How to handle typing of mutated let variables or function parameters?

[justingrant]

A common pattern in our current polyfill code is to reassign variables and parameters. How should we work around TS's limitation that mutating a variable doesn't change its type from TS's perspective?

For example: (TS playground)

function toString(a: unknown) {
  a = String(a);
  return a;
  // expected return type: String
  // actual return type: unknown
}

toString(42).includes('4')
// ^^ Object is of type 'unknown'.(2571)

// explicitly declaring a return type doesn't work either
function toString2(a: unknown): string {
  a = String(a);
  return a;
  // ^^ Type 'unknown' is not assignable to type 'string'.(2322)
}

There is apparently a future TS proposal microsoft/TypeScript#45870 which may fix this. I commented there to ask if it will help us.

cc @12wrigja

[12wrigja]

Hmm - yes this is a bit problematic.

One potential solution we could use is something like this:

playground with type assertion functions.

That being said, this isn't great given that it slightly bloats code-size and runtime as those extra checks have to be included. Internal to Google, we have a system like this that removes these checks in production builds, and we could maybe replicate small parts of it here (using DEBUG) .

Alternatively, could we simply avoid this pattern and rewrite to new variables? Are we really saving that much memory by mutation parameter variables?

[ptomato]

Alternatively, could we simply avoid this pattern and rewrite to new variables? Are we really saving that much memory by mutation parameter variables?

Agreed, we can even turn on no-param-reassign in eslint and just catch all instances of this at once.

[12wrigja]

We fixed this: eslintrc.yml only has that rule turned off in tests.