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no-shadow

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

This rule extends the base eslint/no-shadow rule. It adds support for TypeScript's this parameters and global augmentation, and adds options for TypeScript features.

How to Use​

.eslintrc.cjs
module.exports = {
  "rules": {
    // Note: you must disable the base rule as it can report incorrect errors
    "no-shadow": "off",
    "@typescript-eslint/no-shadow": "error"
  }
};
Try this rule in the playground ↗

Options​

See eslint/no-shadow options.

This rule adds the following options:

interface Options extends BaseNoShadowOptions {
  ignoreTypeValueShadow?: boolean;
  ignoreFunctionTypeParameterNameValueShadow?: boolean;
}

const defaultOptions: Options = {
  ...baseNoShadowDefaultOptions,
  ignoreTypeValueShadow: true,
  ignoreFunctionTypeParameterNameValueShadow: true,
};

ignoreTypeValueShadow​

When set to true, the rule will ignore the case when you name a type the same as a variable.

TypeScript allows types and variables to shadow one-another. This is generally safe because you cannot use variables in type locations without a typeof operator, so there's little risk of confusion.

Examples of correct code with { ignoreTypeValueShadow: true }:

type Foo = number;
const Foo = 1;

interface Bar {
  prop: number;
}
const Bar = 'test';
Open in Playground

ignoreFunctionTypeParameterNameValueShadow​

When set to true, the rule will ignore the case when you name a function type argument the same as a variable.

Each of a function type's arguments creates a value variable within the scope of the function type. This is done so that you can reference the type later using the typeof operator:

type Func = (test: string) => typeof test;

declare const fn: Func;
const result = fn('str'); // typeof result === string

This means that function type arguments shadow value variable names in parent scopes:

let test = 1;
type TestType = typeof test; // === number
type Func = (test: string) => typeof test; // this "test" references the argument, not the variable

declare const fn: Func;
const result = fn('str'); // typeof result === string

If you do not use the typeof operator in a function type return type position, you can safely turn this option on.

Examples of correct code with { ignoreFunctionTypeParameterNameValueShadow: true }:

const test = 1;
type Func = (test: string) => typeof test;
Open in Playground

FAQ​

Why does the rule report on enum members that share the same name as a variable in a parent scope?​

Reporting on this case isn't a bug - it is completely intentional and correct reporting! The rule reports due to a relatively unknown feature of enums - enum members create a variable within the enum scope so that they can be referenced within the enum without a qualifier.

To illustrate this with an example:

const A = 2;
enum Test {
  A = 1,
  B = A,
}

console.log(Test.B);
// what should be logged?

Naively looking at the above code, it might look like the log should output 2, because the outer variable A's value is 2 - however, the code instead outputs 1, which is the value of Test.A. This is because the unqualified code B = A is equivalent to the fully-qualified code B = Test.A. Due to this behavior, the enum member has shadowed the outer variable declaration.

Resources​

Taken with ❤️ from ESLint core