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TypeScript provides two main ways to declare variables: let for values that can change, and const for values that remain constant.

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Using let for Mutable Variables

The let keyword declares variables whose values can be reassigned: 
let firstname: string = 'angel';
let lastname = 'diosdado';

let diceNumber = 5;
diceNumber = 3; // ✓ Valid - can reassign
TypeScript can infer types automatically. In the example above, lastname is inferred as string without an explicit type annotation.

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Using const for Immutable Variables

The const keyword declares variables that cannot be reassigned: 
const containsLetter = lastname.includes('a');
// containsLetter = false; // ✗ Error - cannot reassign const
Use const by default for variables that won’t be reassigned. This makes your code more predictable and easier to reason about.

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Type Inference

TypeScript automatically infers types based on the assigned values: 
let firstname: string = 'angel'; // Explicit type
let lastname = 'diosdado';        // Inferred as string
let diceNumber = 5;               // Inferred as number

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Best Practices

  • Prefer const: Use const for values that won’t change
  • Use let sparingly: Only use let when you need to reassign the variable
  • Avoid var: The old var keyword has confusing scoping rules - stick with let and const
Never use var in modern TypeScript code. It has function-scoping instead of block-scoping, which can lead to bugs.

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