Destructure parameters and type at the same time.

[danm]

name: Destructure function parameters review
about: Allow types to be added to destructured function parameters for better readability

Despite what https://github.com/Microsoft/TypeScript/wiki/FAQ#functions says, can we not type cast parameters using instead?

I destructure my parameters a lot, and having to create another curly brace afterwards makes the code messy. I abstract the parameter type to an interface most of the time, but in some cases, it would just be nicer to add types right there whilst destructuring.

so how about...

function f({x : <number>}) {
  console.log(x);
}

with alias'

function f({ x: aliasX<number>}) {
  console.log(x);
}

with optional

function ({ x?: <number>}) {
  console.log(x);
}

with defaults

function f({ x = 10<number>}) {
  console.log(x);
}

with defaults and alias

function ({ x: aliasX<number> = 10 }) {
  console.log(x);
}

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.

[MartinJohns]

You might want to check out the relevant issue template.

[danm]

thanks @MartinJohns,
I have updated the question.

It still stands though, I haven't seen other comments about why brackets couldn't be used.

[RyanCavanaugh]

See #29526 for discussion of this topic