Introduction to Testing TypeScript

Testing runs code against expected behavior. Unit tests check small boundaries, integration tests check collaborating parts, and end-to-end tests check complete journeys. TypeScript compilation catches certain static mistakes; passing type checks alone cannot establish correct runtime business behavior.

A small assertion compares the function result to the expected value and throws if different. Production projects use a test runner for reporting, fixtures, assertions and automation.

No TypeScript diagnostic guarantees that addNumbers implements the intended arithmetic for every input.

Save as testing.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 addNumbers(a: number, b: number): number { return a + b; }
function assertEqual(actual: number, expected: number): void {
  if (actual !== expected) throw new Error(`Expected ${expected}, got ${actual}`);
}
assertEqual(addNumbers(2, 3), 5);
console.log("1 test passed");

Verified output

1 test passed

Compile: npx tsc --strict --target es2022 --module commonjs testing.ts; run: node testing.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.

A small assertion compares the function result to the expected value and throws if different. Production projects use a test runner for reporting, fixtures, assertions and automation.

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

Practical project lab: These are real configuration and source-file examples, not a TypeScript object that merely describes settings. Place the files at the named paths relative to one project directory.

calc.ts

export const add = (a: number,b: number): number => a+b;

calc.test.ts

import { test } from "node:test";
import { strict as assert } from "node:assert";
import { add } from "./calc";
test("adds", () => assert.equal(add(2,3),5));

Commands

npm i -D typescript @types/node
npx tsc --strict --module commonjs --target es2022 calc.ts calc.test.ts
node --test calc.test.js

Actual test files are discovered and executed by a runner, unlike demonstrations that merely call assertions.

This local compiler or test-runner mini-project was validated in addition to the primary program.

No TypeScript diagnostic guarantees that addNumbers implements the intended arithmetic for every input.

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

Add tests for negative numbers, zero, and large operands; name each case clearly.

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.

For the exact current options and platform behavior, consult the official documentation. These examples were verified using the tool versions described above.

Testing runs code against expected behavior.

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