The Context API in React is used to manage state that needs to be shared between components in a hierarchical manner. It allows you to avoid prop drilling, where you need to pass down props through several layers of components to reach a child component that needs the information.

You should use Context API in React when:

1. You have a need to share state between multiple components that are not directly related to each other in the component tree.
2. You have a need to share state that is read-only (can only be changed by the component that created it) or if you want to prevent certain components from changing the state.
3. You have a need to provide a global configuration or theme to your application.
4. You want to avoid prop drilling and make your code cleaner and easier to maintain.

However, it's important to note that using Context API for everything can lead to complex and difficult-to-maintain code. It's best to use it when it makes sense for your specific use case.

Custom React JS hooks are reusable functions that a React JS software developer can use to add special and unique functionality to the React applications. Usually, if there is a requirement to add a feature, one can install a third-party library and solve the problem. But what if there is no such library with hooks that can be used? This problem is solved by using custom React JS hooks.

A custom hook is a special JavaScript function whose name starts with ‘use’ and can be used to call other hooks. Let’s take a look at some major differences between a custom React JS hook and React JS components:

1. A custom hook does not require a specific signature.
2. A software developer can choose what argument the custom hook has and what should the argument return.
3. A custom hook always starts with the name ‘use’.

In React, useRef is a hook that allows you to create a mutable reference that persists across re-renders of a component. useRef returns a mutable ref object which has a current property. You can set the current property to any value, and it will persist throughout the lifetime of the component.

One common use case for useRef is to reference DOM elements or other values that may change over time. Since the ref object persists across re-renders, it can be used to store a reference to a DOM element that needs to be accessed by multiple functions or event handlers.

In React, useMemo is a hook that memoizes a function, allowing you to avoid unnecessary re-computations of its result. It takes two arguments: a function that returns a value, and an array of dependencies that trigger the re-computation when changed.

The purpose of useMemo is to optimize performance by avoiding expensive calculations that may be needed by a component but only when certain dependencies have changed. By memoizing the function, React can avoid re-computing the value if the dependencies have not changed.