Array.prototype.arrayLength

An exercise in stupidity

How do you get the length of an array in JavaScript?

Array.prototype.arrayLength = function () { return this.length }

I don’t trust <insert-js-impl-here>

Array.prototype.arrayLength = function() {
    let len = 0
    for (let x in this) {
        if (len === 0) len++
        len++
    }
    return len
}

Aside: I think I’d had too much to drink by this point – the above implementation is just wrong. Also amusingly iterating this actually includes arrayLength, which I can’t quite explain currently (I guess I’m using prototype wrong?)

Ok, but that could you make it look like “modern” js at least?

Array.prototype.arrayLength = function() {
    let len = 0
    this.forEach((x, i) => len = i + 1)
    return len
}

Surely you don’t need three lines?

Array.prototype.arrayLength = function() {
    return this.reduce(acc => acc+1, 0)
}

Sorry but that implementation does not account for setting arbitrary indecies:

const xs = [1,2,3]
xs[99] = 100
xs.length == 100
xs.arrayLength() == 4

N.B.: The for (const x in this) solution fails at this as well

Array.prototype.arrayLength = function() {
    return Math.max(...Object.keys(this).map(x => Number.parseInt(x)).filter(k => !Number.isNaN(k))) + 1
}

Clears throught [].arrayLength() === -Infinity

Array.prototype.arrayLength = function() {
    return (Object.keys(this).map(x => Number.parseInt(x)).filter(k => !Number.isNaN(k)).sort((a, b) => b - a)[0]) + 1 || 0
}

I hate you

Array.prototype.arrayLength = function() {
    return this.findLastIndex(this.at(-1))
}

Cries in non-bleeding-edge-impl

N.B.: I haven’t even tested this implementation, since I don’t have any javascripts that support findLastIndex currently.

Footnote

I actually learnt something about how .forEach was implemented by thinking up and testing these dumb implementations, how novel!

Array.prototype.myForEach = function(f) {
    const kvs = Object.entries(this).filter(([k]) => !Number.isNaN(Number.parseInt(k)))
    for (let i = 0; i < kvs.length; i++) {
        f(kvs[i][1], Number.parseInt(kvs[i][0]))
    }
}

It’s quite interesting how far we go in pretending that the numeric indecies we pass in square-braces are actually numbers – ie. we pass back the numeric index in the forEach. Or maybe I just don’t quite understand it

const xs = [1,2,3]
xs[1] === xs['1']