function uniqueId(prefix) {
	      var id = ++idCounter;
	      return toString(prefix) + id;
	    }
Adds two numbers.

@static
@memberOf _
@since 3.4.0
@category Math
@param {number} augend The first number in an addition.
@param {number} addend The second number in an addition.
@returns {number} Returns the total.
@example

_.add(6, 4);
// => 10