A stock implementation of Heap's algorithm as a JS generator.

permutations.js

export function* permutations(array) {
    let mutArray = array.slice();
    if(mutArray.length > 0) yield mutArray.slice();
    const stack = Array(mutArray.length).fill(0);
    
    for(let i = 0; i < mutArray.length;) {
        if(stack[i] < i) {
            if(i % 2 === 0) {
                [mutArray[0], mutArray[i]] = [mutArray[i], mutArray[0]];
            } else {
                [mutArray[stack[i]], mutArray[i]] = [mutArray[i], mutArray[stack[i]]];
            }
            yield mutArray.slice();
            stack[i]++;
            i = 0;
        } else {
            stack[i] = 0;
            i++;
        }
    }
}