Parts of a For Loop

Thefor loopexplicitly forces you to define the start point, stop point, and each step of the loop. In fact, you'll get anUncaught SyntaxError: Unexpected token )if you leave out any of the three required pieces.

for ( start; stop; step ) {
  // do this thing
}

Here's an example of a for loop that prints out the values from 0 to 5. Notice the semicolons separating the different statements of the for loop:var i = 0; i < 6; i = i + 1

for (var i = 0; i < 6; i = i + 1) {
  console.log("Printing out i = " + i);
}

Prints:
Printing out i = 0
Printing out i = 1
Printing out i = 2
Printing out i = 3
Printing out i = 4
Printing out i = 5

Nested Loops

Did you know you can also_nest _loops inside of each other? Paste this nested loop in your browser and take a look at what it prints out:

for (var x = 0; x < 5; x = x + 1) {
  for (var y = 0; y < 3; y = y + 1) {
    console.log(x + "," + y);
  }
}

Prints:
0, 0
0, 1
0, 2
1, 0
1, 1
1, 2
2, 0
2, 1
2, 2
3, 0
3, 1
3, 2
4, 0
4, 1
4, 2

Notice the order that the output is being displayed.

For each value ofxin the outer loop, the inner for loop executes completely. The outer loop starts withx = 0, and then the inner loop completes it's cycle with all values ofy:

x = 0 and y = 0, 1, 2 // corresponds to (0, 0), (0, 1), and (0, 2)

Once the inner loop is done iterating overy, then the outer loop continues to the next value,x = 1, and the whole process begins again.

x = 0 and y = 0, 1, 2 // (0, 0) (0, 1) and (0, 2)
x = 1 and y = 0, 1, 2 // (1, 0) (1, 1) and (1, 2)
x = 2 and y = 0, 1, 2 // (2, 0) (2, 1) and (2, 2)
etc.

NOTE:Nested loops can be tricky at first. We will revisit them again in Lesson 6: Arrays.

Increment and decrement

Here is a summary of operators you've learned so far:

x++ or ++x // same as x = x + 1 
x-- or --x // same as x = x - 1
x += 3 // same as x = x + 3
x -= 6 // same as x = x - 6
x *= 2 // same as x = x * 2
x /= 5 // same as x = x / 5

factorial

var solution = 1;
for (var i = 12; i > 0; i--) { //12!
  solution *= i;
}
console.log(solution); // 12!