code: function map(fn, ctx) {
    assert.equal(typeof fn, 'function')
    return function(val) {
      val = Array.isArray(val) ? val : [val]
      return Promise.all(val.map(fn, ctx))
    }
  }
docs: Apply a function to all values. @param {Mixed|Mixed[]} val @param {Mixed} ctx @return {Function}