如何使用 map 在 jsx 中渲染某个项目一次？

Question

How to render and item once In a map function? I have and array of objects and I want to render the color value only once into another component, I tried to push the color in to an array and checked against it but since it is inside the loop it is getting added instantly,and this check will fail, the point is I don't want the same color to be rendered twice, if found then only once… any good solution for this case?

 const items =[{car: 'bmw', color: 'black'}, {car: 'opel', color:'black'}, {car:'landrover',color:'red'}] {items.map((item, i) => { let arr = []; //arr.push(items[i].color); return ( <React.Fragment> {arr.includes(items[i])? undefined: <colors img={items[i]?.color} /> } <items key={item.id} Car={item.car} /> </React.Fragment> ); })} // Expected result // black color component // 'bmw' // 'opel' // red color component // 'landrover'

Answer 1

I'd suggest you first sort your items by color and then only render the color component when the color differs from the previous item:

items
  .sort((a, b) => a.color.localeCompare(b.color))
  .map((item, index, sortedItems) => {
    const colorChanged = !index || item.color !== sortedItems[index - 1].color
    
    return (
      <>
        { colorChanged && <colors img={items[i]?.color} /> }
        <items key={item.id} Car={item.car} />
      </>
    )
  })

Edit: As index is an integer >= 0, !index will evaluate to true exactly when index === 0 does.

Answer 2

const itemColors = items
  .map(({ color }) => color) // create array of colors
  .filter((value, index, self) => self.indexOf(value) === index) // remove duplicates

// do your rendering here

Or using lodash

Answer 3

 const items =[ {car: 'bmw', color: 'black'}, {car: 'opel', color:'black'}, {car:'landrover',color:'red'} ] //Expected Outcome /* [ { color: 'black' cars: ['bmw','opel'] }, { color: 'black' cars: ['bmw','opel'] } ] */ const finalResult = items.reduce((acc, cur, arr) => { let res = [...acc]; const matchIndex = res.findIndex(obj => obj.color === cur.color); if (matchIndex === -1) { res.push({ color: cur.color, cars: [cur.car] }) } else { res[matchIndex].cars.push(cur.car) } acc = res; return acc; }, [] ) console.log(finalResult); finalResult.forEach(colorObject => { colorObject.cars.forEach(car => { console.log( `Your Component with Color ${colorObject.color} and Car ${car}` ); }) })

Answer 4

So You will need to filter the color value first and make an array of no duplicate color. Then you need to render the cars name by filtering by the color. The simple approach without any third party library will be like this:

const colors = items
   .map((data) => data.color)
   .filter((v, i, s) => s.indexOf(v) === i) 

{colors.map((color) => (
    <div>
        {color}
        {items.filter(item => item.color === color).map((item)=> (
            <>{item.car}</>
        )}
    </div>
)}