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import line from './assets/straight_line.png'; import conditional from './assets/conditional.png'; import forLoop from './assets/for_loop.png'; import whileLoop from './assets/while_loop.png';

2.1.5 JavaScript Control Flow

JavaScript executes a program from top to bottom in order.

Conditional Execution

  • We will very often need a program to do things conditionally.

The if Statement

  • The condition must always evaluate to true for the code inside of the {} to actually run.
if (condition) {
    // do something
}
let number = 20;
if (number > 16) {
    console.log('Hold');
}
let number = 20;
if (number > 16) {
    console.log('Hold');
} else {
    console.log('Hit me');
}

Exercise

Turn the following sentences into valid JavaScript if statements. Use console.log() to "announce" an action.

// 1. If it rains, I stay home.
let currentWeather = 'rainy';

// 2. If I am hungry, I eat.
let hunger = true;

// 3. If it's 10pm, I go to bed. If not, I write code.
let currentHour = 22;

Loops

Here is a program that outputs all even numbers between 0 and 12.

console.log(0);
console.log(2);
console.log(4);
console.log(6);
console.log(8);
console.log(10);
console.log(12);

This is Repetitious, but manageable...

Here is a program that outputs all even numbers between 0 and 1000.

console.log(0);
console.log(2);
// ... on and on and on...

This is where loops come in!

while Loops

Example

let number = 0;
while (number <= 12) {
    console.log(number);
    number = number + 2;
}

// let's break that down.

Let's write a function that outputs 2^10 (two to the power of ten).

// Example

Using while is MUCH better than the repetition that we had before, but it is not great.

We still need to

  • define a counter
  • define the condition for loop to run
  • increment that counter, everytime the while loop runs

I wonder if there is a shorthand for this? 😉

for Loops

Example

for (let number = 0; number <=12; number = number + 1) {
    if (number % 2 === 0) {
        console.log(number);
    }

}

// let's break that down.

⚠️ The ONLY way to break out of the loop is for the condition to be false. ⚠️

  • If you write a for loop that always evaluates to true, your loop will continue forever.
  • This is a bad thing... it will crash your environment (browser window, node env, etc.).
    • To fix it, you will need to force-quit the environment.

Exercise 1

Write a program that output all of the numbers from 0 to 25

// code here

Exercise 2

Write a program that output all of ODD the numbers from 0 to 25

// code here

Exercise 3

Write a program that output all of the numbers from 0 to 25, but replaces all multipes of 5 by the phrase five alive!

// code here