no-shadow
Disallow variable declarations from shadowing variables declared in the outer scope.
Examples
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
module.exports = {
"rules": {
// Note: you must disable the base rule as it can report incorrect errors
"no-shadow": "off",
"@typescript-eslint/no-shadow": "warn"
}
};
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';
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;
Taken with ❤️ from ESLint core