type Utf8ErrorReason: 
  | "UNEXPECTED_CONTINUE"
  | "BAD_PREFIX"
  | "OVERRUN"
  | "MISSING_CONTINUE"
  | "OUT_OF_RANGE"
  | "UTF16_SURROGATE"
  | "OVERLONG";
When using the UTF-8 error API the following errors can be intercepted and processed as the reason passed to the Utf8ErrorFunc. "UNEXPECTED_CONTINUE" - a continuation byte was present where there was nothing to continue. "BAD_PREFIX" - an invalid (non-continuation) byte to start a UTF-8 codepoint was found. "OVERRUN" - the string is too short to process the expected codepoint length. "MISSING_CONTINUE" - a missing continuation byte was expected but not found. The offset indicates the index the continuation byte was expected at. "OUT_OF_RANGE" - the computed code point is outside the range for UTF-8. The badCodepoint indicates the computed codepoint, which was outside the valid UTF-8 range. "UTF16_SURROGATE" - the UTF-8 strings contained a UTF-16 surrogate pair. The badCodepoint is the computed codepoint, which was inside the UTF-16 surrogate range. "OVERLONG" - the string is an overlong representation. The badCodepoint indicates the computed codepoint, which has already been bounds checked.

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Source

encoding/utf8.ts:45