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#### The "for" loop

The "`for`" loop is a JavaScript "method" that allows a certain action (ie: block of code) to be performed continuously in a variably controlled fashion. It is very similar to a "while" loop in which lines of JavaScript codes can be grouped together and repeated until a certain condition. A real life example of a for loop would be as follows:

for length equal to 0 to length equal to 100
run!

Lets see the general syntax of for loops in JavaScript, where y is an arbitrary variable:

```for (var y=0; y<=99; y++){
}```

The above example will alert "hi!" 100 times. Lets look more closely at the heart of a "for" loop:

`for (var y=0; y<=99; y++)`

It consists of three components:

var y=0 //The starting point of a for loop
y<=99 //The ending boundary of a for loop
y++ //How the "for" loop is incremented. y++ means increment it by one step each time until the boundary.

The part that may be confusing is the last part, "y++". Let's see the same above example, only this time, altering that particular part:

```for (var y=0; y<=99; y=y+2){
}```

How many "hi" will be alerted? Well, 50 will, because we are incrementing the "counter" not by 1, but by 2 this time. The point is, you can determine however you want to increment the for loop by assigning a different statement in the third component of the for loop.

Lets see a practical example of a "for" loop than, where it is used to calculate the sum of 1+2+3+.....all the way up to the specified number:

Here's the function that performs this calculation:

```<script type="text/javascript">
function cal(){
var y=1
var temp=prompt("Please input a positive integer:")
for (var x=2;x<=temp;x++){
y=y+x
}