bind, call and apply do not work on unions of function types with different return types. #33815

brainkim · 2019-10-04T18:45:33Z

TypeScript Version: 3.6.3

Search Terms:
call, bind, apply, function, functions, Function.prototype.call, Function.prototype.apply, Function.prototype.bind, union, return, this, strictBindCallApply
Code

type aFn = (this: string, value: string) => string;
type bFn = (this: string, value: string) => Promise<string>;

function callFn(callback: aFn | bFn): Promise<string> | string {
    return callback.call("hello", "world");
}

function applyFn(callback: aFn | bFn): Promise<string> | string {
    return callback.apply("hello", ["world"]);
}

function bindFn(callback: aFn | bFn): () => Promise<string> | string {
    return callback.bind("hello", "world");
}

Expected behavior:
Code compiles. The types in this code are equivalent to the types in the following example, which does compile:

type cFn = (this: string, value: string) => string | Promise<string>;

function callFn1(callback: cFn): Promise<string> | string {
    return callback.call("hello", "world");
}

function applyFn1(callback: cFn): Promise<string> | string {
    return callback.apply("hello", ["world"]);
}

function bindFn1(callback: cFn): () => Promise<string> | string {
    return callback.bind("hello", "world");
}

The only difference between the two examples is that the first uses a type union of function types aFn and bFn, which only differ in terms of return value, whereas the second uses a type union in the return value of cFn. In other words I think the union of type aFn and bFn should behave the same as type cFn.

type aFn = (this: string, value: string) => string;
type bFn = (this: string, value: string) => Promise<string>;
type cFn = (this: string, value: string) => string | Promise<string>;

Actual behavior:

example.ts(5,12): error TS2684: The 'this' context of type 'aFn | bFn' is not assignable to method's 'this' of type '(this: "hello", args_0: string) => string'.
  Type 'bFn' is not assignable to type '(this: "hello", args_0: string) => string'.
    Type 'Promise<string>' is not assignable to type 'string'.

example.ts(9,12): error TS2684: The 'this' context of type 'aFn | bFn' is not assignable to method's 'this' of type '(this: "hello", value: string) => string'.
  Type 'bFn' is not assignable to type '(this: "hello", value: string) => string'.
    Type 'Promise<string>' is not assignable to type 'string'.

example.ts(13,12): error TS2769: No overload matches this call.
  Overload 1 of 6, '(this: (this: "hello", arg0: "world") => string, thisArg: "hello", arg0: "world"): () => string', gave the following error.
    The 'this' context of type 'aFn | bFn' is not assignable to method's 'this' of type '(this: "hello", arg0: "world") => string'.
      Type 'bFn' is not assignable to type '(this: "hello", arg0: "world") => string'.
        Type 'Promise<string>' is not assignable to type 'string'.
  Overload 2 of 6, '(this: (this: "hello", ...args: "world"[]) => string, thisArg: "hello", ...args: "world"[]): (...args: "world"[]) => string', gave the following error.
    The 'this' context of type 'aFn | bFn' is not assignable to method's 'this' of type '(this: "hello", ...args: "world"[]) => string'.
      Type 'bFn' is not assignable to type '(this: "hello", ...args: "world"[]) => string'.
        Type 'Promise<string>' is not assignable to type 'string'.

Playground Link:
https://www.typescriptlang.org/play/index.html?ssl=30&ssc=2&pln=1&pc=1#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-xDfdM1TvTn6h9GQXA87Ac28BWIfya2tNkA

Related Issues:

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brainkim · 2019-11-07T20:28:01Z

@RyanCavanaugh I’m curious why this is marked as a suggestion and not a bug. I don’t mind either way, just thinking the current behavior can’t be expected or correct behavior?

It is needed due to TS error: microsoft/TypeScript#33815

chase-000 · 2022-05-02T15:57:34Z

Changing the call signature from

call<T, A extends any[], R>(this: (this: T, ...args: A) => R, thisArg: T, ...args: A): R;

to

call<T extends ( ...args: any[] ) => any>( this: T, thisArg: ThisParameterType<T>, ...a: Parameters<T> ): ReturnType<T>;

seems to fix this. apply and bind could use the same treatment, I imagine.

It does highlight a small issue though: unions of the exact same tuple types don't get simplified if at least one of them has labels: [a:string]|[a:string] remains as is, so this new call signature looks weird on the call site.

brainkim changed the title ~~Function.prototype call,apply,bind do not work on function type unions~~ Function.prototype.call, Function.prototpe.apply, Function.prototype.bind do not work on function type unions Oct 6, 2019

brainkim changed the title ~~Function.prototype.call, Function.prototpe.apply, Function.prototype.bind do not work on function type unions~~ Function.prototype.call, Function.prototype.apply, Function.prototype.bind do not work on function type unions Oct 6, 2019

brainkim changed the title ~~Function.prototype.call, Function.prototype.apply, Function.prototype.bind do not work on function type unions~~ bind, call, apply do not work on unions of function types with different return types. Oct 6, 2019

brainkim changed the title ~~bind, call, apply do not work on unions of function types with different return types.~~ bind, call and apply do not work on unions of function types with different return types. Oct 6, 2019

RyanCavanaugh added Awaiting More Feedback This means we'd like to hear from more people who would be helped by this feature Suggestion An idea for TypeScript labels Oct 17, 2019

brainkim mentioned this issue Apr 30, 2020

Improve typings to add safety for props bikeshaving/crank#51

Closed

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vlasy pushed a commit to AckeeCZ/cosmas that referenced this issue Oct 6, 2020

🏷 Fix logFn type in express mdw

e166b38

It is needed due to TS error: microsoft/TypeScript#33815

kirkwaiblinger mentioned this issue Mar 31, 2025

fix(eslint-plugin): [unified-signatures] exempt this from optional parameter overload check typescript-eslint/typescript-eslint#11005

Merged

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bind, call and apply do not work on unions of function types with different return types. #33815

bind, call and apply do not work on unions of function types with different return types. #33815

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bind, call and apply do not work on unions of function types with different return types. #33815

bind, call and apply do not work on unions of function types with different return types. #33815

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