JS.BASE.NO.USE.BEFORE.DEFINE

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:

/*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:

/*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) - The flag which shows whether or not this rule checks function declarations. If this is true, this rule warns every reference to a function before the function declaration. Otherwise, ignores those references. Function declarations are hoisted, so it's safe. Default is true.
• classes (boolean) - The flag which shows whether or not this rule checks class declarations of upper scopes. If this is true, this rule warns every reference to a class before the class declaration. Otherwise, ignores those references if the declaration is in upper function scopes. Class declarations are not hoisted, so it might be danger. 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 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:

/*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:

/*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:

/*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:

/*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:

/*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:

/*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:

/*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;