Array.prototype.map() 期望来自箭头的返回值 function array-callback-return

Question

Don't really know why it is saying this but this is the code I believe it is talking about

setUpdate(text, key) {
  const items = this.state.items;
  items.map((item) => {
    if (item.key === key) {
      item.text = text;
    }
  });
  this.setState({
    items: items,
  });
}

Answer 1

Issue

The Array.prototype.map function maps a one-to-one relationship, the resultant array will be the same length as the original. The error is saying that you aren't explicitly returning a value for each element iterated.

You also don't save the resultant mapped array. The result of your code is iteration over the items array and mutation with item.text = text; . In React state and props are to be considered immutable, mutating them directly is very anti-pattern.

setUpdate(text, key){
  const items = this.state.items; // <-- saved state reference
  items.map(item=>{ // <-- missing result array save
    if(item.key===key){
      item.text= text; // <-- state mutation!
    }
    // <-- missing return value (the error)
  })
  this.setState({
    items: items // <-- same reference to previous state!
  })
}

Solution

You will want to shallow copy the array as well as any nested object that are being updated. Use a functional state update to correctly create the next state array from the state array from the previous state. The reason for this is that the next version of the items necessarily depends on the previous array.

setUpdate(text, key) {
  this.setState((prevState) => ({
    items: prevState.items.map((item) =>
      item.key === key ? { ...item, key: text } : item
    )
  }));
}

Answer 2

You have to use like below

setUpdate(text, key) {
  // loop the existing state and find the key value
  const nextItem = this.state.items.map((item) => {
    if (item.key === key) {
      item.text = text;
    }
    // return the modified item
    return item;
  });
  // set the new object into state
  this.setState({items: nextItem });
}

Answer 3

Wrote this answer without realising that React requires you to provide a new array.

You shouldn't be using map just to find and update an item in an array! What's wrong with find ?

find will stop on the first element that matches the condition.

setUpdate(text, key){
  const items = this.state.items;

  const item = items.find(i => i.key === key);

  if (item) item.text = text;

  this.setState({
    items: items
  });
}

Answer 4

The error states that add a return keywork inside of map arrow function not just any HTML or compnent Eg `

  {props.meetups.map((meetup) => {
    return (
      <MeetupItem
        key={meetup.id}
        id={meetup.id}
        image={meetup.image}
        title={meetup.title}
        address={meetup.address}
        description={meetup.description}
      />
    );
  })}
</ul>

`