LibJS: Use more accurate number-to-string method in Number toPrecision

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/5898db8c0f
Pull-request: https://github.com/SerenityOS/serenity/pull/15921
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibJS/Runtime/NumberPrototype.cpp

@@ -386,17 +386,13 @@ JS_DEFINE_NATIVE_FUNCTION(NumberPrototype::to_precision)
     }
     // 10. Else,
     else {
-        // FIXME: The computations below fall apart for large values of 'p'. A double typically has 52 mantissa bits, which gives us
-        //        up to 2^52 before loss of precision. However, the largest value of 'p' may be 100, resulting in numbers on the order
-        //        of 10^100, thus we lose precision in these computations.
-
         // a. Let e and n be integers such that 10^(p-1) ≤ n < 10^p and for which n × 10^(e-p+1) - x is as close to zero as possible.
         //    If there are two such sets of e and n, pick the e and n for which n × 10^(e-p+1) is larger.
         exponent = static_cast<int>(floor(log10(number)));
         number = round(number / pow(10, exponent - precision + 1));
 
         // b. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
-        number_string = decimal_digits_to_string(number);
+        number_string = number_to_string(number, NumberToStringMode::WithoutExponent);
 
         // c. If e < -6 or e ≥ p, then
         if ((exponent < -6) || (exponent >= precision)) {

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toPrecision.js

@@ -87,6 +87,9 @@ describe("correct behavior", () => {
             [1, 4, "1.000"],
             [123, 4, "123.0"],
             [123.45, 4, "123.5"],
+
+            // Disabled for now due to: https://github.com/SerenityOS/serenity/issues/15924
+            // [3, 100, "3." + "0".repeat(99)],
         ].forEach(test => {
             expect(test[0].toPrecision(test[1])).toBe(test[2]);
         });