Loops

Computers are like robots. They are good at doing things over and over and over and over again.

A LOOP is when we ask a program to do something many times.

while

The simplest loop in JavaScript is a while loop.

while (someCondition) {
// this code runs once per loop cycle
doSomeAction()
}

In English this reads, "While some condition is true, do some action".

while true

To loop forever, set the condition to true:

while (true) {
// this will loop forever
// because `true` never changes
console.log("Hello");
}

This means "While true is true, say 'Hello'".

true will always be true, so it loops forever.

To stop the loop, hold down the CONTROL key and press the C key, CRTL+C.

This is called an infinite loop.

One Infinite Loop 1 Infinite Loop
Cupertino, CA 95014

Counting Forever

Let's write a program that counts forever.

Put this in a file called count.js.

let count = 1;
while (true) {
console.log(count);
count = count + 1;
}

Run the program with node count.js.

Remember to CRTL+C to stop the program

LAB: Count to One Hundred

Next, change your count.js program so that it count 1 to 100.

let count = 1;
while (/* count is less than or equal to 100 */) {
console.log(count);
count = count + 1;
}

Solution: Count to 100

Hint Remember, a while loop only runs while the check evaluates true

while(/*this expression is true...*/) {
// ...do this action
}

Solution

let count = 1;
while (count <= 100) {
console.log(count);
count = count + 1;
}

Did your program stop at 100, 101, or 99?

While Loops

• The while statement keeps checking the test expression

• if it's true then it loops back
• if it's false then it stops looping and goes on to the rest of the program

This is fairly complicated, so let's stop here and make sure to understand everything that's happening in this little program.

While breakdown (pt.1)

let count = 1

creates a variable named count and sets its value to 1.

while (count <= 100)

starts a loop and immediately compares count to 100.

1 is less than 100, so the expression is true, so we continue with the block of code starting with the {.

While breakdown (pt.2)

console.log(count);

prints the current value of count.

count = count + 1

increments the count variable... it was 1, so now it's 2

}

goes back to the while line and checks again

While breakdown (pt.3)

while (count <= 100)

compares count to 100.

2 is less than 100, so the expression is true, so we continue with the loop.

Eventually, count becomes 101, and the while expression is false, and so we stop looping and go on.

Break in a While Loop

The magic word break stops looping immediately.

Here is another way to count to 100, but this time using break:

let count = 0;
while (true) {
console.log(count);
count = count + 1;
if (count > 100) {
break;
}
}

For Loops

The while loop is simple, but there is another loop called a for loop that combines multiple operations within the for statement.

for (let count = 1; count <= 100; count++) {
console.log(count);
}

For Loops with Arrays

A for loop is often seen paired with an indexed data structure known as an Array

let poemLines = [
'Roses are red',
'Violets are blue',
'Honey is sweet',
'And so are you'
];

for (let line = 0; line < poemLines.length; line++) {
console.log(poemLines[line]);
}

JavaScript.info Guide for Arrays

MDN Guide for Arrays

For Loop Breakdown (pt.1)

We are using the for loop to get each item in the Array collection poemLines

let poemLines = [
'Roses are red',
'Violets are blue',
'Honey is sweet',
'And so are you'
];

For Loop Breakdown (pt.2)

The for (...) part of a for loop is where the loop:

1. Initializes a variable to track
2. Determines whether to run again
3. Changes the variable after the loop runs.
for (let line = 0; line < poemLines.length; line++)

For Loop Breakdown (pt.3)

Think about the three parts as three separate lines of code.

1. Initialize a variable to track
let line = 0;
1. Determine whether to run again
line < poemLines.length;
1. Change the variable after the loop runs
line++;

For Loop Breakdown (pt.4)

The body of the loop is the code that runs on each time through the loop

for (let line = 0; line < poemLines.length; line++)
/* everything inside the curly braces is the body */
{
// this code will run each time we loop
// since `index` changes each time,
// a different poem line is printed each cycle
console.log(poemLines[line]);
}

LAB: One Potato

Now let's apply what we have learned about while and for loops to print a poem in JavaScript.

Remember this poem?

1 potato,
2 potato,
3 potato,
4!
5 potato,
6 potato,
7 potato,
More!

Please write a program called potato.js that prints that poem, using loops.

(No, you can't just print the entire poem as a single string; that would be cheating.)

Counting Potatoes Solution (pt.1)

Let's separate the solution into two parts.

Create a function poemLine does only one thing; create the correct string for a given line number.

Solution

function poemLine(lineNumber) {
if (lineNumber === 8) {
return "More!"
} else if (lineNumber === 4) {
return "4!"
} else {
return lineNumber + " potato,"
}
}

Counting Potatoes Solution (pt.2)

The second part of the solution only loops, based on a counter lineNumber.

Solution

function poemLine(lineNumber) {
if (lineNumber === 8) {
return "More!"
} else if (lineNumber === 4) {
return "4!"
} else {
return lineNumber + " potato,"
}
}

// New Code Below

let lineNumber = 1;

while (lineNumber <= 8) {
console.log(poemLine(lineNumber));
lineNumber = lineNumber + 1;
}

Counting Potatoes, alternative with break

Another solution using the break to stop looping after eight cycles.

Solution

function poemLine(lineNumber) {
if (lineNumber === 8) {
return "More!"
} else if (lineNumber === 4) {
return "4!"
} else {
return lineNumber + " potato,"
}
}

// New Code Below

let lineNumber = 1;

while (true) {
console.log(poemLine(lineNumber));
lineNumber = lineNumber + 1;
if (lineNumber > 8) {
break;
}
}

Outline