reactjs; after mapping an array nothing renders

问题

I have an input that after the user inserts a text I want to show each word in a button in button-group. I split the sentence and map it that returns

<input .....><button onClick=....>

so I insert the text in input, after clicking the button the text.split(" ") everything works correctly till here. but in the button group part shows nothing

<div className="btn-group" role="group" aria-label="...">
   {this.mapArrayToButtons()}
</div>

   mapArrayToButtons(){
        this.state.textArr.map((word)=>{
            console.log('the word: ',word);
            return(<button key={word} type="button" className="btn btn-default" value={word}>{word}</button>);});
    }

I see console log textArr.length times and word by word. but no button is rendered and nothing shows in the browser.

回答1:

You will need to return a value from your function like so:

   mapArrayToButtons(){
        return this.state.textArr.map((word)=>{
            return(<button key={word} type="button" className="btn btn-default" value={word}>{word}</button>);});
    }

Hope that helps!

回答2:

There are 2 things you need to know / be aware of :

1 - If you have an array of jsx , for instance :

let arrayOfJsx = [ <div>Hello</div>, <div>World</div> ]

When you put this array in your render method , like :

render(){

   let arrayOfJsx = [ <div>Hello</div>, <div>World</div> ] 

   return (
      <div> { arrayOfJsx } </div>
   )
}

is equivalent to

render(){

       return (
          <div>
               <div>Hello</div>
               <div>World</div> 
          </div>
       )
}

2 - Diffence between Array.prototype.forEach and Array.prototype.map :

When you use forEach, what you return from the function you are passing to forEach is not important, it is only important to know that the function stops executing when you return a value, this is not special to forEach. But when we say what you return is not important we want to point out how what you return is important for Array.prototype.map :

You want to use map when you want to create an array from what you return for each item in an array.

Example :

let a = [ 1,2,3 ]
let b = a.map( item => item + 1 )
// b is now [ 2,3,4 ]

To sum up, what you want to do in your example create an array from map and put it in your render method. If you don ' t know about these 2 points, you won't be understanding how it works.

NOTE : you need to pass a unique key for each item you return in your map function. For instance :

let a = [ 1,2,3 ]
let arrayOfJsx = a.map( item => <div key={item} > I am { item } </div> )

So that react doesn t complain

回答3:

try this

mapArrayToButtons(){
    const buttons = this.state.textArr.map((word)=>{
        console.log('the word: ',word);
        return(<button key={word} type="button" className="btn btn-default" value={word}>{word}</button>);});
    return buttons;

}

来源：https://stackoverflow.com/questions/40981918/reactjs-after-mapping-an-array-nothing-renders

标签

javascript

reactjs

react-dom