Why advanced types are needed in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with a string type.

Typescript

let name: [1] = "Alice";

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cany

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean instead of string.

The string type tells TypeScript that the variable name should hold text.

2fill in blank

medium

Complete the code to define a function that returns a number.

Typescript

function getAge(): [1] {
  return 30;
}

Drag options to blanks, or click blank then click option'

Avoid

Bstring

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or void as return type.

The function returns a number, so the return type should be number.

3fill in blank

hard

Fix the error in the code by completing the type for the parameter.

Typescript

function greet(person: [1]) {
  console.log("Hello, " + person);
}

Drag options to blanks, or click blank then click option'

Aany

Bboolean

Cnumber

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using number or boolean causes errors.

The parameter person is used as text, so it should be a string.

4fill in blank

hard

Fill both blanks to create a type that can be either a string or a number.

Typescript

let value: [1] = 42;
value = [2];

Drag options to blanks, or click blank then click option'

Astring | number

B"hello"

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using only string or number alone.

The type string | number means value can be a string or a number. Assigning "hello" is valid.

5fill in blank

hard

Fill all three blanks to create a function type that takes a number and returns a boolean.

Typescript

type Check = ([1]: [2]) => [3];

Drag options to blanks, or click blank then click option'

Anum

Bnumber

Cboolean

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter or return types.

The function type Check takes a parameter named num of type number and returns a boolean.

Practice

1. Why do we need advanced types in TypeScript?

easy

A. To avoid writing any type annotations

B. To make the code run faster at runtime

C. To describe complex data shapes clearly and catch errors early

D. To allow JavaScript code to run without changes

Solution

1. Step 1: Understand the purpose of advanced types

   Advanced types help describe complex data structures and relationships in code.

2. Step 2: Recognize their benefit in error checking

   They allow TypeScript to catch mistakes before running the code by checking data shapes.

3. Final Answer:

   To describe complex data shapes clearly and catch errors early -> Option C

4. Quick Check:

   Advanced types = clear data + early error catch [OK]

Hint: Advanced types describe complex data and catch errors early [OK]

Common Mistakes:

• Thinking advanced types speed up runtime
• Believing they remove the need for any types
• Confusing advanced types with JavaScript features

2. Which of the following is a correct way to define a union type in TypeScript?

easy

A. let value: string | number;

B. let value: string & number;

C. let value: string or number;

D. let value: string, number;

Solution

1. Step 1: Recall union type syntax

   Union types use the pipe symbol | to allow multiple types.

2. Step 2: Check each option

   let value: string | number; uses | correctly; B uses & which means intersection, not union; C and D use invalid syntax.

3. Final Answer:

   let value: string | number; -> Option A

4. Quick Check:

   Union type uses | symbol [OK]

Hint: Union types use | between types, not & or words [OK]

Common Mistakes:

• Using & instead of | for union types
• Writing 'or' instead of |
• Separating types with commas

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

type Result = string | number;
function format(value: Result) {
  if (typeof value === 'string') {
    return value.toUpperCase();
  } else {
    return value.toFixed(2);
  }
}
console.log(format(3.14159));

medium

A. "3.14"

B. "3.14" (number) with two decimals

C. "3.14159"

D. "3.14" (string) with two decimals

Solution

1. Step 1: Understand the function behavior

   If the input is a string, it converts to uppercase. Otherwise, it formats the number to 2 decimals using toFixed(2).

2. Step 2: Apply the function to 3.14159

   3.14159 is a number, so toFixed(2) returns the string "3.14" with two decimals.

3. Final Answer:

   "3.14" (string) with two decimals -> Option D

4. Quick Check:

   Number input uses toFixed(2) = "3.14" [OK]

Hint: Check type guards to predict output format [OK]

Common Mistakes:

• Thinking toFixed returns a number, not string
• Confusing string input with number input
• Ignoring type narrowing in if statement

4. Identify the error in this TypeScript code using advanced types:

type User = { name: string; age: number } | null;
function greet(user: User) {
  console.log("Hello, " + user.name.toUpperCase());
}

medium

A. user can be null, so accessing user.name causes an error

B. toUpperCase() is not a function on string

C. Missing return type in function greet

D. User type should not use union with null

Solution

1. Step 1: Analyze the User type

   User can be an object or null, so user might be null at runtime.

2. Step 2: Check the function usage

   Accessing user.name without checking if user is null causes a runtime error.

3. Final Answer:

   user can be null, so accessing user.name causes an error -> Option A

4. Quick Check:

   Null union requires null check before property access [OK]

Hint: Always check for null before accessing properties [OK]

Common Mistakes:

• Ignoring null possibility in union types
• Assuming string methods work on null
• Thinking missing return type causes error here

5. You have a function that accepts a parameter which can be either a string or an array of strings. You want to return a single string that joins all strings with commas. Which advanced type and code snippet correctly handles this?

hard

A. function joinStrings(input: string[]) { return input.toString(); }

B. function joinStrings(input: string | string[]) { return Array.isArray(input) ? input.join(",") : input; }

C. function joinStrings(input: string | string[]) { return input.join(","); }

D. function joinStrings(input: string & string[]) { return input.join(","); }

Solution

1. Step 1: Understand the parameter type

   The parameter can be a string or an array of strings, so union type string | string[] is correct.

2. Step 2: Handle both cases safely

   Use Array.isArray to check if input is array, then join with commas; else return the string as is.

3. Final Answer:

   function joinStrings(input: string | string[]) { return Array.isArray(input) ? input.join(",") : input; } -> Option B

4. Quick Check:

   Use union type + type guard for safe handling [OK]

Hint: Use union type and Array.isArray to handle both cases [OK]

Common Mistakes:

• Using intersection type & instead of union |
• Calling join on string without checking type
• Ignoring the string input case