Argument expressions should be as const when possible

[kolodny]

Search Terms

"as const" arguments parameters "return type"

Suggestion

With the introduction of as const all expressions to functions should be treated as const

Use Cases/Examples

Consider this case

function takesTuple<T extends readonly any[]>(t: T) {
  return t
}

const tuple1: [number, string] = [1, 'foo'];
const tuple2 = takesTuple(tuple1);

const tuple3 = [1, 'bar'] as const;
const tuple4 = takesTuple(tuple3);

// This should return `[2, 'baz']` or at least `[number, string]`
// instead returns `Array<number | string>`
const bad = takesTuple([2, 'baz']);
bad;

// You shouldn't need to have "as const" for expressions
const good = takesTuple([2, 'baz'] as const);
good

https://www.typescriptlang.org/play/#src=function%20takesTuple%3CT%20extends%20readonly%20any%5B%5D%3E(t%3A%20T)%20%7B%0A%20%20return%20t%0A%7D%0A%0Aconst%20tuple1%3A%20%5Bnumber%2C%20string%5D%20%3D%20%5B1%2C%20'foo'%5D%3B%0Aconst%20tuple2%20%3D%20takesTuple(tuple1)%3B%0A%0Aconst%20tuple3%20%3D%20%5B1%2C%20'bar'%5D%20as%20const%3B%0Aconst%20tuple4%20%3D%20takesTuple(tuple3)%3B%0A%0A%2F%2F%20This%20should%20return%20%60%5B2%2C%20'baz'%5D%60%20or%20at%20least%20%60%5Bnumber%2C%20string%5D%60%0A%2F%2F%20instead%20returns%20%60Array%3Cnumber%20%7C%20string%3E%60%0Aconst%20bad%20%3D%20takesTuple(%5B2%2C%20'baz'%5D)%3B%0Abad%3B%0A%0A%2F%2F%20You%20shouldn't%20need%20to%20have%20%22as%20const%22%20for%20expressions%0Aconst%20good%20%3D%20takesTuple(%5B2%2C%20'baz'%5D%20as%20const)%3B%0Agood

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.

[dragomirtitian]

Duplicate of #30680 I believe

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.