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JavaScript Basics: Making Decisions

JavaScript Basics - Making decisions

Sketchnote by Tomomi Imura

Pre-Lecture Quiz

Pre-lecture quiz

Making decisions and controlling the order in which your code runs makes your code reusable and robust. This section covers the syntax for controlling data flow in JavaScript and its significance when used with Boolean data types

Making Decisions

🎥 Click the image above for a video about making decisions.

You can take this lesson on Microsoft Learn!

A Brief Recap on Booleans

Booleans can have only two values: true or false. Booleans help make decisions on which lines of code should run when certain conditions are met.

Set your boolean to be true or false like this:

let myTrueBool = true let myFalseBool = false

✅ Booleans are named after the English mathematician, philosopher and logician George Boole (1815–1864).

Comparison Operators and Booleans

Operators are used to evaluate conditions by making comparisons that will create a Boolean value. The following is a list of operators that are frequently used.

Symbol Description Example
< Less than: Compares two values and returns the true Boolean data type if the value on the left side is less than the right 5 < 6 // true
<= Less than or equal to: Compares two values and returns the true Boolean data type if the value on the left side is less than or equal to the right 5 <= 6 // true
> Greater than: Compares two values and returns the true Boolean data type if the value on the left side is larger than the right 5 > 6 // false
>= Greater than or equal to: Compares two values and returns the true Boolean data type if the value on the left side is larger than or equal to the right 5 >= 6 // false
=== Strict equality: Compares two values and returns the true Boolean data type if values on the right and left are equal AND are the same data type. 5 === 6 // false
!== Inequality: Compares two values and returns the opposite Boolean value of what a strict equality operator would return 5 !== 6 // true

✅ Check your knowledge by writing some comparisons in your browser's console. Does any returned data surprise you?

If Statement

The if statement will run code in between its blocks if the condition is true.

if (condition) {
  //Condition is true. Code in this block will run.
}

Logical operators are often used to form the condition.

let currentMoney;
let laptopPrice;

if (currentMoney >= laptopPrice) {
  //Condition is true. Code in this block will run.
  console.log("Getting a new laptop!");
}

If..Else Statement

The else statement will run the code in between its blocks when the condition is false. It's optional with an if statement.

let currentMoney;
let laptopPrice;

if (currentMoney >= laptopPrice) {
  //Condition is true. Code in this block will run.
  console.log("Getting a new laptop!");
} else {
  //Condition is false. Code in this block will run.
  console.log("Can't afford a new laptop, yet!");
}

✅ Test your understanding of this code and the following code by running it in a browser console. Change the values of the currentMoney and laptopPrice variables to change the returned console.log().

Switch Statement

The switch statement is used to perform different actions based on different conditions. Use the switch statement to select one of many code blocks to be executed.

switch (expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
  // code block
}
// program using switch statement
let a = 2;

switch (a) {
  case 1:
    a = "one";
    break;
  case 2:
    a = "two";
    break;
  default:
    a = "not found";
    break;
}
console.log(`The value is ${a}`);

✅ Test your understanding of this code and the following code by running it in a browser console. Change the values of the variable a to change the returned console.log().

Logical Operators and Booleans

Decisions might require more than one comparison, and can be strung together with logical operators to produce a Boolean value.

Symbol Description Example
&& Logical AND: Compares two Boolean expressions. Returns true only if both sides are true (5 > 6) && (5 < 6 ) //One side is false, other is true. Returns false
|| Logical OR: Compares two Boolean expressions. Returns true if at least one side is true (5 > 6) || (5 < 6) //One side is false, other is true. Returns true
! Logical NOT: Returns the opposite value of a Boolean expression !(5 > 6) // 5 is not greater than 6, but "!" will return true

Conditions and Decisions with Logical Operators

Logical operators can be used to form conditions in if..else statements.

let currentMoney;
let laptopPrice;
let laptopDiscountPrice = laptopPrice - laptopPrice * 0.2; //Laptop price at 20 percent off

if (currentMoney >= laptopPrice || currentMoney >= laptopDiscountPrice) {
  //Condition is true. Code in this block will run.
  console.log("Getting a new laptop!");
} else {
  //Condition is true. Code in this block will run.
  console.log("Can't afford a new laptop, yet!");
}

Negation operator

You've seen so far how you can use an if...else statement to create conditional logic. Anything that goes into an if needs to evaluate to true/false. By using the ! operator you can negate the expression. It would look like so:

if (!condition) {
  // runs if condition is false
} else {
  // runs if condition is true
}

Ternary expressions

if...else isn't the only way to express decision logic. You can also use something called a ternary operator. The syntax for it looks like this:

let variable = condition ? <return this if true> : <return this if false>

Below is a more tangible example:

let firstNumber = 20;
let secondNumber = 10;
let biggestNumber = firstNumber > secondNumber ? firstNumber : secondNumber;

✅ Take a minute to read this code a few times. Do you understand how these operators are working?

The above states that

  • if firstNumber is larger than secondNumber
  • then assign firstNumber to biggestNumber
  • else assign secondNumber.

The ternary expression is just a compact way of writing the code below:

let biggestNumber;
if (firstNumber > secondNumber) {
  biggestNumber = firstNumber;
} else {
  biggestNumber = secondNumber;
}

🚀 Challenge

Create a program that is written first with logical operators, and then rewrite it using a ternary expression. What's your preferred syntax?

Post-Lecture Quiz

Post-lecture quiz

Review & Self Study

Read more about the many operators available to the user on MDN.

Go through Josh Comeau's wonderful operator lookup!

Assignment

Operators