Using function types to restrict function definition in Typescript

Question

According to doc https://www.typescriptlang.org/docs/handbook/interfaces.html#function-types

I want to restrict the parameters of a function. I have written my function like this....

interface FuncType {
    (one: number, two: number, three: number): number;
}

let myfunc: FuncType = function (num1: number, num2: number, num3: number) {
    return num1 + num2 + num3;
}

let result = myfunc(10,20,30);
console.log(result);

This works great. But the problem is, if I do this...

let myfunc: FuncType = function (num1: number, num2: number) { //num3 is missing
    return num1 + num2;
}

// let result = myfunc(10,20);  //this gives an error.
let result = myfunc(10,20,30); // I had to give three parameters
console.log(result);

This works too. It is not showing any error. I was expecting it should give an error because num3 is not used there in the function definition.

It is also confusing because I cannot call myfunc with two paramters.

Is it not possible to make the function definition strict according to the interface?

I don't want to put my actual code there. But my problem is just like this situation. I want to define an interface that will rule creating functions implementing the interface. So that number of paramters in the function function definition will be same as interface. — Md. Tahmid Mozaffar
– Md. Tahmid Mozaffar, Commented Nov 2, 2020 at 0:59
Probably the way I am thinking there is no work-around. Same discussion here and ended with "Working as Intended" label. github.com/microsoft/TypeScript/issues/17868 — Md. Tahmid Mozaffar
– Md. Tahmid Mozaffar, Commented Nov 2, 2020 at 1:01

Terry · Accepted Answer · 2020-11-01 23:38:41Z

1

That is because TypeScript actually allows arguments to be discarded. From their documentation:

You may be wondering why we allow ‘discarding’ parameters [...]. The reason for this assignment to be allowed is that ignoring extra function parameters is actually quite common in JavaScript.

In your example, function (num1: number, num2: number) is a subtype of function (one: number, two: number, three: number) and is therefore considered valid.

answered Nov 1, 2020 at 23:38

Terry

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1 Comment

Md. Tahmid Mozaffar Over a year ago

That makes sense. But is there a way to restict it? like in my example, i cannot write the function with two parameters as the interface has three?

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