How to get length of string in javascript without using native length method

Question

I am working on a practice problem:

Return the length of a string without using javascript's native string.length method.

The only ways I could think of would be substring or slice, but I'm stumped.

Answer 1

You can loop over the string, testing to see whether there is a non-undefined value at each index (as soon as you get an undefined value you've run past the end of the string):

function strLength(s) {
  var length = 0;
  while (s[length] !== undefined)
    length++;
  return length;
}

console.log(strLength("Hello")); // 5
console.log(strLength("")); // 0

(I'm assuming that if you're not allowed to use the native string .length property that you probably shouldn't use the array .length property either with str.split("").length...)

Answer 2

Given that this is a practice problem, I suspect the OP may not want ES6/ES2015, but, just in case that's an option, and/or for whoever else is looking at this, here's a concise modern approach:

const str = "Hello world!";

console.log([...str].reduce(a => a+1, 0));

(When I posted this, no other answer had proposed this solution. However, I had missed the fact that @MarkoGrešak had essentially proposed this exact solution in a comment to another question.)