Editorial: handle len < seachLen in String.prototype.lastIndexOf #3660
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Great catch! I updated my polyfill to use the latest algorithm, and it immediately exposed this bug, which this PR fixes. |
ljharb
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Jul 24, 2025
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In Step 9 of String.prototype.lastIndexOf, it tries to clamp pos between 0 and len - searchLen and according to clamping,
len - searchLen >= 0. So it cannot be defined whenlen - searchLen < 0.For example:
Even if clamping were defined to allow cases where the lower bound is greater than the upper bound (i.e.,
len - searchLen < 0), Step 10 would result in an invalid state. In Step 10, StringLastIndexOf(S, searchStr, start) is invoked with start, which is required to be a non-negative integer. Whenlen < searchLen, the computed start could become negative, which is invalid.Therefore, this PR adds a step that returns -1 when
len < searchLenbefore Step 9, since it is impossible to find searchString within string whenlen < searchLen.