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:
for (let 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 timesfor/in- loops through the properties of an objectfor/of- loops through the values of any iterablewhile- loops through a block of code while a specified condition is truedo/while- also loops through a block of code while a specified condition is true
The For Loop
The for statement creates a loop with 3 optional expressions:
for (expression 1; expression 2; expression 3) {
// code block to be executed
}
Expression 1 is executed (one time) before the execution of the code block.
Expression 2 defines the condition for executing the code block.
Expression 3 is executed (every time) after the code block has been executed.
From the example above, you can read:
Expression 1 sets a variable before the loop starts (let i = 0).
Expression 2 defines the condition for the loop to run (i must be less than 5).
Expression 3 increases a value (i++) each time the code block in the loop has been executed.
Loop Scope
Using var in a loop:
Using let in a loop:
In the first example, using var, the variable declared in
the loop redeclares the variable outside the loop.
In the second example, using let, the variable declared in
the loop does not redeclare the variable outside the loop.
When let is used to declare the i variable in a loop, the i
variable will only be visible within the loop.
The While Loop
The while loop loops through a block of code as long as a specified condition is true.
Syntax
while (condition) {
// code block to be executed
}
Example
In the following example, the code in the loop will run, over and over again, as long as a variable (i) is less than 10:
If you forget to increase the variable used in the condition, the loop will never end. This will crash your browser.
The Do While Loop
The do while loop is a variant of the while loop. This loop will
execute the code block once, before checking if the condition is true, then it will
repeat the loop as long as the condition is true.
Syntax
do {
// code block to be executed
}
while (condition);
Example
The example below uses a do while loop. The loop will always be
executed at least once, even if the condition is false, because the code block
is executed before the condition is tested:
Do not forget to increase the variable used in the condition, otherwise the loop will never end!
Comparing For and While
If you have read the previous chapter, about the for loop, you will discover that a while loop is much the same as a for loop, with statement 1 and statement 3 omitted.
The loop in this example uses a for loop to collect the car
names from the cars array:
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 0;
let text = "";
for (;cars[i];) {
text += cars[i];
i++;
}
Try it Yourself »
The loop in this example uses a while loop to collect the
car names from the cars array:
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 0;
let text = "";
while (cars[i]) {
text += cars[i];
i++;
}
Try it Yourself »
