classes": false } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "classes": false }]*/

function foo() {
    return new A();
}

class A {
}

variables

Examples of incorrect code for the { "variables": false } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "variables": false }]*/

console.log(foo);
var foo = 1;

f();
const f = () => {};

g();
const g = function() {};

{
    const C = class {
        static x = C;
    }
}

{
    const C = class {
        static x = foo;
    }
    const foo = 1;
}

{
    class C {
        static {
            this.x = foo;
        }
    }
    const foo = 1;
}

Examples of correct code for the { "variables": false } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "variables": false }]*/

function baz() {
    console.log(foo);
}
var foo = 1;

const a = () => f();
function b() { return f(); }
const c = function() { return f(); }
const f = () => {};

const e = function() { return g(); }
const g = function() {}

{
    const C = class {
        x = foo;
    }
    const foo = 1;
}

allowNamedExports

Examples of correct code for the { "allowNamedExports": true } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "allowNamedExports": true }]*/

export { a, b, f, C };

const a = 1;

let b;

function f () {}

class C {}

Examples of incorrect code for the { "allowNamedExports": true } option:

Open in Playground

/*eslint no-use-before-define: ["error", { "allowNamedExports": true }]*/

export default a;
const a = 1;

const b = c;
export const c = 1;

export function foo() {
    return d;
}
const d = 1;

Version

This rule was introduced in ESLint v0.0.9.

Resources

• Rule source
• Tests source