How do I spy on methods?

const spy = spyOn(obj, 'methodName');

Replaces obj.methodName and returns a handle:

• .calls — array of argument arrays, one per call
• .callCount — calls.length
• .lastCall — args of the most recent call, or undefined
• .andCallFake(fn) — delegate subsequent calls to fn
• .andReturn(value) — short-circuit subsequent calls to return value
• .restore() — puts the original method back

With neither .andCallFake nor .andReturn set, calls pass through to the original implementation while still being recorded — spies default to transparent, not silent.

See How do I fake timers?.