Is it possible to provide default parameter to interface function in typescript?

Question

I extend the Number class as follows:

interface Number {
    evolution(now: number, before: number): string;
}


Number.prototype.magnitude = function(d=1, suffix="") {
    //…
}

And I like to provide default parameters.

But when using it with no explicit parameter ass follows:

label = "÷ " + show.magnitude();

I got an error "The supplied parameters do not match signature"

Related: developer.mozilla.org/en-US/docs/Web/JavaScript/… — Paleo
– Paleo, Commented Feb 17, 2017 at 15:37

Seamus · Accepted Answer · 2017-02-17 15:50:28Z

3

You need to tell the TypeScript compiler that the parameters are optional:

In JavaScript, every parameter is optional, and users may leave them off as they see fit. When they do, their value is undefined. We can get this functionality in TypeScript by adding a ? to the end of parameters we want to be optional.

Here is an example similar to what you want to accomplish:

interface ISum {
    (baz?: number, buz?: number): number;
}

let sum: ISum = (baz = 1, buz = 2) => {
    return baz + buz;
}

console.log(sum()); //Console: 3
console.log(sum(2)); //Console: 4
console.log(sum(2, 7)); //Console: 9

answered Feb 17, 2017 at 15:50

Seamus

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