Super constructor return ignored

[whitneyit]

TypeScript Version:

nightly (1.9.0-dev.20160313)

Code

class Collection extends Array {

    constructor(...args) {
        const first = arguments[0];
        if (args.length === 1 && typeof first === 'number') {
            super(first, null);
            this.pop();
        } else {
            super(...args);
        }
    }

}

const a = new Collection(3, 2, 1);
const b = new Collection(3);
console.log(a, b);

Expected behavior:
The Array is correctly subclassed and the instances are created correctly.

Actual behavior:
The instances are not created correctly.

Notes:

• This code works in the babel repl.
• Not only does the above code not work in the playground, it needs to following addition to IArrayConstructor to get it to not squark.
• This seems related to Return value of super() calls not used for this #7574.

Questions:

• Should this be part of core?

interface ArrayConstructor {
    new (...items: any[]): any[];
}

[whitneyit]

The following can be used as a workaround:

class Collection extends Array {

    constructor(...args) {
        super();
        const first = arguments[0];
        if (args.length === 1 && typeof first === 'number') {
            super.push.call(this, first);
        } else {
            super.push.apply(this, args);
        }
    }

}

const a = new Collection(3, 2, 1);
const b = new Collection(3);
console.log(a, b);

[jeffreymorlan]

This code works in the babel repl.

Actually, it doesn't - it's impossible to subclass builtin classes like Array in ES5, because there's no way to tell the Array constructor to use your subclass's prototype.

If you transpile this with Babel, new Collection creates a plain array which does not inherit from Collection.prototype, defeating the whole purpose of subclassing.

[Arnavion]

(Since it wasn't explicitly answered) The reason the compiler complains about the two super() calls is that Collection extends Array and not Array<T>. The construct signature that takes in array elements only exists in the generic form new<T>(...items: T[]): T[] .

So Collection should either be class Collection<T> extends Array<T> or class Collection extends Array<any> depending on what the intended use is.