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Inference with const

Learn literal type inference with const.

Inference with const

With const, TypeScript infers a literal type, meaning the exact value becomes the type. In const x = 5, the type of x is not number but the literal type 5. This is because const variables cannot be reassigned.

const x = 5;          // type: 5
const y = "merhaba";   // type: "merhaba"

const uses a literal type because the value never changes.

let vs const inference difference

With let, TypeScript widens the type: let x = 5 gives type number. With const, the literal type is preserved: const x = 5 gives type 5. This difference arises because let variables can receive other values of the same type, while const never changes.

let a = 5;      // type: number
const b = 5;    // type: 5

let gives a wide type; const gives a narrow literal type.

Practice Multiplication with const and let

Define a const named carpan with value 3. Define a let named deger with value 7. Print the result of carpan * deger with console.log.

Starter code

index.ts
// carpan adında bir const tanımla, değeri 3
// deger adında bir let tanımla, değeri 7
// carpan * deger sonucunu yazdır

Sample solution

index.ts
const carpan = 3;
let deger = 7;
console.log(carpan * deger);

Expected output

21

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 In <code>const renk = "kırmızı";</code>, what is the inferred type of <code>renk</code>?

  • string (general type)
  • const (keyword)
  • "kırmızı" (literal type) Correct answer
  • renk (variable name)

Explanation: Since <code>const</code> does not change, TypeScript infers the exact value as a literal type; the type of <code>renk</code> is <code>"kırmızı"</code>, not the general <code>string</code>.

2 Find the const literal type

Question 
const kod = 200;
// kod'un türü: ____
Answer 
const kod = 200;
// kod'un türü: 200

Explanation: When declared with <code>const</code>, TypeScript makes the exact value the literal type; since <code>kod</code>'s value is <code>200</code>, its type is <code>200</code>, not the general <code>number</code>.

3 const inference concepts

Term Meaning
const x = "a" Literal type inference
let x = "a" General type inference
const x = 5 Type: 5 (literal)
const inference Produces exact value type

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app