Find length of largest substring with unique characters
I am more concerned this code does not look "javascript" enough 


There's no such thing as "javascript enough". If that meant anything, for me it would be keeping it simple. The best code is one that's written in a way that you never have to run to debug. Errors will show themselves voluntarily.


  returns the length of the largest substring


But your function isn't named that way. It's named LongestSubstringNoRepeats which sounds like it returns the substring, not the length. Always name appropriately. Do not give ambiguous or confusing names. It may be popular among devs that naming is one of the two hardest problems in computer science, but that only happens because it's a detail easily missed.

substring.split("").map((char) =&gt; {
  newChars[char] = true;    
});


Never use functions outside their intended purpose. array.map is intended to create another array based on the values of an existing array, a 1:1 transform. In this case, you used it as a loop. If you intend to do that, use array.forEach instead.

Also, avoid unnecessary creation of objects. In this case string.split creates an array, array.map also creates another. This operation is probably possible using only string manipulation.

let currSubstring = {
    length: 1,
    substring: s[0],
}

currSubstring.chars = {};
currSubstring.chars[s[0]] = true;

// to

let currentSubString = {
  length: 1,
  substring: s[0],
  chars: { [s[0]]: true }
}


Can be shortened by using computed property names.

One way to check for repeats is to use Set. It's like an array, but will only hold unique values. Once you add in something that already exists, Set never changes size - something you can check if a character already repeated.

Always name your variables fully and explicitly. Abbreviations should be avoided as it will only cause headaches down the road. JS can be compressed with tools so code size is never an issue. Write for a human, not for the machine.

Always provide test cases. In this case, just looking at the code doesn't give me the impression that it works. It may not work at all or has edge cases you never thought of. TDD is always a good start, especially for API-ish code/code that doesn't involve anything else other than a function that returns a value.