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Loops can execute a block of code a number of times
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:
text += cars[0] + "<br>";
text += cars[1] + "<br>";
text += cars[2] + "<br>";
text += cars[3] + "<br>";
text += cars[4] + "<br>";
text += cars[5] + "<br>";
for (let i = 0; i < cars.length; i++) {
text += cars[i] + "<br>";
}
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 an
iterable object while - 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 trueThe for statement creates a loop with 3 optional expressions:
for (let i = 0; i < 5; i++) {
text += "The number is " + i + "<br>";
}
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.
Normally you will use expression 1 to initialize the variable used in the loop (let i = 0).
This is not always the case, JavaScript doesn't care. Expression 1 is optional.
You can initiate many values in expression 1 (separated by comma):
for (let i = 0, len = cars.length, text = ""; i < len; i++) {
text += cars[i] + "<br>";
}
And you can omit expression 1 (like when your values are set before the loop starts):
let i = 2;
let len = cars.length;
let text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}
Often expression 2 is used to evaluate the condition of the initial variable.
This is not always the case, JavaScript doesn't care. Expression 2 is also optional.
If expression 2 returns true, the loop will start over again, if it returns false, the loop will end.
If you omit expression 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.
Often expression 3 increments the value of the initial variable.
This is not always the case, JavaScript doesn't care, and expression 3 is optional.
Expression 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.
Expression 3 can also be omitted (like when you increment your values inside the loop):
let i = 0;
let len = cars.length;
let text = "";
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}
Using var in a loop:
var i = 5;
for (var i = 0; i < 10; i++) {
// some code
}
// Here i is 10
Using let in a loop:
let i = 5;
for (let i = 0; i < 10; i++) {
// some code
}
// Here i is 5
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 for/in loop and the for/of loop are explained in the next chapter.
The while loop and the do/while are explained in the next chapters.
Create a loop that runs from 0 to 9.
let i;
( = ; < ; ) {
console.log(i);
}