rel.js

/*! (c) Andrea Giammarchi - ISC */
const Rel = typeof Map === 'undefined' ?
  function () {
    const k = [], v = [];
    return {
      has: value => -1 < k.indexOf(value),
      get: value => v[k.indexOf(value)],
      set: value => {
        const i = k.indexOf(value);
        v[i < 0 ? (k.push(value) - 1) : i] = value;
      }
    };
  } :
  Map
;