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Showing posts from September, 2019

100% code coverage is false safety

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It is really easy to lie to yourself when doing Test Driven Development that your code is good, safe, predictable and well understood, just because you have that magic 100% code coverage number.
If you test just the happy case it is easy to get to 100% code coverage, while still leaving code paths untested and unexpected behaviour unexplored.

Consider this simple function that takes in an object with 2 properties, an array of strings and a separating string to concatenate between them.

1: export const concatStrings = ({ strings, joiner }) => { 2: return strings.join(joiner); 3: };
Testing the Happy Path is really easy.
1: import { concatStrings } from "./stringConcat"; 2: describe(concatStrings, () => { 3: it("Should concatinate strings", () => { 4: const actual = concatStrings({ joiner: ", ", strings: ["one", "two"] }); 5: expect(actual).toBe("one, two"); 6: }); 7: })…