Function key

docs/app/js/sanddance-app.js:107023–107046

(fields, flat, opt)

    return typeof _ === "string";
}
function key(fields, flat, opt) {
    if (fields) fields = flat ? array(fields).map((f)=>f.replace(/\\(.)/g, "$1")) : array(fields);
    const len = fields && fields.length, gen = opt && opt.get || getter, map = (f)=>gen(flat ? [
            f
        ] : splitAccessPath(f));
    let fn;
    if (!len) fn = function() {
        return "";
    };
    else if (len === 1) {
        const get = map(fields[0]);
        fn = function(_) {
            return "" + get(_);
        };
    } else {
        const get = fields.map(map);
        fn = function(_) {
            let s = "" + get[0](_), i = 0;
            while(++i < len)s += "|" + get[i](_);
            return s;
        };
    }
    return accessor(fn, fields, "key");
}
function lerp(array4, frac) {
    const lo = array4[0], hi = peek(array4), f = +frac;
    return !f ? lo : f === 1 ? hi : lo + f * (hi - lo);