# For...Loop
There is probably a programming language out there that does not have some sort of implementation of the for loop, but they are few and far between. It is almost a requirement in order to have the computer do code repeatedly **a certain number** of times.
The loop contains a counter or **index** that the programmer defines. The code will run and loop back to the top as long as the "**continue condition**" is true. Each loop iteration needs to modify the index somehow or else you have an infinite loop. Typically you increase the index by 1 but you can do any sort of mathematical operation on the index.
### Syntax
The syntax is fairly simple:
```javascript
for (index start value; continue condition; index modifier) {
// code
}
```
### Example 1 - [Hello World](https://tech.io/snippet/HU3d1Dk)
```javascript
// The following prints "Hello World!" 5 times, showing the line number
// Start at 1, end before 6 (so, 5), increase by 1 every interation
for ( let index = 1; index < 6; index++ ) {
console.log(`${index}: Hello world!`);
}
```
### Example 2 - Animation
Here is an animation of a for loop in action. The code prints the powers of 2 from a power of zero to ten (inclusive). It also keeps track of a running total and prints that at the end.
.gif?alt=media\&token=6758e361-d14b-49e4-8777-1575315873a1)
{% hint style="info" %}
For a faster animation, [click here](https://drive.google.com/file/d/0BzyP6cTwQ57eaEFjX2lkdW9zWDQ/view?usp=sharing) (and for a really fast one, [click here](https://drive.google.com/file/d/0BzyP6cTwQ57eaFBIMXAtQ05nYjg/view?usp=sharing)).
{% endhint %}
### Example 3 - [Decrement](https://tech.io/snippet/Ra72fAC)
While most examples and definitions show `i++` to *increment* the index, it does not need to be counting *up.* You can *decrement* (subtract one), add 2, multiply by a value... it all depends on the needs of the programmer. Here is an example of a for loop that counts *down* by 1:
```javascript
// Print from 20 (inclusive) to -10 (exclusive)
// Start at 20, loop if greater than -10 (so stop after -9), decrease by 1 each loop
for (let i = 20; i > -10; i--) {
console.log(i);
}
```
### Example 4 - [Nested Loops](https://tech.io/snippet/P1NODsX)
Because a for loop can contain any amount of code (scope block `{ }`), you can write *another* for loop inside (and another inside that, etc). This is called *nested looping*. An example would be printing a multiplication table or other 2-dimensional work:
```javascript
// Print the multiplication table from 1 to 10 (inclusive)
let output;
// Rows (1 top, 10 bottom)
for (let y = 1; y <= 10; y++) {
output = ''; // Make sure it is a string
// Columns (1 left, 10 right)
for (let x = 1; x <= 10; x++) {
// Build the output row
output += x*y + "\t";
}
// Print the current row
console.log(output);
}
```
### More Information
#### If you need more information, here are some other sources:
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