Refactoring TypeScript Code

Refactoring changes internal code structure while preserving externally observable behavior. Strong types make many unsafe changes visible, but tests are still needed to detect semantic regressions. Use small steps, keep behavior-focused tests green, and favor named functions when they clarify intent.

The extraction of a subtotal calculation makes the overall formula easier to read without changing the final result. A test can assert 33.00 before and after refactoring.

A refactor should not quietly change business rules or data contracts, even if the compiler still accepts the code.

Save as refactoring-code.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

function totalWithTax(prices: readonly number[], rate: number): number {
  const subtotal = prices.reduce((sum, price) => sum + price, 0);
  return subtotal * (1 + rate);
}
console.log(totalWithTax([10, 20], 0.1).toFixed(2));

Verified output

33.00

Compile: npx tsc --strict --target es2022 --module commonjs refactoring-code.ts; run: node refactoring-code.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

The extraction of a subtotal calculation makes the overall formula easier to read without changing the final result. A test can assert 33.00 before and after refactoring.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

A refactor should not quietly change business rules or data contracts, even if the compiler still accepts the code.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Extract the subtotal reduction into a separate function and re-run the same tests.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

Refactoring changes internal code structure while preserving externally observable behavior.

Run the primary example, then complete the practice task or environment-specific lab.