How TypeScript infers types automatically - Quick Revision & Summary

Recall & Review

beginner

What is type inference in TypeScript?

Type inference is TypeScript's ability to automatically figure out the type of a variable or expression without you explicitly telling it.

beginner

How does TypeScript infer the type of a variable declared with <code>let x = 5;</code>?

TypeScript sees the value 5 is a number, so it infers the type of x as number.

intermediate

Can TypeScript infer types for function return values? Give an example.

Yes. For example, <code>function add(a: number, b: number) { return a + b; }</code> TypeScript infers the return type as <code>number</code> because <code>a + b</code> is a number.

beginner

What happens if you declare a variable without initializing it, like <code>let y;</code>?

TypeScript infers the type as any because it has no information about what type y should be.

intermediate

How does TypeScript infer types in arrays? Example with <code>const nums = [1, 2, 3];</code>

TypeScript looks at the elements and infers the array type as number[] because all elements are numbers.

What type does TypeScript infer for let name = "Alice";?

A boolean

B number

C any

D string

TypeScript sees the value is a string, so it infers the type as string.

If you write let data; without assigning a value, what type does TypeScript infer?

A any

B string

C undefined

D never

Without an initial value, TypeScript infers the type as any.

What is the inferred return type of this function?

function greet() { return "Hi"; }

A string

B void

C any

D number

The function returns a string, so TypeScript infers the return type as string.

Given const flags = [true, false, true];, what type does TypeScript infer for flags?

A number[]

B boolean[]

C string[]

D any[]

All elements are booleans, so TypeScript infers the array type as boolean[].

Which of these is NOT a benefit of type inference in TypeScript?

A Less need to write explicit types

B Catch errors early

C Code runs faster at runtime

D Better code readability

Type inference helps with development but does not affect runtime speed.

Explain how TypeScript infers the type of a variable when you assign a value to it.

Think about how TypeScript guesses the type from the value you give.

Describe what happens when you declare a variable without initializing it in TypeScript.

What does TypeScript do when it has no clues about the variable's type?

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