Question 309560 in Reactjs Question posted in
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Reactjs – Using .map with numbers not an array, or ignoring the first parameter in a map and just using index

I am making a react app and I was trying to do a map to loop over and display multiple images. I was trying to make this dynamic so that if the number of images is different in each folder it will display the appropriate amount of images.

<div className="rotating-gallery-image-container" style={{transform: currentTransform} as React.CSSProperties}>
   {[...Array(numberOfPictures)].map((x, index) =>
       <span style={{ "--i": index } as React.CSSProperties} key={index}>
          <img
              src={`/images/gallery-images/${imageGroup}/${index}.jpg`}
          />
       </span>
    )}
</div>

I’m only used to mapping over arrays, but in this case numberOfPictures is just a number. Is there a better way for mapping over this or doing a foreach?

Also the only way to get the index is to have it as the second parameter but then I don’t have any use for the "x" in this situation. I end up with the warning that x is declared but its value is never read, which I would want to get rid of.

Is there a way to just have the index and no first parameter in the map? Or a better way to render multiple times based on a number.

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2

Answers


  1. 0

    It is not good practice to put the index in key, as react’s document says:

    You might be tempted to use an item’s index in the array as its key.
    In fact, that’s what React will use if you don’t specify a key at all.
    But the order in which you render items will change over time if an
    item is inserted, deleted, or if the array gets reordered. Index as a
    key often leads to subtle and confusing bugs.

    The best approach is, using a unique id from the data you are trying to map over, then you’ll have to use the x and you don’t get the mentioned error.

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  2. 0

    In my opinion it would be better to map over you pictures Array if it’s possible, especially if those pictures have an id which you could use as a key inside your loop instead of the index.

    In all cases you can use an underscore if you don’t need to use the first param to avoid the warning.

    [...Array(numberOfPictures)].map((_, index) => {
    ...
})
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