Trailing Commas in Function Param Lists

[ewinslow]

https://jeffmo.github.io/es-trailing-function-commas/

This proposal is Stage 3 according to https://github.com/tc39/ecma262, so seems like its at an appropriate stability for TypeScript team to implement it.

This would allow function definitions and calls in the following style:

function foo(
  bar: Bar, // Comment about bar
  baz: Baz, // Comment about baz
) {
  // Implementation...
}

foo(
  bar,
  baz,
);

Benefits: https://github.com/jeffmo/es-trailing-function-commas/blob/master/proposal_presentation_slides.pdf

• Helps with VCS authorship attribution in case of added/removed parameters
• Consistency with object and array literals
• Lintable (if you don't like the style, can ban it in your codebase with a linter)
• Supported by other languages

In addition to all of the above, I suspect the extra "verbosity" of TypeScript's inline types makes this feature even more attractive in TypeScript than in vanilla JS. I feel this particularly in class constructors, which can also have private annotations, and possibly a readonly annotation coming down the pipe.

class Foo {
  constructor(
    private bar: Bar = new BasicBar(),
    private baz: Baz = new BasicBaz(),
  ) {}
}

[RyanCavanaugh]

Approved for whatever the Stage 3 proposal says

[mhegazy]

PRs are welcomed of course. the change should be limited to the parser, so would be a rather easy change to make.

[DanielRosenwasser]

Actually, signature help and overload resolution could potentially be affected by this.