Count how many times the loop body runs as the input grows. A loop that visits every one of n items runs n times. If each pass does a fixed amount of work, the loop is O(n).
for (let i = 0; i < n; i++) {
total += arr[i]; // runs n times, O(1) each
}
// whole loop: O(n)
The loop bound is what matters, not the constant work inside. A loop from zero to n stays O(n) even if the body does ten small steps.
Look at what the counter depends on. If it runs a fixed number of times regardless of input, that part is O(1). A bound that grows differently, like stopping at the square root of n, changes the class.
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