Why do i need put -1 instead of 0 to get correct code?

Question

    let str = "Hello world";
    let target = "o";
    let pos = -1;
    while ((pos = str.indexOf(target, pos + 1)) != -1) {
      console.log( pos );
    }

When pos is - 1 it works fine, but when I set 0 and remove + 1 it returns endless 4. Why does it happen?

[How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) and [What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/q/25385173) — VLAZ, Nov 30 '22 at 14:56
string.indexOf(searchvalue, start), https://www.w3schools.com/jsref/jsref_indexof.asp . Will work depending on the size of target and the start parameter. — MZM, Nov 30 '22 at 15:01

score 2 · Accepted Answer · answered Nov 30 '22 at 15:01

If you do not increase the position, it repeats 4 indefinitely because you are always doing the exact same check. Removing variables and using just this part:

pos = str.indexOf(target, pos)

This would mean:

str.indexOf(target, 0) // Returns 4
str.indexOf(target, 4) // Returns 4
str.indexOf(target, 4) // etc

That's because indexOf will start looking at the exact index that it just found so it will find the o again. Adding +1 makes sure that it continues looking PAST last occurence found.

Why do i need put -1 instead of 0 to get correct code?

1 Answers1