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prefer-function-type

Enforce using function types instead of interfaces with call signatures.

🔒

Extending "plugin:@typescript-eslint/strict" in an ESLint configuration enables this rule.

🛠

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

TypeScript allows for two common ways to declare a type for a function:

  • Function type: () => string
  • Object type with a signature: { (): string }

The function type form is generally preferred when possible for being more succinct.

This rule suggests using a function type instead of an interface or object type literal with a single call signature.

.eslintrc.cjs
module.exports = {
"rules": {
"@typescript-eslint/prefer-function-type": "warn"
}
};

Examples

interface Example {
(): string;
}
function foo(example: { (): number }): number {
return example();
}
interface ReturnsSelf {
// returns the function itself, not the `this` argument.
(arg: string): this;
}

Options

This rule is not configurable.

When Not To Use It

If you specifically want to use an interface or type literal with a single call signature for stylistic reasons, you can disable this rule.

Resources