combinations.html

<script type="text/javascript">
function getCombos(chars, k, indent) {
    if (indent === undefined) {
        indent = 0;
    }
    let debugMsg = ".".repeat(indent) + "In getCombos('" + chars + "', " + k + ")";
    document.write(debugMsg + ", start.<br />");
    if (k == 0) {
        // BASE CASE
        document.write(debugMsg + " base case returns ['']<br />");
        // If k asks for 0-combinations, return '' as the selection of
        // zero letters from chars.
        return [""];
    } else if (chars == "") {
        // BASE CASE
        document.write(debugMsg + " base case returns []<br />");
        return [];  // A blank chars has no combinations, no matter what k is.
    }

    // RECURSIVE CASE
    let combinations = [];
    //  First part, get the combos that include the head:
    let head = chars.slice(0, 1);
    let tail = chars.slice(1, chars.length);
    document.write(debugMsg + " part 1, get combos with head '" + head + "'<br />");
    let tailCombos = getCombos(tail, k - 1, indent + 1);
    document.write(".".repeat(indent) + "Adding head '" + head + "' to tail combos:<br />");
    for (tailCombo of tailCombos) {
        document.write(".".repeat(indent) + "New combination " + head + tailCombo + "<br />");
        combinations.push(head + tailCombo);
    }
    // Second part, get the combos that don't include the head:
    document.write(debugMsg + " part 2, get combos without head '" + head + "')<br />");
    combinations = combinations.concat(getCombos(tail, k, indent + 1));

    document.write(debugMsg + " results are " + combinations + "<br />");
    return combinations;
}

document.write('<pre>2-combinations of "ABC":<br />');
document.write("Results: " + getCombos("ABC", 2) + "<br /></pre>");
</script>