Number.isInteger is the modern, type-aware integer check; it rejects strings even if they look like numbers, which is usually the safe behavior. The looser isWholeNumber helper is for when you have already accepted a number-or-numeric-string input and just want "no fractional part". For digit length, the math form (floor(log10(abs(n))) + 1) is fast and avoids allocating a string, but it has a degenerate case at zero (the formula yields -Infinity), so guard with an explicit zero check. The string form (.toString().length) is shorter and gets the zero case for free; use it unless you are in a hot loop.

Iterative exponent (pow) shows the principle but does O(exp) multiplications; binary exponentiation (powFast) squares and conditionally multiplies, doing only O(log exp) work and matching the algorithm libraries use under the hood. Euclidean GCD walks (a, b) -> (b, a mod b) until the remainder is zero; the destructuring swap keeps the body to one line. LCM uses the identity a * b = gcd(a, b) * lcm(a, b), derived directly from prime factorizations. For prime listing in a small range, trial division up to sqrt(n) is fast enough; if you need every prime up to a large N, switch to a Sieve of Eratosthenes for O(N log log N).

All three digit tricks share the same shape: convert to string, manipulate as a character array, convert back to a number. sortDigitsDesc uses a numeric comparator (Number(b) - Number(a)) because the default sort would compare characters lexicographically (which happens to match numerics for single digits, but is fragile to depend on). reverseDigits carries the sign explicitly because string reversal would otherwise leave the minus sign at the end; note the trailing-zero gotcha (-9870 reversed becomes -789 when parsed back to a number). sumOfDigits is a one-liner via reduce. For large numbers (above Number.MAX_SAFE_INTEGER, ~9e15), use BigInt and a different conversion pipeline.

The hex color trick combines Math.random() with 0xffffff (16,777,215, the max 6-digit hex value) and toString(16) to produce a hex string. The padStart(6, '0') is critical: small integers like 0x42ff produce a 4-character string and the result #42ff is not a valid CSS color. The countdown helper returns the values as an array so the caller can format or render them; for an animation, drive a setInterval that decrements a counter. Random integer in a range and unbiased array shuffle are deliberately not duplicated here; both already live as their own snippets in the catalog and have additional context (modulo bias, Fisher-Yates correctness) you should not lose.