no-redeclare

Disallow variable redeclaration

In JavaScript, it’s possible to redeclare the same variable name using var. This can lead to confusion as to where the variable is actually declared and initialized.

Rule Details

This rule is aimed at eliminating variables that have multiple declarations in the same scope.

Examples of incorrect code for this rule:

/*eslint no-redeclare: "error"*/

var a = 3;
var a = 10;

class C {
    foo() {
        var b = 3;
        var b = 10;
    }

    static {
        var c = 3;
        var c = 10;
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Examples of correct code for this rule:

/*eslint no-redeclare: "error"*/

var a = 3;
a = 10;

class C {
    foo() {
        var b = 3;
        b = 10;
    }

    static {
        var c = 3;
        c = 10;
    }
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

Options

This rule takes one optional argument, an object with a boolean property "builtinGlobals". It defaults to true. If set to true, this rule also checks redeclaration of built-in globals, such as Object, Array, Number…

builtinGlobals

The "builtinGlobals" option will check for redeclaration of built-in globals in global scope.

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

/*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/

var Object = 0;
1
2
3

Examples of incorrect code for the { "builtinGlobals": true } option and the browser environment:

/*eslint no-redeclare: ["error", { "builtinGlobals": true }]*/
/*eslint-env browser*/

var top = 0;
1
2
3
4

The browser environment has many built-in global variables (for example, top). Some of built-in global variables cannot be redeclared.

Note that when using the node or commonjs environments (or ecmaFeatures.globalReturn, if using the default parser), the top scope of a program is not actually the global scope, but rather a “module” scope. When this is the case, declaring a variable named after a builtin global is not a redeclaration, but rather a shadowing of the global variable. In that case, the no-shadow rule with the "builtinGlobals" option should be used.

Handled by TypeScript

It is safe to disable this rule when using TypeScript because TypeScript's compiler enforces this check.

Note that while TypeScript will catch let redeclares and const redeclares, it will not catch var redeclares. Thus, if you use the legacy var keyword in your TypeScript codebase, this rule will still provide some value.

Related Rules

• no-shadow

Version

This rule was introduced in ESLint v0.0.9.

Resources

• Rule source
• Tests source