Passing an array to a spread operator parameter with TypeScript

Question

I've got a class that inherits from Array:

class List<T> extends Array<T> {
    constructor(items: T[]) {
        super(items); // error here...
    }
}

Argument of type 'T[]' is not assignable to parameter of type 'T'.

Assuming this is because Array's constructor takes (...items: T[])

So, how do I pass a standard array to something that takes a spread operator?

Ben Fogel · Accepted Answer · 2017-01-09 20:47:23Z

3

The Array Constructor takes any number of parameters. If you pass in an array into new Array() it gets assigned to the first index.

const newArray = new Array([1, 2, 3]);
console.log(newArray); // [[1, 2, 3]];

Use the spread operator to expand the array out to the parameters.

const newArray = new Array(...[1, 2, 3]); // equivalent to new Array(1, 2, 3);
console.log(newArray); // [1, 2, 3];

So your class would look like this:

class List<T> extends Array<T> {
    constructor(items: T[]) {
        super(...items); // Spread operator here
    }
}

answered Jan 9, 2017 at 20:47

Ben Fogel

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