Variables in TypeScript

Variables give names to values, while TypeScript tracks which kinds of values those names may hold.

In this lesson, you will study the rule, run a complete TypeScript example, explain its output, and test a small change.

let declares a reassignable binding; const declares a binding that cannot be reassigned. A type can be inferred from the initializer or explicitly annotated. Variables hold JavaScript values at runtime; the type checker tracks permitted assignments during development.

Save this code as main.ts or paste it into the TypeScript Playground. The example uses fixed values so it runs without prompting for input.

TypeScript code

let score: number = 10;
 score = score + 5;
 const player: string = "Ravi";
 console.log(`${player}: ${score}`);

Verified output

Ravi: 15

How to run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. When using a project configuration, run the matching project build instead.

score starts at 10 and is reassigned to 15. player is initialized once with a string. The template literal reads the final values.

Predict the output before you run the sample. Compare each line with the source expression that produced it, and distinguish the type checker’s work from what happens in JavaScript at runtime.

const does not freeze objects; it only prevents rebinding that variable. A separate lesson covers the differences among let, const and var.

Debugging habit: when a change is rejected, read the exact compiler diagnostic and inspect the operand types. When it compiles but behaves unexpectedly, follow the values through the JavaScript expressions.

Change the starting score and add another reassignment. Try score = "high" and read the checker message.

Challenge: write down your prediction first, run the changed source, and explain any difference. Return to the verified version before continuing.

Variables in TypeScript: let declares a reassignable binding; const declares a binding that cannot be reassigned.

let creates a mutable binding; a const binding cannot be reassigned.

Before continuing, be able to explain what is checked before execution and what actually changes at runtime in the example above.