Pattern matching with template literals in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of pattern matching with template literals

What is the output of this TypeScript code using pattern matching with template literals?

Typescript

const input = "user_123";

const match = input.match(/^user_(\d+)$/);

console.log(match ? `ID: ${match[1]}` : "No match");

A ID: 123

B No match

C ID: user_123

D SyntaxError

💡 Hint

Look at the regular expression and what part is captured.

The regex /^user_(\d+)$/ matches strings starting with 'user_' followed by digits only. The digits are captured and printed as ID.

❓ Predict Output

intermediate

Extracting parts with template literal types

What will be the output of this TypeScript code using template literal types and pattern matching?

Typescript

type EventName = `on${string}Click`;

function handleEvent(event: EventName) {
  if (event.startsWith("on") && event.endsWith("Click")) {
    console.log(`Handled event: ${event}`);
  } else {
    console.log("Unknown event");
  }
}

handleEvent("onButtonClick");

A Handled event: ButtonClick

B Unknown event

C Handled event: onButtonClick

D TypeError

💡 Hint

Check the string passed and the conditions in the function.

The string 'onButtonClick' starts with 'on' and ends with 'Click', so it matches the condition and prints the handled event.

🔧 Debug

advanced

Why does this pattern matching fail?

This TypeScript code tries to extract a version number from a string using template literal types but fails. What is the cause?

Typescript

type VersionString = `v${number}`;

function parseVersion(v: string) {
  if (v.match(/^v\d+$/)) {
    const version: VersionString = v;
    return version;
  }
  return "Invalid version";
}

console.log(parseVersion("v2"));

A Type 'string' is not assignable to type 'VersionString' without assertion

B Regex pattern is incorrect and does not match 'v2'

C Function returns undefined instead of string

D Syntax error in template literal type

💡 Hint

Check the type assignment inside the if block.

Even though the string matches the pattern, TypeScript cannot guarantee the string type matches the template literal type without a type assertion.

📝 Syntax

advanced

Identify the syntax error in template literal pattern matching

Which option contains a syntax error in TypeScript template literal pattern matching?

A type File = `${string}.txt`;

B type Greeting = `Hello, ${string}`;

C type Path = `/home/${string}`;

D type Invalid = `${number`;

💡 Hint

Look carefully at the backticks and braces.

Option D has a missing closing brace '}' after 'number', causing a syntax error.

🚀 Application

expert

Count how many strings match this template literal type

Given the type `type Code = `ERR_${number}_${string}`;` and the array below, how many strings match the `Code` type pattern?

Typescript

type Code = `ERR_${number}_${string}`;

const codes = [
  "ERR_404_NotFound",
  "ERR_500_ServerError",
  "WARN_300_SlowResponse",
  "ERR_abc_Invalid",
  "ERR_123_",
  "ERR_0_ZeroError"
];

A 3

B 4

C 5

D 2

💡 Hint

Check which strings start with 'ERR_', followed by a number, then an underscore and any string.

Strings matching the pattern must start with 'ERR_', then a number, then '_', then any string (including empty). 'ERR_abc_Invalid' fails because 'abc' is not a number. 'WARN_300_SlowResponse' fails prefix. 'ERR_123_' matches because empty string after underscore is allowed.

Practice

1. What does pattern matching with template literals in TypeScript primarily help you do?

easy

A. Create new variables dynamically

B. Automatically generate functions from strings

C. Change the type of a variable at runtime

D. Check if a string fits a specific shape or format

Solution

1. Step 1: Understand the purpose of pattern matching

   Pattern matching with template literals is used to verify if strings follow a certain pattern or format.

2. Step 2: Relate to template literals usage

   Template literals allow embedding expressions and patterns to check string shapes easily.

3. Final Answer:

   Check if a string fits a specific shape or format -> Option D

4. Quick Check:

   Pattern matching = Check string shape [OK]

Hint: Pattern matching checks string shapes, not variable types [OK]

Common Mistakes:

• Confusing pattern matching with variable creation
• Thinking it changes variable types
• Assuming it generates functions automatically

2. Which of the following is the correct syntax to use a template literal pattern in a TypeScript type check?

easy

A. type ID = `${string}-${number}`;

B. type ID = '${string}-${number}';

C. ;`}rebmun{$-}gnirts{$` = DI epyt

D. type ID = `${string}-${number}`

Solution

1. Step 1: Identify correct template literal syntax

   Template literals use backticks (`) and embed types inside ${} without quotes.

