opentick | fast tick database for financial timeseries data | Database library

You're not modifying the original string, you're modifying the local variable called string. That's why you have to reassign the result of the addition back to string each iteration.

If you're not familiar with the concept of passing by reference vs passing by value, you can skip this paragraph. In JS, strings are passed by value. What this effectively means is you're getting a copy of the string, so you can't modify the original value. If JS passed strings by reference, you would be able to modify the original string and wouldn't have to return a new one. (Disclaimer: This is very simplified, I'm trying to explain this in terms that those new to these concepts can hopefully understand.)

Instead of trying to mutate a parameter, it's usually better to return a new parameter anyways. This is true even for objects where you can modify the original object (to an extent). This is a paradigm of Functional programming that attempts to avoid difficult to understand (and debug) side effects.

To get this to work, you can do something like