Function Declarations

A function declaration introduces a named function with the function keyword. Its parameters and return type form a reusable checked contract.

What you will learn: the purpose of function declarations, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

Function declarations are initialized before normal statement execution in their enclosing scope, so this example can call greet before the source declaration. This is often described as hoisting. Prefer readable definition order despite that capability.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

console.log(greet("Mira"));
function greet(name: string): string {
  return `Hello, ${name}!`;
}

Verified output

Hello, Mira!

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

The call resolves the declared function. Mira becomes the local name argument. The template literal returns the greeting, which console.log displays.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Do not confuse a hoisted function declaration with a const holding a function expression: that const cannot be used before initialization.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Add a second call with another name. Then rewrite the declaration as a const arrow function below the call and observe the compiler error.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Function Declarations: A function declaration introduces a named function with the function keyword.

The call resolves the declared function. Mira becomes the local name argument. The template literal returns the greeting, which console.log displays.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.