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The expect function is used every time you want to test a value. You will rarely call expect by itself. Instead, you will use expect along with a "matcher" function to assert something about a value.

Use .toBe to compare primitive values or to check referential identity of object instances. It calls Object.is to compare values, which is even better for testing than === strict equality operator.

Use .toEqual to compare recursively all properties of object instances (also known as "deep" equality). It calls Object.is to compare primitive values, which is even better for testing than === strict equality operator.

If you know how to test something, .not lets you test its opposite.