let, const, and var

Modern TypeScript favors const for bindings that will not be reassigned and let for bindings that will.

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

let and const are block-scoped; var is function-scoped and has older hoisting behavior that can be surprising. Prefer const by default, changing to let when reassignment is intended. A const object can still have mutable properties unless the type or runtime behavior prevents that.

Use the diagram to locate the checks, values or branches involved in the example.

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

const user = { name: "Maya", points: 2 };
 user.points = 3;
 let attempts = 1;
 attempts += 1;
 console.log(`${user.name}: ${user.points}, tries: ${attempts}`);

Verified output

Maya: 3, tries: 2

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.

The const binding still refers to the same object while its points field changes. let allows attempts to be reassigned from 1 to 2.

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.

Do not infer that const makes every nested property immutable. Nor should you use var simply because it works in older JavaScript.

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.

Try reassigning user itself and observe the diagnostic; then use a block with let inside and see why its name is unavailable outside.

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

let, const, and var: let and const are block-scoped; var is function-scoped and has older hoisting behavior that can be surprising.

Binding immutability and object mutability are different concepts.

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