Using Matchers in Jest
test.js
Jest provides a variety of built-in matchers to help you write clear and meaningful assertions in your tests. Choosing the right matcher is important for expressing exactly what you want to check.
Use the toBe matcher when you need to check that two values are exactly the same, both in value and type. This is known as strict equality, similar to using === in JavaScript. For instance, expect(5).toBe(5) passes, but expect({ a: 1 }).toBe({ a: 1 }) fails because the two objects are not the same reference.
The toEqual matcher is used for deep equality checks. It works well when you want to compare the contents of objects or arrays, not just their references. For example, expect({ name: 'Alice' }).toEqual({ name: 'Alice' }) passes because the objects have the same properties and values.
When you want to verify that a value is truthy—meaning it would evaluate to true in a conditional—use toBeTruthy. This matcher is helpful for checking that a value exists or is not false, 0, '', null, or undefined.
The toBeNull matcher checks specifically for the null value. Use this when you need to ensure something is set to null and not just any falsy value.
Finally, the toContain matcher is useful for checking if an array contains a certain element or if a string contains a particular substring. This matcher helps you confirm that specific data is present in collections or text.
Understanding these matchers allows you to write more precise and expressive tests, making your test suite easier to read and maintain.
1. Which matcher would you use to check if a value is exactly equal to another value in Jest?
2. What does the toContain matcher check for?
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test.js
Jest provides a variety of built-in matchers to help you write clear and meaningful assertions in your tests. Choosing the right matcher is important for expressing exactly what you want to check.
Use the toBe matcher when you need to check that two values are exactly the same, both in value and type. This is known as strict equality, similar to using === in JavaScript. For instance, expect(5).toBe(5) passes, but expect({ a: 1 }).toBe({ a: 1 }) fails because the two objects are not the same reference.
The toEqual matcher is used for deep equality checks. It works well when you want to compare the contents of objects or arrays, not just their references. For example, expect({ name: 'Alice' }).toEqual({ name: 'Alice' }) passes because the objects have the same properties and values.
When you want to verify that a value is truthy—meaning it would evaluate to true in a conditional—use toBeTruthy. This matcher is helpful for checking that a value exists or is not false, 0, '', null, or undefined.
The toBeNull matcher checks specifically for the null value. Use this when you need to ensure something is set to null and not just any falsy value.
Finally, the toContain matcher is useful for checking if an array contains a certain element or if a string contains a particular substring. This matcher helps you confirm that specific data is present in collections or text.
Understanding these matchers allows you to write more precise and expressive tests, making your test suite easier to read and maintain.
1. Which matcher would you use to check if a value is exactly equal to another value in Jest?
2. What does the toContain matcher check for?
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