Insert element inside array Code Answer

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I have a function

checkName(output) {

  output.filter((NewData) => {
    return this.props.elements.filter((OldData) => {
      if (NewData.key == OldData.key) { =,
          //there i need to add another element 
          // Need to add newData.number = OldData.number
      return NewData
  return output

and I call this function like:

const named = this.checkName(product.rows)

Now I need to add to my product’s array that I passed to checkName the value “OldData.Number” to “newData.Number” that is not defined in product (so I need to create this field)

For example:

Product before the checkName function

product.rows = []

Product after the checkName function

product.rows = ["value of", NewData.number="value of OldData.number"]

How can I obtain this result?


There are 2 confusing things in your code:

  • You are using filter to execute an action in each member of the output array. However, filter should be used to… well, filter that array, meaning that is should not modify it, just return a sub-set of it. Instead, you might want to use forEach. However, taking into accound the next bullet, probably you want to use map.
  • You are modifying the array passed to the checkName function. This is confusing and can lead to hard-to-find bugs. Instead, make your function “pure”, meaning that it should not mutate its inputs, instead just return the data you need from it.

I would suggest some implementation like this one:

    return => {
        // find the old data item corresponding to the current NewData
        const OldData = this.props.elements.find(x => x.key === NewData.key);
        if (OldData) {
            // If found, return a clone of the new data with the old data name
            // This uses the spread syntax:
            return {
                ...NewData, // Clone the NewData object
                name:, // set the value found in in the "name" field of the cloned object
                number: OldData.number, // You can do the same for each field for which you want to replace the value cloned from NewValue
        } else {
            // Otherwise, just return a clone of the NewData
            return { ...NewData };

The usage would be like this:

const named = this.checkName(product.rows)

Be aware that the product.rows array won’t be modified!

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