Question

Many loops follow the pattern seen in the previous while examples. First, a “counter” variable is created to track the progress of the loop. Then comes a while loop, whose test expression usually checks whether the counter has reached some boundary yet. At the end of the loop body, the counter is updated to track progress.

Because this pattern is so common, JavaScript and similar languages provide a slightly shorter and more comprehensive form, the for loop.

for (var number = 0; number <= 12; number = number + 2)
  console.log(number);
// → 0
// → 2
//   … etcetera

This program is exactly equivalent to the earlier even-number-printing example. The only change is that all the statements that are related to the “state” of the loop are now grouped together.

The parentheses after a for keyword must contain two semicolons. The part before the first semicolon initializes the loop, usually by defining a variable. The second part is the expression that checks whether the loop must continue. The final part updates the state of the loop after every iteration. In most cases, this is shorter and clearer than a while construct.

Here is the code that computes 2¹⁰, using for instead of while :

var result = 1;
for (var counter = 0; counter < 10; counter = counter + 1)
  result = result * 2;
console.log(result);
// → 1024

Note that even though no block is opened with a { , the statement in the loop is still indented two spaces to make it clear that it “belongs” to the line before it.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.