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

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

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

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

Hint

Run the program and check the console output.

Practice

1. What does equality narrowing in TypeScript help you do?

easy

A. It lets TypeScript know the exact type after checking equality.

B. It automatically converts types without checks.

C. It disables type checking for variables.

D. It only works with numbers, not strings.

Solution

1. Step 1: Understand equality narrowing concept

   Equality narrowing means TypeScript uses equality checks (like ===) to know the exact type of a variable.

2. Step 2: Compare options with definition

   Only It lets TypeScript know the exact type after checking equality. correctly describes this behavior. Others are incorrect or unrelated.

3. Final Answer:

   It lets TypeScript know the exact type after checking equality. -> Option A

4. Quick Check:

   Equality narrowing = exact type after equality check [OK]

Hint: Equality narrowing means type known after === check [OK]

Common Mistakes:

• Thinking it converts types automatically
• Believing it disables type checking
• Assuming it only works for numbers

2. Which of the following is the correct syntax to check equality for narrowing in TypeScript?

easy

A. if (value === 10) { }

B. if (value = 10) { }

C. if (value == 10) { }

D. if (value != 10) { }

Solution

1. Step 1: Identify correct equality operator for narrowing

   TypeScript uses strict equality === for safe type narrowing.

2. Step 2: Check each option

   if (value === 10) { } uses ===, which is correct. if (value == 10) { } uses loose equality, which is less safe. if (value = 10) { } is assignment, not comparison. if (value != 10) { } is inequality, not equality.

3. Final Answer:

   if (value === 10) { } -> Option A

4. Quick Check:

   Use === for equality narrowing [OK]

Hint: Use === for equality narrowing, not == or = [OK]

Common Mistakes:

• Using = instead of ===
• Using == which is not strict
• Confusing != with ===

3. What will be the output of this TypeScript code?

function check(x: string | number) {
  if (typeof x === 'string') {
    return x.toUpperCase();
  } else {
    return x.toFixed(2);
  }
}
console.log(check(3.14159));

medium

A. "3.14" (error)

B. "3.14" (string)

C. "3.14"

D. "3.14" (number)

Solution

1. Step 1: Understand type narrowing in function

   The function checks if x is a string or number using typeof. For number, it calls toFixed(2).

2. Step 2: Evaluate output for input 3.14159

   Since input is number, it returns 3.14159.toFixed(2) which is string "3.14".

3. Final Answer:

   "3.14" -> Option C

4. Quick Check:

   Number input returns toFixed string "3.14" [OK]

Hint: toFixed returns string, so output is "3.14" [OK]

Common Mistakes:

• Thinking toFixed returns number
• Confusing string and number output
• Ignoring type narrowing with typeof

4. Identify the error in this TypeScript code using equality narrowing:

function process(value: string | number) {
  if (value === 'hello') {
    return value.toUpperCase();
  } else {
    return value.toUpperCase();
  }
}

medium

A. Error: should use == instead of ===.

B. No error, code is correct.

C. Error: missing else if for number type.

D. Error: calling toUpperCase on number in else block.

Solution

1. Step 1: Analyze type narrowing with equality check

   The check value === 'hello' narrows value to string only if it equals 'hello'. Else block still has value as string or number.

2. Step 2: Identify error in else block

   Calling toUpperCase() on value in else block is unsafe because value could be number, causing runtime error.

3. Final Answer:

   Error: calling toUpperCase on number in else block. -> Option D

4. Quick Check:

   toUpperCase on number causes error [OK]

Hint: Check types before calling string methods [OK]

Common Mistakes:

• Assuming else block is string only
• Using == instead of === for narrowing
• Ignoring runtime errors from wrong method calls

5. Given this TypeScript code, what is the correct way to safely handle both types using equality narrowing?

function formatValue(val: string | number | null) {
  if (val === null) {
    return "No value";
  } else if (typeof val === 'string') {
    return val.trim();
  } else {
    return val.toFixed(1);
  }
}

hard

A. Missing check for undefined, so unsafe.

B. Code is correct and safely narrows all types.

C. Should use == null instead of === null for null check.

D. toFixed should be called only after checking val is number.

Solution

1. Step 1: Check null handling

   The first if checks val === null, safely handling null case.

2. Step 2: Check type narrowing for string and number

   The else if uses typeof val === 'string' to narrow string, else block handles number with toFixed(1).

3. Step 3: Confirm all cases covered

   All possible types (null, string, number) are handled safely with proper narrowing.

4. Final Answer:

   Code is correct and safely narrows all types. -> Option B

5. Quick Check:

   All types handled safely with === and typeof [OK]

Hint: Check null first, then typeof for string, else number [OK]

Common Mistakes:

• Forgetting null check
• Using == null which includes undefined
• Calling toFixed without type check