You can (and should) declare and initialize the loop index variable i inside the parentheses of the for loop, like this:

for (let i = startIndex; i <= stopIndex; i++) {

Apart from this code being more concise, it also ensures that the i variable is created inside the block scope of the for loop body (because you are, correctly, using let and not var). The way you have done it creates the variable in the local scope of the function where it is not needed.

Got it and put it inside of for loop as you show.

You can simplify this code by using two if statements in a row, rather than chaining them in an if-then-else sequence. Each condition is then tested independently:

if (i % 3 === 0) {
  threeCallback(i);
}
if (i % 5 === 0) {
  fiveCallback(i);
}

I was also uncomfortable with these if statement block and then solved more logically as you say.

You did not completely follow the assignment here as it asked to create the array first and then loop through the elements (requiring to for loops in a row rather one for loop:

The function should first generate an array containing values from start value to end value (inclusive).

Then the function should take the newly created array and iterate over it, and calling the first callback if the array value is divisible by 3.

You're right Sir. I changed sequence and added another for loop for creating array and iterating it.

Just a comment on the English language: 'can divide by 3' -> 'can be divided by 3.'

Grammar error, corrected.

There is no need for this if statement as the test num > 0 is already done by the for loop. The for loop will execute the loop body only while num > 0.

Absolutely Sir. If statement is deleted.

Here, you do need to test before entering the loop whether num <= 0. This is because in the case of the do-while loop, the condition is tested at the end of the loop and therefore the loop body is executed at least once. This also makes the do-while loop rather useless in practice, and it can always be rewritten as a regular while loop.

Got it. I couldn't think that. do-while loop is implemented in if statement.

You could just return base + sum directly without creating an intermediate variable sum. However, sometimes it makes sense to create an intermediate variable as you did here if you want to make it absolutely clear how the human reader should interpret the result of the computation.

I understand. It is shorthened.

See my comment below with respect to creating the variable unique.

Also this one is shorthened.