TS1256: A rest element must be last in a tuple type

[ethan-fraser]

Describe the issue/behavior that seems buggy When compiling with tsc v4.9.3, it fails with the following error:

> tsc

node_modules/highlight.js/types/index.d.ts:55:54 - error TS1256: A rest element must be last in a tuple type.

55             either: (...args: (RegExp | string)[] | [...(RegExp | string)[], RegexEitherOptions]) => string,
                                                         ~~~~~~~~~~~~~~~~~~~~~~

Found 1 error.

Expected behavior Compiling should not fail.

Additional context At index.d.ts:55, if you swap RegexEitherOptions and ...(RegExp | string)[], the error is resolved.

i.e.

<<<<<<
            either: (...args: (RegExp | string)[] | [...(RegExp | string)[], RegexEitherOptions]) => string,
======
            either: (...args: (RegExp | string)[] | [RegexEitherOptions, ...(RegExp | string)[]]) => string,
>>>>>>

[joshgoebel]

It's not supposed to be a tuple, it's supposed to be any number of arguments followed by RegexEitherOptions

[RicardoPhilippe]

Same here with tsc 3.9.7.

[joshgoebel]

Can someone recommend the right way to type this?

https://github.com/highlightjs/highlight.js/blob/main/src/lib/regex.js#L77

The type of args is literally (RegExp | string)[] except the last params can optinally be a RegexEitherOptions.

[joshgoebel]

type RegexEitherOptions = 
{
    more: boolean;
}

    interface PublicApi {
        regex: {
            concat: (...args: (RegExp | string)[]) => string,
            lookahead: (re: RegExp | string) => string,
            either: (...args: (RegExp | string)[] | [...(RegExp | string)[], RegexEitherOptions]) => string,
            optional: (re: RegExp | string) => string,
            anyNumberOfTimes: (re: RegExp | string) => string
        }
    }

Seems to work in the 4.9.5 playground without issue?

[joshgoebel]

This snippet also seems to work just fine:

    type RegexEitherOptions = {
        capture?: boolean
    }


export function either(...args: (RegExp | string)[] | [...(RegExp | string)[], RegexEitherOptions]) {
  const opts : RegexEitherOptions = stripOptionsFromArgs(args);
  args = args as (RegExp | string)[];
  const joined = '('
    + (opts.capture ? "" : "?:")
    + args.map((x) => source(x)).join("|") + ")";
  return joined;
}

function stripOptionsFromArgs(args: (RegExp | string)[] | [...(RegExp | string)[], RegexEitherOptions]) : RegexEitherOptions {
  const opts = args[args.length - 1] as RegexEitherOptions;

  if (typeof opts === 'object' && opts.constructor === Object) {
    args.splice(args.length - 1, 1);
    return opts;
  } else {
    return {};
  }
}

export function source(re : RegExp | string) {
  if (!re) return null;
  if (typeof re === "string") return re;

  return re.source;
}

[PeterWone]

I'm sure this comment won't endear me to the rest of the Typescript community, but usability is more important than technical compliance. Perhaps you could use an interim any.

[joshgoebel]

So right now I can't reproduce this error with the web version - is it no longer an issue in the latest TS versions?

[PeterWone]

VS Code 1.76 uses 4.95 but I can't tell from here what version of tsc is used by gulp in my CICD, I'll have to look into that after work.

[joshgoebel]

I only write the types to use the linting features in VS Code... so if it worked when I wrote it - and evidently it works in the latest tsc - does that make 4.95 sound like perhaps a bug or regression?