code: function NumberToPrecisionJS(precision) {
    var x = this;
    if (!IS_NUMBER(this)) {
      if (!IS_NUMBER_WRAPPER(this)) {
        throw MakeTypeError(kIncompatibleMethodReceiver,
                            "Number.prototype.toPrecision", this);
      }
      // Get the value of this number in case it's an object.
      x = %_ValueOf(this);
    }
    if (IS_UNDEFINED(precision)) return TO_STRING(x);
    var p = TO_INTEGER(precision);
  
    if (NUMBER_IS_NAN(x)) return "NaN";
    if (x == INFINITY) return "Infinity";
    if (x == -INFINITY) return "-Infinity";
  
    if (p < 1 || p > 21) {
      throw MakeRangeError(kToPrecisionFormatRange);
    }
    return %NumberToPrecision(x, p);
  }
docs: ES6 20.1.3.5 Number.prototype.toPrecision(precision)