Repeated calculations and nested loops

A loop can repeat any operation, not only printing. To work out 2×2×2×2×2, start with num = 1 and multiply it by 2 five times; the variable carries the result from one round to the next:

let num = 1;
for (let i = 0; i < 5; i++) {
  num = num * 2;
  console.log(num);
}

2
4
8
16
32

You can use the counter itself in the calculation. Here i takes the values 1 to 5, so the program works out 1×2×3×4×5 and prints only the final result, because the print is after the loop:

let num = 1;
for (let i = 1; i <= 5; i++) {
  num = num * i;
}
console.log(num);

120

For adding, start at 0. The shortcut total += i means total = total + i:

let total = 0;
for (let i = 1; i <= 10; i++) {
  total += i;
}
console.log(total);

55

if inside a loop

Putting if inside a loop runs a command on chosen rounds only. Here there are five sit-ups, with a run after the third one. The counter starts at 0, so the third time is i == 2:

for (let i = 0; i < 5; i++) {
  console.log("sit-up");
  if (i == 2) {
    console.log("running");
  }
}

sit-up
sit-up
sit-up
running
sit-up
sit-up

A loop inside a loop

You can put a whole loop inside another one. On every round of the outer loop, the inner loop runs from start to finish. This prints a small times table:

for (let i = 1; i <= 3; i++) {
  for (let j = 1; j <= 3; j++) {
    console.log(i + " x " + j + " = " + i * j);
  }
}

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9

To draw a shape, the inner loop builds each line by adding "*" to an empty text, and the outer loop prints it:

for (let row = 1; row <= 3; row++) {
  let line = "";
  for (let col = 1; col <= 5; col++) {
    line = line + "*";
  }
  console.log(line);
}

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