Truthiness narrowing in Typescript - Mini Project: Build & Apply

Truthiness Narrowing in TypeScript

📖 Scenario: You are building a simple program that checks user input values and decides if they are valid or not based on whether they are "truthy" or "falsy" in TypeScript.

🎯 Goal: Learn how to use truthiness narrowing to check if a variable has a meaningful value before using it.

📋 What You'll Learn

Create a variable with a value that can be truthy or falsy

Create a boolean variable to hold the check result

Use an if statement to narrow the type by checking truthiness

Print the result to the console

💡 Why This Matters

🌍 Real World

Checking if user inputs or data values exist before using them is common in web forms and applications.

💼 Career

Understanding truthiness narrowing helps avoid errors and write safer TypeScript code in real projects.

1

Create a variable with a value that can be truthy or falsy

Create a variable called userInput and assign it the value "Hello".

Typescript

# Create a variable called userInput and assign it the value "Hello"
// Your code here

Hint

Use let userInput: string | null = "Hello"; to allow userInput to be a string or null.

2

Create a boolean variable to hold the check result

Create a boolean variable called isValid and set it to false.

Typescript

let userInput: string | null = "Hello";
// Create a boolean variable called isValid and set it to false
// Your code here

Hint

Use let isValid: boolean = false; to create the boolean variable.

3

Use an if statement to narrow the type by checking truthiness

Use an if statement to check if userInput is truthy. Inside the if, set isValid to true.

Typescript

let userInput: string | null = "Hello";
let isValid: boolean = false;
// Use if statement to check if userInput is truthy
// Inside if, set isValid to true
// Your code here

Hint

Use if (userInput) { isValid = true; } to check truthiness and update isValid.

4

Print the result to the console

Write a console.log statement to print the value of isValid.

Typescript

let userInput: string | null = "Hello";
let isValid: boolean = false;
if (userInput) {
    isValid = true;
}
// Print isValid to the console
// Your code here

Hint

Use console.log(isValid); to show the result.

Practice

1. What does truthiness narrowing in TypeScript help you do inside an if statement?

easy

A. It forces TypeScript to ignore type errors.

B. It converts all values to strings automatically.

C. It tells TypeScript the value is not null, undefined, or falsy, so you can use it safely.

D. It makes the code run faster by skipping checks.

Solution

1. Step 1: Understand what truthiness means

   Truthiness means a value is considered true in a boolean context, so it is not null, undefined, 0, empty string, or false.

2. Step 2: Apply truthiness narrowing in TypeScript

   Inside an if statement, TypeScript narrows the type by excluding falsy values, letting you safely use the value without extra checks.

3. Final Answer:

   It tells TypeScript the value is not null, undefined, or falsy, so you can use it safely. -> Option C

4. Quick Check:

   Truthiness narrowing = safe value use [OK]

Hint: If inside if(value), value is truthy and safe to use [OK]

Common Mistakes:

• Thinking it converts values to strings
• Believing it skips type checks
• Assuming it improves runtime speed

2. Which of the following TypeScript if statements correctly uses truthiness narrowing?

easy

A. if (value == false) { console.log(value); }

B. if (value === 0) { console.log(value); }

C. if (value = true) { console.log(value); }

D. if (value !== null && value !== undefined) { console.log(value); }

Solution

1. Step 1: Check each if condition for truthiness narrowing

   if (value !== null && value !== undefined) { console.log(value); } explicitly checks for not null and not undefined, which is a common way to narrow truthy values.

2. Step 2: Identify incorrect syntax or logic

   if (value == false) { console.log(value); } checks if value is false (falsy), which is opposite of truthiness narrowing. if (value = true) { console.log(value); } uses assignment (=) instead of comparison (===). if (value === 0) { console.log(value); } checks if value equals 0, which is falsy, so it does not narrow to truthy.

3. Final Answer:

   if (value !== null && value !== undefined) { console.log(value); } -> Option D

4. Quick Check:

   Correct syntax and logic = if (value !== null && value !== undefined) { console.log(value); } [OK]

Hint: Use !== null && !== undefined for safe truthiness checks [OK]

Common Mistakes:

• Using assignment (=) instead of comparison (===)
• Checking for falsy instead of truthy
• Confusing equality operators

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

function printLength(str?: string) {
  if (str) {
    console.log(str.length);
  } else {
    console.log('No string');
  }
}
printLength('hello');
printLength('');
printLength();

medium

A. 5, No string, No string

B. 5, 0, undefined

C. 5, No string, undefined

D. 5, 0, No string

Solution

1. Step 1: Analyze the if condition with truthiness narrowing

   The condition if (str) is true only if str is truthy. An empty string ('') is falsy, so it goes to else.

2. Step 2: Check each function call output

   printLength('hello') prints 5 because 'hello' is truthy and length is 5.
   printLength('') prints 'No string' because '' is falsy.
   printLength() prints 'No string' because str is undefined (falsy).

3. Final Answer:

   5, No string, No string -> Option A

4. Quick Check:

   Falsy empty string triggers else [OK]

Hint: Empty string is falsy, so else block runs [OK]

Common Mistakes:

• Assuming empty string is truthy
• Expecting undefined to print length
• Confusing falsy values with null only

4. Find the error in this TypeScript code using truthiness narrowing:

function process(value: string | null) {
  if (value !== null) {
    console.log(value.toUpperCase());
  } else {
    console.log('No value');
  }
}
process(null);
process(undefined);

medium

A. The if condition should check for undefined as well as null.

B. The code will cause a runtime error when value is undefined.

C. The toUpperCase method is not valid for strings.

D. There is no error; the code works fine.

Solution

1. Step 1: Check the type of value and condition

   Value can be string or null, but the function is called with undefined, which is not handled.

2. Step 2: Identify missing check for undefined

   The if condition only excludes null, so if value is undefined, it will cause an error when calling toUpperCase.

3. Final Answer:

   The if condition should check for undefined as well as null. -> Option A

4. Quick Check:

   Check all falsy types to avoid errors [OK]

Hint: Check both null and undefined for safe narrowing [OK]

Common Mistakes:

• Ignoring undefined values
• Assuming null check covers undefined
• Not testing all input cases

5. Given this TypeScript code, what is the safest way to narrow the type of input to avoid errors when accessing input.length?

function handleInput(input: string | null | undefined | number) {
  if (/* condition here */) {
    console.log(input.length);
  } else {
    console.log('Invalid input');
  }
}

hard

A. if (input) { console.log(input.length); }

B. if (typeof input === 'string' && input) { console.log(input.length); }

C. if (input !== null) { console.log(input.length); }

D. if (input != undefined) { console.log(input.length); }

Solution

1. Step 1: Identify all possible types of input

   Input can be string, null, undefined, or number. Only strings have a length property.

2. Step 2: Narrow input to string and check truthiness

   Using typeof input === 'string' ensures input is a string type. Adding && input excludes empty strings (falsy) if needed.

3. Final Answer:

   if (typeof input === 'string' && input) { console.log(input.length); } -> Option B

4. Quick Check:

   Check type and truthiness for safe property access [OK]

Hint: Check type string before accessing length [OK]

Common Mistakes:

• Only checking truthiness without type
• Ignoring number type causing errors
• Assuming null check covers undefined