Why test prev !== undefined instead of if (prev)?

A character first seen at index 0 is stored as 0, which is falsy. if (prev) would treat that as never seen and leave left behind the repeat, so "aa" would return 2. Comparing with undefined narrows the number | undefined type and keeps index 0.

Why check prev >= left before moving the left edge?

The map remembers every character ever seen, including ones that have already slid out of the window. Without the check a stale index drags left backwards: "noon" would return 3 instead of 2. left = Math.max(left, prev + 1) says the same thing in one line.

Is an Int32Array faster than a Map here?

Usually, for ASCII input. const last = new Int32Array(128).fill(-1) indexed by s.charCodeAt(right) avoids hashing string keys, and the loop body becomes left = Math.max(left, last[c] + 1); last[c] = right;. It only works while every code is below 128; a Map accepts any character.

Why does a string of emoji give the wrong answer?

s.length and s[i] count UTF-16 code units, and an emoji outside the Basic Multilingual Plane is two of them, so the window can split one emoji in half. Run the same loop over Array.from(s), which yields whole code points, when the input is not plain ASCII.

How do I return the substring itself?

Change the return type to string, keep a start next to best, and replace the Math.max line with if (right - left + 1 > best) { start = left; best = right - left + 1; }. Finish with return s.slice(start, start + best);. The strict > keeps the earliest of several equally long runs, so "noon" gives "no".

Longest Substring Without Repeating Characters in TypeScript: Set and Map Windows

length_of_longest_substring.ts

TypeScript 5.9

Not run yet

+ 8 hidden

Input

s = "streetcar"

Expected

The double e breaks the word up. After it, "etcar" has five different characters.