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eslint/no-shadow Suspicious

What it does

Disallows variable declarations from shadowing variables declared in the outer scope.

Why is this bad?

Shadowing is the process by which a local variable shares the same name as a variable in its containing scope. This can cause confusion, as it may be unclear which variable is being referenced, and can lead to bugs that are difficult to diagnose.

Examples

Examples of incorrect code for this rule:

var x = 1;
function foo() {
  var x = 2; // x shadows the outer x
}

Examples of correct code for this rule:

var x = 1;
function foo() {
  var y = 2; // different name, no shadowing
}

Configuration

This rule accepts a configuration object with the following properties:

allow

type: string[]

default: []

List of variable names that are allowed to shadow.

builtinGlobals

type: boolean

default: false

Whether to report shadowing of built-in global variables.

hoist

type: "all" | "functions" | "functions-and-types" | "never" | "types"

default: "functions-and-types"

Controls how hoisting is handled when checking for shadowing.

"all"

Report shadowing even before the outer variable is declared (due to hoisting).

"functions"

Only report shadowing for function declarations that are hoisted.

"functions-and-types"

Report shadowing for both function and type declarations that are hoisted.

"never"

Never report shadowing before the outer variable is declared.

"types"

Only report shadowing for type declarations that are hoisted.

ignoreFunctionTypeParameterNameValueShadow

type: boolean

default: true

If true, ignore when a function type parameter shadows a value. Example: const T = 1; function foo<T>() {}

ignoreOnInitialization

type: boolean

default: false

Whether to ignore the variable initializers when the shadowed variable is presumably still uninitialized.

ignoreTypeValueShadow

type: boolean

default: true

If true, ignore when a type and a value have the same name. This is common in TypeScript: type Foo = ...; const Foo = ...;

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "rules": {
    "no-shadow": "error"
  }
}

CLI

oxlint --deny no-shadow

References

• Rule Source