Run this code:

let n = 5;
let str = '';
for (let i = 0; i < n; i++) {
    for (let j = 0; j < i; j++)
        str += j;
   
   console.log("Outer: "+i+" Inner: "+str);
   str = '';
}

Output is:

Outer: 0 Inner: 
Outer: 1 Inner: 0
Outer: 2 Inner: 01
Outer: 3 Inner: 012
Outer: 4 Inner: 0123

As you can see, in the output above, inner loop (the one with variable j) doesn't run, because if you replace the variables with numbers it would be 0 < 0 (i < 0), which isn't true.

The best way for you to understand how nested loops work is to write each step and variable values on the paper, like so:

n = 5

STEP 1:

• i = 0
• i < n (0 < 5) TRUE
  • j = 0
  • j < i (0 < 0) FALSE inner loop doesn't execute
• OUTPUT: "Outer: 0 Inner:"
• str = ''

STEP 2:

• i = 1
• i < n (1 < 5) TRUE
  • j = 0
  • j < i (0 < 1) TRUE
  • str = 0
  • j = 1 (j++)
  • j < i (1 < 1) FALSE
• OUTPUT: "Outer: 1 Inner: 0"
• str = ''

And so on... Keep repeating this until the argument in the outer loop is false (i < n).

You must remember, in for loop the sequence of orders executed:

for(let i = 0; i < 5; i++) {
  console.log('It works');
}

1. let i = 0; (this executes only once)

2. i < 5 (if true run 3 and 4)

3. run the code in loop

4. i ++

5. i < 5 (if true run 6 and 7)

6. run the code in loop

7. i++

etc.