Skip to main content

no-inferrable-types

Disallow explicit type declarations for variables or parameters initialized to a number, string, or boolean.

✅

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 is able to infer the types of parameters, properties, and variables from their default or initial values. There is no need to use an explicit : type annotation on one of those constructs initialized to a boolean, number, or string. Doing so adds unnecessary verbosity to code -making it harder to read- and in some cases can prevent TypeScript from inferring a more specific literal type (e.g. 10) instead of the more general primitive type (e.g. number)

.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/no-inferrable-types": "error"
  }
};

Examples​

const a: bigint = 10n;
const a: bigint = BigInt(10);
const a: boolean = !0;
const a: boolean = Boolean(null);
const a: boolean = true;
const a: null = null;
const a: number = 10;
const a: number = Infinity;
const a: number = NaN;
const a: number = Number('1');
const a: RegExp = /a/;
const a: RegExp = new RegExp('a');
const a: string = `str`;
const a: string = String(1);
const a: symbol = Symbol('a');
const a: undefined = undefined;
const a: undefined = void someValue;

class Foo {
  prop: number = 5;
}

function fn(a: number = 5, b: boolean = true) {}

Options​

This rule accepts an options object with the following properties:

interface Options {
  ignoreParameters?: boolean;
  ignoreProperties?: boolean;
}

const defaultOptions: Options = [
  { ignoreParameters: false, ignoreProperties: false },
];

ignoreParameters​

When set to true, the following pattern is considered valid:

function foo(a: number = 5, b: boolean = true) {
  // ...
}

ignoreProperties​

When set to true, the following pattern is considered valid:

class Foo {
  prop: number = 5;
}

When Not To Use It​

If you do not want to enforce inferred types.

Further Reading​

TypeScript Inference

Resources​