The Steinhaus-Johnson-Trotter permutation algorithm in Javascript

SJT-Permutation.js

function permute(array) {
    // Identity
    if(!array.length) {
        return [];
    }
    
    var ret = [array],
        len = array.length,
        modlen = len - 1,
        mover = array[modlen],
        which,
        subset = permute(array.slice(0, -1)),
        permlen = subset.length;
    
    for(var iy = 0; iy < permlen; iy++) {
        which = subset[iy];
        if(iy % 2) {
            for(var ix = 0; ix <= modlen; ix ++) {
                ret.push(which.slice(0, ix).concat([mover], which.slice(ix)));
            }
        } else {
            for(var ix = modlen - 1; ix >= 0; ix --) {
                ret.push(which.slice(0, ix).concat([mover], which.slice(ix)));
            }
        }
    }
    return ret;
}

The result is 22 values when i input '1234'. It is wrong. In this case, it has 24 values. Please check again

thanks. silly thing is that I was looking for this a couple weeks ago and ran into my own old code and saw your comment and thought "oh this bozo implementation is wrong. I can't use it". And it took me a while to realize that it was MY implementation and I was the bozo here.

Anyway, I should fix this regardless ... the implementation I ended up using I lifted from a "framework" thing that was really top-heavy