Skip to main content

no-unsafe-function-type

Disallow using the unsafe built-in Function type.

✅

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

🔧

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

TypeScript's built-in Function type allows being called with any number of arguments and returns type any. Function also allows classes or plain objects that happen to possess all properties of the Function class. It's generally better to specify function parameters and return types with the function type syntax.

"Catch-all" function types include:

  • () => void: a function that has no parameters and whose return is ignored
  • (...args: never) => unknown: a "top type" for functions that can be assigned any function type, but can't be called

Examples of code for this rule:

let noParametersOrReturn: Function;
noParametersOrReturn = () => {};

let stringToNumber: Function;
stringToNumber = (text: string) => text.length;

let identity: Function;
identity = value => value;
Open in Playground
.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/no-unsafe-function-type": "error"
  }
};

Try this rule in the playground ↗

Options​

This rule is not configurable.

When Not To Use It​

If your project is still onboarding to TypeScript, it might be difficult to fully replace all unsafe Function types with more precise function types. You might consider using ESLint disable comments for those specific situations instead of completely disabling this rule.

Resources​