Java
Script 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:
for
(i = 0; i < cars.length; i++) {
text += cars[i] + "<br>";
}
»
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
is often the tool you will use when you want to create a loop.
The for loop
has the following syntax:
for
(statement 1; statement 2; statement 3) {
code block to be executed
}
Statement 1
is executed before the loop (the code block) starts.
Statement 2
defines the condition for running the loop (the code block).
Statement 3
is executed each time after the loop (the code block) has been executed.
Example
for
(i = 0; i < 5; i++) {
text += "The number is " + i +
"<br>";
}
»
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>";
}
»
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>";
}
»
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++;
}
»
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];
}
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