A typed array of functions

Question

I'm struggling to figure out if it's possible in TypeScript to declare a statically typed array of functions.

For example, I can do this:

const foo: (data: string) => void = function (data) {};

But if I want foo to be an array of functions that take a string and return nothing, how do I do that?

const foo: (data: string) => void [] = [];

Doesn't work because TypeScript thinks it's a function that takes a string and returns an array of void, and it doesn't seem to like me trying to wrap the function in brackets.

Any ideas?

Updated answer from 2025:

const foo: ((data: string) => void)[] = [];

function doFoo() {
  for (var i = 0; i < this.foo.length; i++) {
     this.foo[i]("test");
  }
}

foo.push((bar) => { alert(bar) });
foo.push((bar) => { alert(bar.length.toString()) });

doFoo();

possible duplicate of In TypeScript how do I declare an array of functions that accept a string and return a string? — Fenton
– Fenton, Commented Oct 4, 2012 at 21:26

schmijos · Accepted Answer · 2021-11-09 15:26:43Z

144

You can find this in the language spec section 3.6.4:

foo: { (data: string): void; } []

edited Nov 9, 2021 at 15:26

schmijos

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answered Oct 4, 2012 at 19:55

mohamed hegazy

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10 Comments

Matt Burland Over a year ago

Yep, that seemed to do it. Thanks.

Peter Olson Over a year ago

@asawyer It has to be inside of a class or interface, and it has to have a semicolon at the end.

Fenton Over a year ago

It can be a variable too: var foo: { (data: string): void; } [];

phuzi Over a year ago

link to language spec is broken

Ini Over a year ago

Broken link: The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.

|

Ninjakannon · Accepted Answer · 2025-11-13 17:23:22Z

94

Since this question was originally posted, there are now newer and more readable ways to type an array of functions:

const foo: ((data: string) => void)[];
const bar: Array<(data: string) => void>;

edited Nov 13 at 17:23

Ninjakannon

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answered Apr 23, 2017 at 15:28

Saravana

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1 Comment

JSideris Over a year ago

The first option here was the only syntax I could get to work with AssemblyScript.

Ninjakannon · Accepted Answer · 2025-11-13 17:25:57Z

17

Or const foo: ((data: string) => void)[]

edited Nov 13 at 17:25

Ninjakannon

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answered Mar 1, 2020 at 12:36

aaron

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Comments

Janderson Silva · Accepted Answer · 2020-02-01 21:06:52Z

9

If you wish declare an array of callable function in TypeScript, you can declare a type:

type Bar = (
  (data: string) => void
);

And then use it:

const foo: Bar[] = [];

const fooFn = (data: string) => console.log(data);
foo.push(fooFn);
foo.forEach((fooFn: Bar) => fooFn("Something");

answered Feb 1, 2020 at 21:06

Janderson Silva

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Comments

Jonathan Dsouza · Accepted Answer · 2022-07-28 09:18:32Z

1

type fn = (...args: any[]) => void

answered Jul 28, 2022 at 9:18

Jonathan Dsouza

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