A traditional for-loop

A traditional for loop has three components:

1. The initialization: executed before the look block is executed the first time
2. The condition: checks a condition every time before the loop block is executed, and quits the loop if false
3. The afterthought: performed every time after the loop block is executed

These three components are separated from each other by a ; symbol. Content for each of these three components is optional, which means that the following is the most minimal for loop possible:

for (;;) {
    // Do stuff
}

Of course, you will need to include an if(condition === true) { break; } or an if(condition === true) { return; } somewhere inside that for-loop to get it to stop running.

Usually, though, the initialization is used to declare an index, the condition is used to compare that index with a minimum or maximum value, and the afterthought is used to increment the index:

for (var i = 0, length = 10; i < length; i++) {
    console.log(i);
}

Using a traditional for loop to loop through an array

The traditional way to loop through an array, is this:

for (var i = 0, length = myArray.length; i < length; i++) {
    console.log(myArray[i]);
}

Or, if you prefer to loop backwards, you do this:

for (var i = myArray.length - 1; i > -1; i--) {
    console.log(myArray[i]);
}

There are, however, many variations possible, like for example this one:

for (var key = 0, value = myArray[key], length = myArray.length; key < length; value = myArray[++key]) {
    console.log(value);
}

… or this one …

var i = 0, length = myArray.length;
for (; i < length;) {
    console.log(myArray[i]);
    i++;
}

… or this one: