Storm Kiernan Storm Kiernan - 1 year ago 61
TypeScript Question

How can I get type safety for returned functions in TypeScript?

This TypeScript compiles fine:

abstract class Animal {
Any extension of Animal MUST have a function which returns
another function that has exactly the signature (string): void
abstract getPlayBehavior(): (toy: string) => void;

class Cat extends Animal {
Clearly does not have a function which returns a function
that has the correct signature. This function returns a function with
the signature (void) : void
getPlayBehavior() {
return () => {
console.log(`Play with toy_var_would_go_here!`);

class Program {
static main() {
let cat: Animal = new Cat();


I am expecting an error because the Cat class definitely does not implement the abstract Animal class properly. I expect that the Cat class must have a function which returns another function of the exact signature specified in the abstract Animal class.

Running the code, I get:

> node index.js
> Play with toy_var_would_go_here!

Is there anything I can do to make sure the compiler enforces this kind of policy?

Answer Source

You're not getting an error because in javascript/typescript you're not forced to declare the arguments if you don't want to use them, as long as there's no contradiction.

For example, the signature of the Array.forEach is:

forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void;

But this will compile just fine:

let a = [1, 2, 3];
a.forEach(item => console.log(item));

And that's a good thing, it would have been horrible if I had to have all arguments even if I don't use them.
The same goes here:

type MyFn = (s: string) => void;
let fn: MyFn = () => console.log("hey");

If I don't need to use the string argument then I can neglect it, or I can even do:

let fn: MyFn = () => console.log(arguments);

If you'll change the signature of the function that you return in Cat.getPlayBehavior to something that contradicts the definition in Animal then you'll get an error:

class Cat extends Animal {
    getPlayBehavior() {
        return (n: number) => {
            console.log(`Play with toy_var_would_go_here!`);


Class 'Cat' incorrectly extends base class 'Animal'.
  Types of property 'getPlayBehavior' are incompatible.
    Type '() => (n: number) => void' is not assignable to type '() => (toy: string) => void'.
      Type '(n: number) => void' is not assignable to type '(toy: string) => void'.
        Types of parameters 'n' and 'toy' are incompatible.
          Type 'string' is not assignable to type 'number'.
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