Disallow Shadowing (no-shadow)

Shadowing is the process by which a local variable shares the same name as a variable in its containing scope. For example:

var a = 3;
function b() {
    var a = 10;
}

In this case, the variable a inside of b() is shadowing the variable a in the global scope. This can cause confusion while reading the code and it’s impossible to access the global variable.

Rule Details

This rule aims to eliminate shadowed variable declarations.

The following patterns are considered warnings:

var a = 3;
function b() {
    var a = 10;
}

var a = 3;
var b = function () {
    var a = 10;
}

var a = 3;
function b(a) {
    a = 10;
}
b(a);

var a = 3;

if (true) {
    let a = 5;
}

Options

This rule takes one option, an object, with properties "builtinGlobals", "hoist".

{
    "no-shadow": [2, {"builtinGlobals": false, "hoist": "functions"}]
}

builtinGlobals

false by default. If this is true, this rule checks with built-in global variables such as Object, Array, Number, …

When {"builtinGlobals": true}, the following patterns are considered warnings:

function foo() {
    var Object = 0; // shadowed the built-in globals.
}

hoist

The option has three settings:

all - reports all shadowing before the outer variables/functions are defined.
functions (by default) - reports shadowing before the outer functions are defined.
never - never report shadowing before the outer variables/functions are defined.

Thought with the following codes:

if (true) {
    let a = 3;
    let b = 6;
}

let a = 5;
function b() {}

all - Both let a and let b in the if statement are considered warnings.
functions - let b is considered warnings. But let a in the if statement is not considered warnings. Because there is it before let a of the outer scope.
never - Both let a and let b in the if statement are not considered warnings. Because there are those before each declaration of the outer scope.

Version

This rule was introduced in ESLint 0.0.9.