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 problems:
/*eslint no-shadow: 2*/
/*eslint-env es6*/
var a = 3;
function b() {
var a = 10; /*error 'a' is already declared in the upper scope.*/
var b = function () {
var a = 10; /*error 'a' is already declared in the upper scope.*/
function b(a) { /*error 'a' is already declared in the upper scope.*/
a = 10;
b(a);
if (true) {
let a = 5; /*error 'a' is already declared in the upper scope.*/
Options
This rule takes one option, an object, with properties "builtinGlobals", "hoist" and "allow".
"no-shadow": [2, {"builtinGlobals": false, "hoist": "functions", "allow": []}]
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 problems:
/*eslint no-shadow: [2, { "builtinGlobals": true }]*/
function foo() {
var Object = 0; /*error 'Object' is already declared in the upper scope.*/
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.
{ "hoist": "all" }
With "hoist" set to "all", both let a and let b in the if statement are considered problems.
/*eslint no-shadow: [2, { "hoist": "all" }]*/
/*eslint-env es6*/
if (true) {
let a = 3; /*error 'a' is already declared in the upper scope.*/
let b = 6; /*error 'b' is already declared in the upper scope.*/
let a = 5;
function b() {}
{ "hoist": "functions" } (default)
With "hoist" set to "functions", let b is considered a warning. But let a in the if statement is not considered a warning, because it is before let a of the outer scope.
/*eslint no-shadow: [2, { "hoist": "functions" }]*/
/*eslint-env es6*/
if (true) {
let a = 3;
let b = 6; /*error 'b' is already declared in the upper scope.*/
let a = 5;
function b() {}
{ "hoist": "never" }
With "hoist" set to "never", neither let a nor let b in the if statement are considered problems, because they are before the declarations of the outer scope.
/*eslint no-shadow: [2, { "hoist": "never" }]*/
/*eslint-env es6*/
if (true) {
let a = 3;
let b = 6;
let a = 5;
function b() {}
allow
The option is an array of identifier names to be allowed (ie. “resolve”, “reject”, “done”, “cb” etc.):
"rules": {
"no-shadow": [2, {"allow": ["done"]}]
Allows for the following code to be valid:
import async from 'async';
function foo(done) {
async.map([1, 2], function (e, done) {
done(null, e * 2)
}, done);
foo(function (err, result) {
console.log({ err, result });
Further Reading
Variable Shadowing
Version
This rule was introduced in ESLint 0.0.9.
Resources
Rule source
Documentation source
