O(n) JavaScript + TypeScript with detailed explanatory comments
\n// We can solve this using a dynamic "sliding window" that \n// shrinks or grows whenever certain conditions are met\n\nconst lengthOfLongestSubstring = funct
```\n// We can solve this using a dynamic "sliding window" that \n// shrinks or grows whenever certain conditions are met\n\nconst lengthOfLongestSubstring = function(characters: string) {\n    // Setting to 0 takes care of the edge case where "characters" is \'\'\n    let length = 0\n    \n    // Map will store each character and the last index they were looped over at\n    let characterMap = new Map()\n    \n    // We declare a left index since a sliding window requires two indices\n    // (left and right) to track the window of elements the indices surround\n    let leftIndex = 0\n    \n    // Begin looping through each character in the string.\n    // rightIndex will always be greater the leftIndex to create a "window" of elements\n    for (let rightIndex = 0; rightIndex < characters.length; rightIndex++) {\n        \n        // Get current character for better readability\n        const character = characters[rightIndex]\n        \n        // Check if character exists and if its last index is greater than the\n        // current leftIndex. If we don\'t check it\'s leftIndex, we\'re setting\n        // it to a previous result which\'ll provide incorrect values \n        // when calculating the window length\n        if (characterMap.has(character) && characterMap.get(character) >= leftIndex) {\n            // set left index to last index where the character was found plus one\n            leftIndex = characterMap.get(character) + 1\n        }\n        \n        // Which is greater, the last iteration\'s window length, or the current\n        // interation\'s window length? We decide here, then set our result length\n        // to that number. We add 1 since we need to take into account every element\n        // within the window (2 - 0 = 2, although there are three elements, so add 1) \n        length = Math.max(length, rightIndex - leftIndex + 1)\n            \n        // Add character and its index to map, or, if it exists,\n        // automatically overwrite its index with the current\n        characterMap.set(character, rightIndex)\n    }\n    \n    return length\n};\n```
['Sliding Window', 'TypeScript', 'JavaScript']