if (!Array.prototype.flat) {
  Object.defineProperty(Array.prototype, "flat", {
    configurable: true,
    value: function flat() {
      var depth = isNaN(arguments[0]) ? 1 : Number(arguments[0])

      return depth
        ? Array.prototype.reduce.call(
            this,
            function(acc, cur) {
              if (Array.isArray(cur)) {
                acc.push.apply(acc, flat.call(cur, depth - 1))
              } else {
                acc.push(cur)
              }

              return acc
            },
            []
          )
        : Array.prototype.slice.call(this)
    },
    writable: true,
  })
}