I'm reading the ECMAScript abstract operation ToString. In Step 5 (m is the number we want to convert to a string):
- Otherwise, let n, k, and s be integers such that k ≥ 1, 10^(k−1) ≤ s < 10^k, the Number value for s × 10^(n−k) is m, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
I can't figure out in which case the least significant digit of s would not be uniquely determined. Any example?
The answer is - as always with floating point math - rounding at the edge of the available precision.
Let's take s = 7011750883285835, k=16 and some n (let's say n=0). Now determining m, we'll get the floating point number 0x3FE67006BD248487 (somewhere around 0.70117508832858355…). However, we could also have chosen s = 7011750883285836 and it would be equal to m as well.
The point is that if we had converted the double to decimal representation exactly, we would've gotten 0.701175088328583551167128007364. That is much longer than necessary (and implies higher precision than available), so the ToString algorithm specifies to make a decimal representation of m with the least amount of siginificant digits ("k is as small as possible") that still parses to the number m we want. Sometimes, we can both round up or round down to get that, and both ways are allowed.
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