JSでRangeを扱うことは闇が深い(クソ)

00. range.js

function* range(from, to, _step=1){
    const step = ((step)=> {
        if(typeof(step) === "function") return step;
        if(!Number.isFinite(step) || !step) return;
        return _=> step;
    })(_step);
    
    if(!step) return;
    
    const isString = (typeof(from) === "string");
    const f1 = isString ? (s)=> s.codePointAt(0) : (n)=> n;
    const f2 = isString ? (s)=> String.fromCodePoint(s) : (n)=> n;
    
    if(to == null) [from, to] = [isString ? "\u0000" : 0, from];
    
    if(typeof(from) !== typeof(to)) return;
    
    const a = f1(from);
    const b = f1(to);
    
    let i = a;
    let s = step(i);
    
    while(Number.isFinite(s) && (!s || (s>0) ? i<=b : i>=b)){
      yield f2(i);
      
      s = step(i);
      i += s;
    }
}

01. index.js

console.log(Array.from(range(1, 10, 2)));                         // [1, 3, 5, 7, 9]
console.log(Array.from(range("a", "c")));                         // ["a", "b", "c"]
console.log(Array.from(range("z", "x")));                         // []
console.log(Array.from(range("z", "x", -1)));                     // ["z", "y", "x"]
console.log(Array.from(range("z", "x", 0)));                      // [] (これ無限ジェネレータにすべきか悩んでる)
console.log(Array.from(range("z", "x", NaN)));                    // []
console.log(Array.from(range("a")));                              // ["", "", ..., "_", "`", "a"]
console.log(Array.from(range(0, 20, (i)=> (Math.random()*4|0)))); // [0, 0, 0, 3, 6, 7, 8, 11, 12, 14, 14, 17, 17, 19] (random)