React basic interview questions (share)

[Related topic recommendations: react interview questions (2020)]

1. The role of keys in React is What?

key is used in React to track which elements in the list have been modified, deleted or added's auxiliary identifier . In diff algorithm, key is used to determine whether the element node has been moved or a newly created element, reducing unnecessary elementsduplication Render .

2. What are the ways to build components in React?

What's the difference?

• The function component seems to be just a function whose return value is a DOM structure. In fact, behind it is the idea of ​​stateless components.
• In function components, you cannot use State, nor can you use the component's life cycle method. This determines that function components are all display components, receiving Props, rendering DOM, and not paying attention to other logic
• There is no this in the function component
• The function component is easier to understand. When you see a function component, you know that its function is just to receive properties and render the page. It does not perform logical processing that has nothing to do with the UI. It is just a pure function. Regardless of how complex the DOM structure it returns is

3. What happens after calling setState?

4. React diff principle

• Decompose the tree structure according to levels, and only compare elements at the same level
• to the list Each unit of the structure adds a unique key attribute to facilitate comparison
• React will only match components of the same class (the class here refers to the name of the component)
• Selective subtree rendering. Developers can override shouldComponentUpdate to improve diff performance

5. Why is it recommended that the parameter passed to setState is a callback instead of an object

becausethis. The updates of props and this.state may be asynchronous, and their values ​​cannot be relied upon to calculate the next state

6. About this Binding?

7.The role of the second parameter of setState

This function will be called when the setState function call is completed and the component starts to re-render. , we can use this function to monitor whether rendering is completed

8. (In the constructor) What is the purpose of calling super(props)

In super() Subclasses cannot use this before being called. In ES5, subclasses must call super() in the constructor. The reason for passing props to super() is to facilitate (in subclasses) the idea of ​​​​accessing this.props

9.flux in

constructor

And process?

The biggest feature of Flux is the "one-way flow" of data.
Process:
User accessview
viewSend the user’s action
dispatcher Receive action and require store to make corresponding updates
storeAfter the update, send a change event
viewAfter receiving the change event, update the page.

10. What is the difference between Element and Component in React?

• The createElement function is a function used to create React Element after JSX compilation.
• cloneElement is used to copy an element and pass in new Props

11.What is the difference between createElement and cloneElement

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