A return value is data a function gives back to its caller when it finishes. In testing, it is a publicly visible exit point: a result the caller can inspect without reaching into private implementation details. A useful return value is one that is not `undefined`. section 1.4 section 1.3
A unit of work can have several exit points, including a return value, a state change, or a call to a third party. A unit test invokes the work through an entry point and checks one specific exit point. When a return value is available, a value-based test can usually assert it directly. section 1.4 section 1.3
Returning the result of a calculation
A `sum` function splits two comma-separated numbers, converts them, adds them, and returns the result. A test can call `sum("1,2")` and check the returned value, rather than inspecting how the calculation was performed internally. section 1.3
Prefer checking a return value over verifying calls to protected or internal functions. Testing an internal call couples a test to implementation details, so harmless refactoring can force test changes. If `verify()` returns a boolean, assert that boolean; do not mock and verify its internal `findFailedRules()` call. section 8.3.1
Listing 8.11 Production code that calls a protected function
export class PasswordVerifier4 {
private _rules: ((input: string) => boolean)[];
private _logger: IComplicatedLogger;
constructor(rules: ((input) => boolean)[],
logger: IComplicatedLogger) {
this._rules = rules;
this._logger = logger;
}
verify(input: string): boolean {
const failed = this.findFailedRules(input); #1
if (failed.length === 0) {
this._logger.info("PASSED");
return true;
}
this._logger.info("FAIL");
return false;
}
protected findFailedRules(input: string) { #2
const failed = this._rules
.map((rule) => rule(input))
.filter((result) => result === false);
return failed;
}
}
#1 Call to the internal function
#2 Internal function
The `verify` method illustrates a clear return-value exit point: it returns `true` when no rules fail and `false` otherwise, while its helper remains internal. section 8.3.1
