Testing in Node.js with Jest

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Asserts and Matchers in Jest

In this section, we will delve into the assertions and matchers that Jest provides for evaluating conditions in our tests. Assertions and matchers are fundamental for comparing the expected result with the actual result of a function or process.

Assertions in Jest

In Jest, assertions are made using the expect() function, which takes care of comparing values. The expect() function takes a value as an argument and returns an object with several matchers that you can use to make specific comparisons.

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Main Matchers in Jest

Below are some of the most common matchers:

Equality

toBe: Compares using the === operator.
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toEqual: Checks value equality, useful for objects and arrays.
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Truthiness

toBeNull: Verifies if the value is null.
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toBeUndefined: Verifies if the value is undefined.
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toBeDefined: Verifies if the value is not undefined.
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toBeTruthy: Verifies if the value is true in a boolean context.
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toBeFalsy: Verifies if the value is false in a boolean context.
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Numbers

toBeGreaterThan: Verifies if a number is greater than another.
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toBeGreaterThanOrEqual: Verifies if a number is greater than or equal to another.
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toBeLessThan: Verifies if a number is less than another.
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toBeLessThanOrEqual: Verifies if a number is less than or equal to another.
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toBeCloseTo: Verifies if two numbers are close to each other, useful for decimal comparison.
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Strings

toMatch: Verifies if a string matches a regular expression.
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Arrays and Sets

toContain: Verifies if an element is in an array or Set.
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toContainEqual: Verifies if an equivalent object is in an array.
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Exceptions

toThrow: Verifies if a function throws an error when executed.
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Placeholder for an image: Comparative table showing different matchers with short examples and expected results.

Complete Example with Matchers

Below is an example where several of the mentioned matchers are used:

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Conclusion

Mastering matchers and assertions in Jest will allow you to write more effective and precise tests. In the following chapters, we will explore advanced testing techniques, including TDD, mocks, and testing asynchronous functions.

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