function uniqueId(prefix) {
      var id = ++idCounter;
      return baseToString(prefix) + id;
    }
Calculates `n` rounded down to `precision`.

@static
@memberOf _
@category Math
@param {number} n The number to round down.
@param {number} [precision=0] The precision to round down to.
@returns {number} Returns the rounded down number.
@example

_.floor(4.006);
// => 4

_.floor(0.046, 2);
// => 0.04

_.floor(4060, -2);
// => 4000