React语法理解：新手对render方法代码存疑

Hey there! Let’s break down this React render method code piece by piece—since you already have basic JavaScript knowledge, this should click into place quickly.

First, let’s restate the code clearly for reference:

render() {
  let person = null;
  if (this.state.showPerson) {
    person = (
      <div>
        { this.state.person.map((el, index) => {
            return <Person 
              key={el.id} 
              click={this.deletePersonHandler.bind(index)} 
              name={el.name} 
              age={el.age} 
              changed={(event) =>... } 
            />
          }) 
        }
      </div>
    )
  }
  // 后续通常会有return语句，比如 return (<div>{person}</div>)
}

Let’s go step by step:

1. 初始化空变量：let person = null; #

We start by declaring a person variable and setting it to null initially. This is just a placeholder—we’ll only give it actual renderable JSX content if a specific condition is met later.

2. 条件渲染开关：if (this.state.showPerson) #

This is React’s way of doing conditional rendering. The component checks the boolean value of this.state.showPerson:

• If showPerson is true, we run the code inside the if block and assign a chunk of JSX to the person variable.
• If it’s false, person stays null, so nothing from this section gets rendered when the component outputs its UI.

3. 遍历数组生成组件列表：this.state.person.map((el, index) => {...}) #

Here’s where JavaScript’s map() method does the heavy lifting:

• this.state.person is an array of objects (each object has id, name, age properties, based on the props we’re passing).
• map() loops through every element (el) in the array, along with its position in the array (index).
• For each element, we return a <Person /> component—this converts every data object in the array into a reusable React component that can be rendered to the page.

4. 详解组件的每个属性 #

Let’s break down the props we’re passing to each <Person> component:

• key={el.id}: React requires every element in a list to have a unique key prop. This helps React efficiently update the list when items are added, removed, or reordered. Using el.id is best practice (better than index, since index can shift if the array changes).
• click={this.deletePersonHandler.bind(index)}: We’re passing a click event handler to the <Person> component. bind(index) attaches the current element’s index to the deletePersonHandler function—so when the component triggers this click event, we know exactly which person in the array to delete.
• name={el.name} & age={el.age}: These are just passing raw data from the array element (el) to the <Person> component as props. The <Person> component can then use these props to display the person’s name and age.
• changed={(event) =>... }: This is an arrow function that handles input changes (like editing the person’s name or age). Arrow functions are used here because they preserve the correct this context (pointing to the parent component), and they let us access the event object to get the input value from event.target.value.

5. 包裹组件列表 #

We wrap all the <Person> components generated by map() inside a <div>. React requires that multiple sibling elements be wrapped in a single parent element—this div serves that purpose to keep the JSX valid.

整体流程 Recap #

When the component renders:

1. It checks if showPerson is true.
2. If yes, it loops through the person array in state, creates a <Person> component for each entry, wraps them in a div, and assigns this to the person variable.
3. Finally, the component’s return statement (not shown in your snippet) will include {person}—so if showPerson is true, all the <Person> components are rendered; if false, nothing shows up for this section.

内容的提问来源于stack exchange，提问作者user9504869