fixed params last in variable argument functions

[JeroMiya]

Extracting this suggestion from this issue:
#1336

Currently the variable arguments list supports variable arguments only as the last argument to the function:

function foo(arg1: number, ...arg2: string[]) {
}

This compiles to the following javascript:

function foo(arg1) {
 var arg2 = [];
 for (var _i = 1; _i < arguments.length; _i++) {
 arg2[_i - 1] = arguments[_i];
 }
}

However, variable argument functions are limited to appearing only as the last argument and not the first argument. I propose support be added for having a variable argument appear first, followed by one or more fixed arguments:

function subscribe(...events: string[], callback: (message: string) => void) {
}

// the following would compile
subscribe(message => alert(message)); // gets all messages
subscribe('errorMessages', message => alert(message));
subscribe(
 'errorMessages',
 'customMessageTypeFoo123', 
 (message: string) => {
 alert(message);
 });

// the following would not compile
subscribe(); // supplied parameters do not match any signature of call target
subscribe('a1'); // argument of type 'string' does not match parameter of type '(message: string) => void'
subscribe('a1', 'a2'); // argument of type 'string' does not match parameter of type '(message: string) => void'

subscribe compiles to the following JavaScript:

function subscribe() {
 var events= [];
 var callback = arguments[arguments.length - 1];
 for(var _i = 0; _i < arguments.length - 2; _i++) {
 events[_i] = arguments[_i];
 }
}

notes: it should be impossible for typescript code to call this function with zero arguments when typechecking. If JS or untyped TS code calls it without arguments, callback will be undefined. However, the same is true of fixed arguments at the beginning of the function.

edit: used a more realistic/motivating example for the fixed-last/variable-arguments-first function.

[danquirk]

I'm assuming by fixed arguments you mean required arguments since this certainly couldn't work when mixed with other rest params or optional arguments. While this proposal would work I'm not sure I see this adding enough value. Are there a lot of existing JavaScript APIs that would require this sort of signature? If not, this isn't really adding any expressivity or much convenience, just reordering some arguments in a subset of signatures that use rest params. Offhand I can't think of any other languages with varargs that support using them in any position but the last.

[RyanCavanaugh]

A cataloging of libraries that use this pattern would be useful.

[noobiept]

Createjs uses this, for example:

http://createjs.com/Docs/EaselJS/classes/Container.html#method_addChildAt
http://createjs.com/Docs/EaselJS/files/easeljs_display_Container.js.html#l200

[mhesler]

One example is String.prototype.replace. Although this occurs in the middle and not exclusively at the beginning or end.
lib.d.ts does this:

replace(searchValue: RegExp, replaceValue: (substring: string, ...args: any[]) => string): string;

should be this:

replace (searchValue: RegExp, replaceValue: (match: string, ...refs: string[], index: number, input: string) => string): string;

[pgrm]

I just came across this when updating TypeDefinitions for rethinkdb. getAll (http://rethinkdb.com/api/javascript/get_all/) should look like this:

getAll(key: string, ...keys: string[]): Selection
getAll(key: string, ...keys: string[], index: {index: string}): Selection

The problem here is index in the end being optional, but that is there problem, in an interface this still should work. Another example from rethinkdb client is the map function (http://rethinkdb.com/api/javascript/map/) which doesn't have the last field as optional.

For now I'm kind of "solving" it like Action and Func are defined in C# by simply creating new functions with more parameters:

getAll(key: string, ...keys: string[]): Selection;
getAll(key: string, key2: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, key5: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, key5: string, key6: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, key5: string, key6: string, key7: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, key5: string, key6: string, key7: string, key8: string, index?: {index: string}): Selection;
getAll(key: string, key2: string, key3: string, key4: string, key5: string, key6: string, key7: string, key8: string, key9: string, index?: {index: string}): Selection;

but this isn't just a pain to do, it's also not the representation of the underlying javascript library

[aholmes]

As mentioned in #3686, to add to the list of libraries doing this, lodash uses this pattern in extend/assign.

I also want to call out that lodash makes the callback and thisArg arguments optional.

[Xananax]

I'm getting stumped by something. Trying to create the typings for Flyd; it's a library that creates streams. It has a function combine that combines a variable amount of streams in one. It works like this:

const combinedSignal = flyd.combine(function(s1:Signal,s2:signal2,s3:signal3....,self:signal,changed:signal[]){})

I've tried the method described by Peter Grman (@pgrm), but I'm getting Argument of type '(s: FlydStream, self: FlydStream) => void' is not assignable to parameter of type 'FlydCombinator', even if one of the overloaded calls matches this pattern exactly.

It seems the method works as long as I don't have another interface with overload that uses an overloaded interface. But the library is entirely made of curried functions calling each other, I just can't not have overloads using overloads.

I think I've managed to recreate a minimal test case here

[olee]

Same here.
Tried to write type definitions for an library function which accepts a list of string arguments for filtering and as a last argument a callback. Only way to solve this right now I know is to prepare many copies of the function so it can be used in most cases.

Considering this issue was first opened in Dec 2014, I think this should definitely be added in the near future!

[DanielRosenwasser]

It looks like Ember.computed uses it.

I wonder if allowing rest arguments in non-implementation signatures would be acceptable. That way we could potentially support this at the type level but we'd leave the runtime implementation to be decided by users.