Equality narrowing in Typescript - Mini Project: Build & Apply

Equality narrowing

📖 Scenario: Imagine you are building a simple program that checks the status of a device. The device can be either "on" or "off". You want to write code that behaves differently depending on the device's status.

🎯 Goal: You will create a variable to hold the device status, then use equality narrowing to check if the device is "on" or "off" and print a message accordingly.

📋 What You'll Learn

Create a variable called deviceStatus with the exact value "on" or "off".

Create a variable called isOn that checks if deviceStatus is equal to "on".

Use an if statement with deviceStatus === "on" to print "The device is ON".

Use an else block to print "The device is OFF".

💡 Why This Matters

🌍 Real World

Checking device or system status is common in apps that control hardware or monitor conditions, like smart home devices or servers.

💼 Career

Understanding equality narrowing helps you write safer and clearer code that reacts correctly to different states or inputs.

1

Create the device status variable

Create a variable called deviceStatus and set it to the string "on".

Typescript

# Your code here

Need a hint?

Use let deviceStatus = "on"; to create the variable.

2

Create a boolean variable to check if device is on

Create a variable called isOn and set it to the result of checking if deviceStatus is equal to "on" using ===.

Typescript

let deviceStatus: string = "on";
# Your code here

Need a hint?

Use let isOn = deviceStatus === "on"; to create the boolean.

3

Use equality narrowing with an if statement

Write an if statement that checks if deviceStatus === "on". Inside the if, write a console.log to print "The device is ON". Use an else block to print "The device is OFF".

Typescript

let deviceStatus: string = "on";
let isOn: boolean = deviceStatus === "on";
# Your code here

Need a hint?

Use if (deviceStatus === "on") { console.log("The device is ON"); } else { console.log("The device is OFF"); }

4

Print the device status message

Run the program so it prints the correct message based on deviceStatus. The output should be exactly The device is ON.

Typescript

let deviceStatus: string = "on";
let isOn: boolean = deviceStatus === "on";

if (deviceStatus === "on") {
    console.log("The device is ON");
} else {
    console.log("The device is OFF");
}
# Your code here

Need a hint?

Run the program and check the console output.