How TypeScript infers types automatically - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the inferred type of variable 'x'?

Consider this TypeScript code snippet. What is the inferred type of variable x?

Typescript

let x = 42;

A number

B any

C string

D unknown

💡 Hint

TypeScript looks at the value assigned to infer the type.

Since 42 is a number, TypeScript infers the type of x as number.

❓ Predict Output

intermediate

What is the inferred type of the function parameter?

Look at this function. What type does TypeScript infer for the parameter name?

Typescript

function greet(name = "Guest") {
  return `Hello, ${name}!`;
}

A any

B unknown

C string

D number

💡 Hint

Default value helps TypeScript guess the parameter type.

The default value "Guest" is a string, so TypeScript infers the parameter type as string.

❓ Predict Output

advanced

What is the inferred type of the array elements?

What type does TypeScript infer for the elements of this array?

Typescript

const fruits = ["apple", "banana", "cherry"];

A string[]

B any[]

C Array<number>

D unknown[]

💡 Hint

All elements are strings, so TypeScript infers an array of strings.

Since all elements are strings, TypeScript infers the type as string array (string[]).

❓ Predict Output

advanced

What is the inferred return type of this arrow function?

What type does TypeScript infer as the return type of this arrow function?

Typescript

const multiply = (a: number, b: number) => a * b;

A void

B number

C any

D string

💡 Hint

The function returns the product of two numbers.

The function returns the product of two numbers, so the return type is inferred as number.

❓ Predict Output

expert

What is the inferred type of the variable 'result'?

Given this code, what is the inferred type of result?

Typescript

const data = [1, "two", 3];
const result = data.map(item => typeof item === "number" ? item * 2 : item.toUpperCase());

A any[]

B number[]

C string[]

D (string | number)[]

💡 Hint

The map returns either a number or a string depending on the item type.

The map function returns numbers doubled or strings uppercased, so the result is an array of strings or numbers.

Practice

1. What type does TypeScript automatically assign to the variable declared as let count = 10;?

easy

A. number

B. string

C. any

D. boolean

Solution

1. Step 1: Look at the assigned value

   The variable count is assigned the value 10, which is a number.

2. Step 2: Understand TypeScript inference

   TypeScript infers the type from the assigned value, so count is inferred as number.

3. Final Answer:

   number -> Option A

4. Quick Check:

   Value 10 means type number [OK]

Hint: Check the assigned value type to guess variable type [OK]

Common Mistakes:

• Thinking unassigned variables get a type
• Confusing number with string
• Assuming type any by default

2. Which of the following is the correct way to declare a variable with automatic type inference in TypeScript?

easy

A. let name: string = 'Alice';

B. let score: any = 100;

C. let isActive: boolean;

D. let age = 25;

Solution

1. Step 1: Identify automatic type inference

   TypeScript infers types when you assign a value without explicitly declaring the type.

2. Step 2: Check each option

   let age = 25; declares age with an assigned value 25 and no explicit type, so TypeScript infers number.

3. Final Answer:

   let age = 25; -> Option D

4. Quick Check:

   Assigned value without explicit type means inference [OK]

Hint: Look for variables declared with value but no type [OK]

Common Mistakes:

• Confusing explicit type with inferred type
• Choosing variables declared without assignment
• Assuming any type is inferred by default

3. What will be the inferred type and output of this code?

let message = 'Hello';
message = 123;

medium

A. TypeScript error: cannot assign number to string

B. message becomes 123 and type is number

C. message becomes '123' as string

D. No error, message is any type

Solution

1. Step 1: Identify inferred type from initial assignment

   Variable message is assigned a string 'Hello', so TypeScript infers type string.

2. Step 2: Check reassignment compatibility

   Assigning a number 123 to a variable inferred as string causes a type error.

3. Final Answer:

   TypeScript error: cannot assign number to string -> Option A

4. Quick Check:

   Inferred string type rejects number assignment [OK]

Hint: Inferred type fixes variable type, no mixed types allowed [OK]

Common Mistakes:

• Assuming variable type changes on reassignment
• Thinking TypeScript converts types automatically
• Ignoring type errors on incompatible assignments

4. Find the error in this TypeScript code:

let data = [1, 2, 3];
data.push('four');

medium

A. No error, array can hold any type

B. Error: cannot push string into number array

C. Error: push method does not exist

D. Error: data is not initialized

Solution

1. Step 1: Identify inferred array type

   Variable data is assigned an array of numbers, so TypeScript infers type number[].

2. Step 2: Check push argument type

   Trying to push a string 'four' into a number[] array causes a type error.

3. Final Answer:

   Error: cannot push string into number array -> Option B

4. Quick Check:

   Array type enforces element types [OK]

Hint: Array inferred type fixes allowed element types [OK]

Common Mistakes:

• Assuming arrays accept mixed types by default
• Thinking push method is unavailable
• Ignoring type errors on array element types

5. Given this code, what is the inferred type of result and why?

const numbers = [1, 2, 3];
const result = numbers.map(n => n * 2);

hard

A. void, because map does not return anything

B. any[], because map returns any type by default

C. number[], because map returns an array of numbers

D. number, because map returns a single number

Solution

1. Step 1: Identify input array type

   numbers is an array of numbers, so type is number[].

2. Step 2: Understand map return type

   Map applies a function to each element and returns a new array of the same length with transformed elements. Since the function returns n * 2 (a number), the result is number[].

3. Final Answer:

   number[], because map returns an array of numbers -> Option C

4. Quick Check:

   map returns array of transformed elements [OK]

Hint: map returns array with same length and inferred element type [OK]

Common Mistakes:

• Thinking map returns a single value
• Assuming map returns any[] by default
• Confusing void return with map's return