2. Step 2: Check for correct punctuation

   type ID = `${string}-${number}`; correctly uses backticks and ${} with no extra quotes.

3. Final Answer:

   type ID = `${string}-${number}`; -> Option A

4. Quick Check:

   Backticks + ${} = correct syntax [OK]

Hint: Use backticks and ${} without quotes for template types [OK]

Common Mistakes:

• Using single or double quotes instead of backticks
• Putting quotes inside ${} expressions
• Missing backticks around the pattern

3. What will be the result of this TypeScript code snippet?

type Email = `${string}@${string}.com`;

function isEmail(value: string): value is Email {
  return /.+@.+\.com$/.test(value);
}

console.log(isEmail('[e-mail]'));
console.log(isEmail('[e-mail]'));

medium

A. true\ntrue

B. false\ntrue

C. true\nfalse

D. false\nfalse

Solution

1. Step 1: Understand the Email type pattern

   The Email type expects strings like '[e-mail]'.

2. Step 2: Analyze the isEmail function

   The regex checks if the string ends with '.com' after an '@' symbol.

3. Step 3: Check the console.log outputs

   '[e-mail]' matches the regex, so true; '[e-mail]' does not, so false.

4. Final Answer:

   true false -> Option C

5. Quick Check:

   Regex matches .com only = true, false [OK]

Hint: Regex matches only .com emails, others return false [OK]

Common Mistakes:

• Assuming .org matches .com pattern
• Ignoring regex anchors
• Confusing true/false order in output

4. Identify the error in this TypeScript code using template literal pattern matching:

type Phone = `${number}-${number}-${number}`;

function isPhone(value: string): value is Phone {
  return /^\d{3}-\d{3}-\d{4}$/.test(value);
}

medium

A. The Phone type pattern does not match the regex format

B. The regex pattern is missing anchors

C. Template literals cannot use number types inside ${}

D. The function should return a boolean, not a type predicate

Solution

1. Step 1: Compare Phone type and regex pattern

   Phone type expects three numbers separated by dashes, but each ${number} matches any number, not fixed digits.

2. Step 2: Check regex pattern details

   The regex expects exactly 3 digits, then 3 digits, then 4 digits, which doesn't align with the Phone type's generic number placeholders.

3. Final Answer:

   The Phone type pattern does not match the regex format -> Option A

4. Quick Check:

   Type pattern and regex must align [OK]

Hint: Match template literal types exactly with regex patterns [OK]

Common Mistakes:

• Assuming ${number} matches fixed digit counts
• Ignoring regex anchors (though present here)
• Confusing return type of type predicate functions

5. Given this TypeScript code, what will be the output?

type UserID = `user_${number}`;

function parseUserID(id: string): UserID | null {
  if (id.match(/^user_\d+$/)) {
    return id as UserID;
  }
  return null;
}

console.log(parseUserID('user_123'));
console.log(parseUserID('admin_123'));

hard

A. null\nnull

B. 'user_123'\nnull

C. 'user_123'\n'user_123'

D. null\n'user_123'

Solution

1. Step 1: Understand UserID type pattern

   UserID must start with 'user_' followed by a number.

2. Step 2: Analyze parseUserID function

   The function returns the id cast as UserID if it matches the regex /^user_\d+$/, else returns null.

3. Step 3: Evaluate console.log outputs

   'user_123' matches regex, so returns 'user_123'; 'admin_123' does not, so returns null.

4. Final Answer:

   'user_123' null -> Option B

5. Quick Check:

   Regex match returns id, else null [OK]

Hint: Regex match controls return; unmatched returns null [OK]

Common Mistakes:

• Assuming 'admin_123' matches user pattern
• Confusing return types with casting
• Ignoring regex anchors and pattern