Loops execute a block of code a specified number of times, or while a specified condition is true.
Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
for - loops through a block of code a specified number of times
while - loops through a block of code while a specified condition is true
The for Loop
The for loop is used when you know in advance how many times the script should run.
code to be executed
The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs.
Note: The increment parameter could also be negative, and the <= could be any comparing statement.
document.write("The number is " + i);