no-shadow
Disallow variable declarations from shadowing variables declared in the outer scope
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.
Examples of incorrect code for this rule:
Open in Playground
/*eslint no-shadow: "error"*/
var a = 3;
function b() {
var = 10;
}
var c = function () {
var = 10;
}
function d() {
a = 10;
}
d(a);
if (true) {
let = 5;
}
Options
This rule takes one option, an object, with properties "builtinGlobals", "hoist", "allow" and "ignoreOnInitialization".
{
"no-shadow": ["error", { "builtinGlobals": false, "hoist": "functions", "allow": [], "ignoreOnInitialization": false }]
}
builtinGlobals
The builtinGlobals option is false by default.
If it is true, the rule prevents shadowing of built-in global variables: Object, Array, Number, and so on.
Examples of incorrect code for the { "builtinGlobals": true } option:
Open in Playground
/*eslint no-shadow: ["error", { "builtinGlobals": true }]*/
function foo() {
var = 0;
}
hoist
The hoist option has three settings:
functions(by default) - reports shadowing before the outer functions are defined.all- reports all shadowing before the outer variables/functions are defined.never- never report shadowing before the outer variables/functions are defined.
hoist: functions
Examples of incorrect code for the default { "hoist": "functions" } option:
Open in Playground
/*eslint no-shadow: ["error", { "hoist": "functions" }]*/
if (true) {
let = 6;
}
function b() {}
Although let b in the if statement is before the function declaration in the outer scope, it is incorrect.
Examples of correct code for the default { "hoist": "functions" } option:
Open in Playground
/*eslint no-shadow: ["error", { "hoist": "functions" }]*/
if (true) {
let a = 3;
}
let a = 5;
Because let a in the if statement is before the variable declaration in the outer scope, it is correct.
hoist: all
Examples of incorrect code for the { "hoist": "all" } option:
Open in Playground
/*eslint no-shadow: ["error", { "hoist": "all" }]*/
if (true) {
let = 3;
let = 6;
}
let a = 5;
function b() {}
hoist: never
Examples of correct code for the { "hoist": "never" } option:
Open in Playground
/*eslint no-shadow: ["error", { "hoist": "never" }]*/
if (true) {
let a = 3;
let b = 6;
}
let a = 5;
function b() {}
Because let a and let b in the if statement are before the declarations in the outer scope, they are correct.
allow
The allow option is an array of identifier names for which shadowing is allowed. For example, "resolve", "reject", "done", "cb".
Examples of correct code for the { "allow": ["done"] } option:
Open in Playground
/*eslint no-shadow: ["error", { "allow": ["done"] }]*/
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 });
});
ignoreOnInitialization
The ignoreOnInitialization option is false by default. If it is true, it prevents reporting shadowing of variables in their initializers when the shadowed variable is presumably still uninitialized.
The shadowed variable must be on the left side. The shadowing variable must be on the right side and declared in a callback function or in an IIFE.
Examples of incorrect code for the { "ignoreOnInitialization": "true" } option:
Open in Playground
/*eslint no-shadow: ["error", { "ignoreOnInitialization": true }]*/
var x = => x;
Because the shadowing variable x will shadow the already initialized shadowed variable x.
Examples of correct code for the { "ignoreOnInitialization": true } option:
Open in Playground
/*eslint no-shadow: ["error", { "ignoreOnInitialization": true }]*/
var x = foo(x => x)
var y = (y => y)()
The rationale for callback functions is the assumption that they will be called during the initialization, so that at the time when the shadowing variable will be used, the shadowed variable has not yet been initialized.
Related Rules
Version
This rule was introduced in ESLint v0.0.9.
Further Reading