TypeScript Inference (original) (raw)

Last Updated : 9 Sep, 2025

TypeScript's type inference automatically determines the types of variables, function return values, objects, and arrays based on their assigned values and usage.

This feature reduces the need for explicit type annotations, simplifying code while maintaining type safety.
By analyzing the context and initial values, TypeScript ensures that variables and functions operate with consistent and expected types throughout the codebase. JavaScript ``

let age = 25; let name = "John";

console.log(Age: ${age}); console.log(Name: ${name});

**In this Example

TypeScript infers age as a number and name as a string based on their assigned values.
This automatic detection ensures type safety without requiring explicit annotations.

Inference of Variable Type

Variable type inference means the programming language automatically deduces the type of a variable from the value assigned to it, without the programmer explicitly specifying the type.

JavaScript `

let x = 10; // TypeScript infers x as a number console.log(typeof x);

**In this Example,

TypeScript infers the type of x as number based on the initial value 10.
This ensures x can only hold numerical values, improving type safety.

**Output:

number

**Inference of Array Type

JavaScript `

let fruits = ["Apple", "Banana", "Cherry"]; // TypeScript infers fruits as string[] console.log(fruits);

**In this Example,

TypeScript infers the type of fruits as an array of strings (string[]) based on the initial values.
This prevents adding elements of other types, maintaining array consistency.

**Output:

[ 'Apple', 'Banana', 'Cherry' ]

**Inference of Function Return Type

Array type inference means the compiler or interpreter automatically determines the type of an array based on the elements it contains. Instead of explicitly declaring the array’s type, the language infers it from the assigned values.

JavaScript `

function add(a: number, b: number) { return a + b; // TypeScript infers the return type as number } console.log(add(5, 10));

**In this Example,

The add function's return type is inferred as number because it returns the sum of two numbers.
This ensures the function always returns a numerical value, avoiding type-related errors.

**Output: