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?<br>

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?