Mocking Dependencies

Mocking replaces a dependency at a clearly defined boundary so a unit test can control its responses. Interfaces describe the dependency shape; test doubles supply behavior. Too much mocking can make tests pass even when the real components are incompatible, so keep integration coverage too.

The production function receives a Clock dependency rather than reading global time directly. The test injects deterministic time 100, making both expiration branches reproducible.

A mock verifies behavior under its simulated assumptions, not actual network availability or database guarantees.

Save as mocking-dependencies.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

type Clock = { now(): number };
function isExpired(deadline: number, clock: Clock): boolean {
  return clock.now() > deadline;
}
const fakeClock: Clock = { now: () => 100 };
console.log(isExpired(90, fakeClock));
console.log(isExpired(110, fakeClock));

Verified output

true
false

Compile: npx tsc --strict --target es2022 --module commonjs mocking-dependencies.ts; run: node mocking-dependencies.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 production function receives a Clock dependency rather than reading global time directly. The test injects deterministic time 100, making both expiration branches reproducible.

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

A mock verifies behavior under its simulated assumptions, not actual network availability or database guarantees.

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

Create a fake email sender that records messages; test whether the correct recipient receives a call.

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.

Mocking replaces a dependency at a clearly defined boundary so a unit test can control its responses.

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