I'm trying to use the map function inside the toString overwritten in my Array subclass and I'm seeing a strange behaviour. It looks like that .map is calling the constructor of my subclass for no apparent reason and throwing a TypeError.

class Seq extends Array {
    constructor(seq = '') {
        super();
        (seq.match(/.{2}/g) || []).forEach((mi) => this.push(mi));
    }
    toString() {
        return this.map(it=>it).join('');
    }
}
new Seq('1643').toString() //expects to see '1643' as the output but getting a TypeError

Recreating the array in the toString method fixes the problem, but I'd like to understand why.

toString() {
    return [...this].map(it=>it).join('');
}