Flash cards
Review the key moves
What is the main idea behind JavaScript For Loop?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ block to be executedPut the learning moves in the order that makes the concept easiest to apply.
For Loops can execute a block of code a number of times.
For Loops are fundamental for tasks like performing an action multiple times.
The For Loop
The for statement creates a loop with 3 optional expressions:
for (
exp 1
;
exp 2
;
exp 3
) {
//
code block to be executed
}exp 1 is executed (one time) before the execution of the code block.
exp 2 defines the condition for executing the code block.
exp 3 is executed (every time) after the code block has been executed.
Example
for (let i = 0; i < 5; i++) {
text += "The number is " + i + "<br>";
}From the example above, you can read:
exp 1 sets a variable before the loop starts (let i = 0).
exp 2 defines the condition for the loop to run (i must be less than 5).
exp 3 increases a value (i++) after the code block has been executed.
forHow to use exp 1
exp 1 is used to initialize the variable(s) used in the loop (let i = 0).
exp 1 is optional.
You can omit exp 1 if the value is set before the loop starts:
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 2;
let text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}How to use exp 2
exp 2 is used to evaluate the condition of the initial variable (i < len).
exp 2 is also optional.
If exp 2 returns false , the loop will end.
If you omit exp 2, you must provide a break inside the loop.
Otherwise the loop will never end.
This will crash your browser.
How to use exp 3
exp 3 increments the value of the initial variable (i++).
exp 3 is optional.
exp 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.
exp 3 can be omitted (if you increment the value inside the loop):
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 0;
let text = "";
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}Loop Scope
Example
var i = 5;
for (var i = 0; i < 10; i++) {
// some code
}
// Here i is 10Example
let i = 5;
for (let i = 0; i < 10; i++) {
// some code
}
// Here i is 5Using 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.