Function Expressions

A function expression creates a function value that can be assigned to a variable or passed to another function. The variable holding it follows normal initialization rules.

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

Here the variable format has a function type: it accepts a number and returns a string. TypeScript checks that the assigned expression is compatible. The JavaScript value itself is a callable function object.

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

const format: (value: number) => string = function (value) {
  return `₹${value.toFixed(2)}`;
};
console.log(format(12.5));

Verified output

₹12.50

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 function expression is evaluated and stored in format. Calling format(12.5) invokes it, runs toFixed(2) on the number, then returns the formatted string.

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 call a const-defined function before its initialization. The left-hand function type is a description, not another runtime function.

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.

Replace the currency symbol with $ and call format(3). Which part determines the input type?

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 Expressions: A function expression creates a function value that can be assigned to a variable or passed to another function.

The function expression is evaluated and stored in format. Calling format(12.5) invokes it, runs toFixed(2) on the number, then returns the formatted string.

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