Codefix: removing inferable types

Search Terms

inferable types no-inferable-types no-inferrable-types removal

Suggestion

It is somewhat unnecessary to add : type declarations for member, parameter, and/or variable types already inferable from code.

// This type declaration actually requests a lower amount of type information,
// as `number` is less specific (narrow) than `3`
const value: number = 3;

It'd be nice to have a codefix to remove them to clean up code.

Use Cases

These unnecessary type declarations can easily get introduced in code as it gets changing during editing, and can cause visual clutter.

We should note that some users choose to prefer including these inferable type declarations as a style preference to increase the amount of type information visible purely from source code.

The @typescript-eslint/no-inferable-types already exists with an auto-fixer for a good starting reference.

Examples

Variables such as the above value can generally have their type removed if either is true:

• The variable is let, contains an initializer, and the type is the same as what it would be inferred as

  // We can remove `: number` here
  let value: number = 0;
  
  // We cannot remove `: number | string`, as `| string` is not added by TypeScript
  let otherValue: number | string = 0;
  

• The variable is const and the type is less narrow than what the type is inferred as

Parameters can similarly be treated as let parameters:

// The `: string` can be safely removed
function takesStringWithDefault(input: string = "") { }

Parameters can also have type declarations removed if their parent function is already declared to be a type that provides the same type declaration for the parameter:

type TakesString = (input: string) => void;

// We can remove the `: string` here too
const takesString: TakesString = (input: string) => {};

Class members can be treated as let variables here too.

class WithValue {
    // We can remove the `: string` here
    value: string = "";
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

No repository files, tests, or entry points are named in the issue. Start by reviewing the variable, parameter, and class-member examples, then compare the proposed behavior with the @typescript-eslint/no-inferable-types auto-fixer; done means a codefix safely removes only the described inferable annotations without changing runtime output.