10 Questions
What happens if you do not provide the dependencies array to useEffect and only provide a function?
It will run after every render.
What is the consequence of forgetting to provide dependencies correctly when setting a piece of local state?
An infinite loop will occur.
Why is an infinite loop created when useEffect runs after every render without the dependencies array?
The effect function runs, state is updated, causing a re-render, which causes the effect function to run again.
What should you do if you are updating state within your useEffect?
Provide an empty dependencies array.
What is the recommended default approach when using useEffect?
Provide an empty dependencies array.
What is the purpose of the useEffect hook in the example of a simple React app that fetches users from JSONPlaceholder?
To fetch users from JSONPlaceholder.
What is imported from React in the example of a simple React app that fetches users from JSONPlaceholder?
useState and useEffect.
What is the name of the functional component defined in the example of a simple React app that fetches users from JSONPlaceholder?
UsersList.
What is the consequence of not providing the dependencies array to useEffect when making API calls?
The effect function will run after every render, causing infinite loops.
Why is it important to be aware of the subtle details when using useEffect?
To avoid common mistakes and prevent infinite loops.
Study Notes
Introducing React Hooks
- Hooks are a new feature introduced in React 16.8 that allows using state and other React features without writing a class.
- Hooks are functions that "hook into" React state and lifecycle features from function components.
- Hooks do not work inside classes and are backward-compatible, which means they do not contain any breaking changes.
Rules of Hooks
- There are two rules to follow when using Hooks:
- Only call Hooks at the top level
- Only call Hooks from React functions
State in React
- State refers to the data that represents the current condition of a component.
- State is an object that holds some information that may change over the lifetime of the component.
- State allows React components to create dynamic and interactive UIs.
Built-in Hooks
- There are several built-in Hooks in React, including:
- useState
- useEffect
- useContext
- useReducer
- useCallback
- useMemo
- useRef
- useImperativeHandle
- useLayoutEffect
- useDebugValue
useState Hook
- useState is a Hook that allows having state variables in functional components.
- You pass the initial state to this function, and it returns a variable with the current state value and another function to update the state value.
- useState can be used with simple types, objects, and arrays.
- Each call to useState creates a single piece of state, holding a single value of any type.
Implementation of useState Hook
- We can create a React project that uses useState to implement a counter application.
- The useState Hook is used to declare a new state variable, and the state is initialized to zero.
Review of Counter App
- The useState Hook lets us keep local state in a function component.
- We declare a new state variable by calling the useState Hook, which returns a pair of values.
- The first value is the current state, and the second value is a function that lets us update the state.
Using Multiple State Variables
- We can use array destructuring to declare multiple state variables.
- React assumes that if you call useState multiple times, you do it in the same order during every render.
useEffect Hook
- useEffect adds the ability to perform side effects from a function component.
- The Effect Hook serves the same purpose as componentDidMount, componentDidUpdate, and componentWillUnmount in React classes, but unified into a single API.
- The useEffect Hook takes two arguments: a function that contains the side effect code and an optional array of dependencies.
This quiz covers the basics of React Hooks, including the rules of hooks, using state hooks, and using effect hooks. It's a fundamental concept in React 16.8 and beyond.
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