filter.js

function match(straw, needle, key = identity) {
    let penalty = 0;
    let start = 0;
    let chars = [];
    let text = key(straw);
    for (char of needle) {
        let pos = text.substr(start).search(char);
        if (pos < 0) {
            return false, chars];
        } else {
            chars.push(start + pos);
            penalty += pos;
            start += pos + 1;
        }
    }
    return [penalty, chars];
}