JavaScript For Loop
Loops can execute a block of code a number of times.
JavaScript Loops
Loops are handy, if you want to run the same code over and over again, each time with a different value.
Often this is the case when working with arrays:
Instead of writing:
text += cars[0] + "<br>";
text += cars[1] + "<br>";
text += cars[2] + "<br>";
text += cars[3] + "<br>";
text += cars[4] + "<br>";
text += cars[5] + "<br>";
You can write:
var i;
for (i = 0; i < cars.length; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
Different Kinds of Loops
JavaScript supports different kinds of loops:
- for - loops through a block of code a number of times
- for/in - loops through the properties of an object
- while - loops through a block of code while a specified condition is true
- do/while - also loops through a block of code while a specified condition is true
The For Loop
The for loop has the following syntax:
for (statement 1; statement 2; statement 3) {
code block to be executed
}
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
From the example above, you can read:
Statement 1 sets a variable before the loop starts (var i = 0).
Statement 2 defines the condition for the loop to run (i must be less than 5).
Statement 3 increases a value (i++) each time the code block in the loop has been executed.
Statement 1
Normally you will use statement 1 to initialize the variable used in the loop (i = 0).
This is not always the case, JavaScript doesn't care. Statement 1 is optional.
You can initiate many values in statement 1 (separated by comma):
Example
for (i = 0, len = cars.length, text = ""; i < len; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
And you can omit statement 1 (like when your values are set before the loop starts):
Example
var i = 2;
var len = cars.length;
var text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
Statement 2
Often statement 2 is used to evaluate the condition of the initial variable.
This is not always the case, JavaScript doesn't care. Statement 2 is also optional.
If statement 2 returns true, the loop will start over again, if it returns false, the loop will end.
If you omit statement 2, you must provide a break inside the loop. Otherwise the loop will never end. This will crash your browser. Read about breaks in a later chapter of this tutorial.
Statement 3
Often statement 3 increments the value of the initial variable.
This is not always the case, JavaScript doesn't care, and statement 3 is optional.
Statement 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.
Statement 3 can also be omitted (like when you increment your values inside the loop):
Example
var i = 0;
var len = cars.length;
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}
Try it Yourself »
The For/In Loop
The JavaScript for/in statement loops through the properties of an object:
Example
var person = {fname:"John", lname:"Doe", age:25};
var text = "";
var x;
for (x in person) {
text += person[x];
}
Try it Yourself »
The While Loop
The while loop and the do/while loop will be explained in the next chapter.
Test Yourself with Exercises!
Exercise 1 » Exercise 2 » Exercise 3 » Exercise 4 » Exercise 5 » Exercise 6 »
