TypeScript Array<T>.splice override

Question

Within a class that derives from Array<T>, I have a splice override:

public splice(start?: number, deleteCount?: number, ...items: T[]): T[] {
    return super.splice(start, deleteCount, ...items);
}

Which compiles to...

SuperArray.prototype.splice = function (start, deleteCount) {
    var items = [];
    for (var _i = 2; _i < arguments.length; _i++) {
        items[_i - 2] = arguments[_i];
    }

    return _super.prototype.splice.apply(this, [start, deleteCount].concat(items));
};

This doesn't work at all. It completely breaks splice! Is there something wrong with the way it compiles this .apply(this, [start, deleteCount].concat(items)) and how do I fix it?

What is happened with splice? Why it is broken?

array.splice(0); // array unaffected

Your "super splice" frankly just wouldn't work as defined, it's totally ambiguous. Suppose you had a super array of numbers, how exactly is that function supposed to behave? — Jeff Mercado
– Jeff Mercado, Commented Jan 11, 2017 at 23:56

Nitzan Tomer · Accepted Answer · 2017-01-11 22:46:10Z

It seems that the reason this happens is that deleteCount is undefined, and if you try this:

let a = [1, 2, 3];
a.splice(0, undefined); // []
console.log(a); /// [1, 2, 3]

Exactly the same thing.
To get over this, you'll need to build the arguments array yourself, something like this:

class MyArray<T> extends Array<T> {
    public splice(start?: number, deleteCount?: number, ...items: T[]): T[] {
        if (start == undefined) {
            start = 0; // not sure here, but you wanted it to be optional
        }

        if (deleteCount == undefined) {
            deleteCount = this.length - start; // the default
        }

        const args = ([start, deleteCount] as any[]).concat(items);
        return super.splice.apply(this, args);
    }
}

Or something like:

public splice(start?: number, deleteCount?: number, ...items: T[]): T[] {
    if (deleteCount != undefined) {
        return super.splice(start, deleteCount, ...items);
    } else {
        return super.splice(start, ...items);
    }
}

But I haven't tested this one.

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