How to describe expectations in a test?
Expectations in a test are described using the construct:
javascript
expect(actualResult).matcher(expectedValue);It compares what the function returned with what was supposed to happen.
The simplest example
js
expect(sum(2, 3)).toBe(5);Here:
expect(sum(2, 3))- the actual result.toBe(5)- the expectation (matcher) that the result should equal5
The most common matchers
| Matcher | What it checks |
|---|---|
toBe(value) | strict equality (like ===) |
toEqual(obj) | compares objects and arrays by content |
toBeTruthy() | the value is truthy |
toBeFalsy() | the value is falsy |
toBeNull() | strictly null |
toBeUndefined() | strictly undefined |
toContain(item) | the item is present in an array or string |
toBeGreaterThan(num) | greater than… |
toBeLessThan(num) | less than… |
Example with different expectations
js
expect([1, 2, 3]).toContain(2);
expect({ name: "Tom" }).toEqual({ name: "Tom" });
expect(10).toBeGreaterThan(5);
expect(false).toBeFalsy();The meaning of expectations
Every expectation is a statement that must be true. If even one of them does not match reality, the test fails and shows exactly what went wrong.
In other words, expectations describe how the code is supposed to behave, while Jest automatically checks whether the actual behavior matches the expected one.
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