assert(toUCSindex(`hello world`, 7) == 7); assert(toUCSindex(`hello world`w, 7) == 7); assert(toUCSindex(`hello world`d, 7) == 7); assert(toUCSindex(`Ma Chérie`, 7) == 6); assert(toUCSindex(`Ma Chérie`w, 7) == 7); assert(toUCSindex(`Ma Chérie`d, 7) == 7); assert(toUCSindex(`さいごの果実 / ミツバチと科学者`, 9) == 3); assert(toUCSindex(`さいごの果実 / ミツバチと科学者`w, 9) == 9); assert(toUCSindex(`さいごの果実 / ミツバチと科学者`d, 9) == 9);
Given index into str and assuming that index is at the start of a UTF sequence, toUCSindex determines the number of UCS characters up to index. So, index is the index of a code unit at the beginning of a code point, and the return value is how many code points into the string that that code point is.