What is the best way to retrieve the last 3 elements that have a specific name value using JMESPATH and JavaScript?

Below is a JSON data array that I have:

[{"name":"Button1","date":"1596959802144"},{"name":"Button2","date":"1596959804238"},{"name":"Button3","date":"1596959809334"},{"name":"Button1","date":"1597000878135"},{"name":"Button2","date":"1597000896335"},{"name":"Button3","date":"1597000901536"},{"name":"Button2","date":"1597000904437"},{"name":"Button3","date":"1597000909535"},{"name":"Button1","date":"1597000912250"},{"name":"Button2","date":"1597000939937"},{"name":"Button3","date":"1597000957940"},{"name":"Button2","date":"1597000964640"},{"name":"Button1","date":"1597001005141"},{"name":"Button2","date":"1597001010240"},{"name":"Button3","date":"1597001014845"},{"name":"Button2","date":"1597001021644"},{"name":"Button1","date":"1597001025738"},{"name":"Button2","date":"1597001049030"},{"name":"Button3","date":"1597001054139"},{"name":"Button1","date":"1597001057741"},{"name":"Button2","date":"1597001060340"},{"name":"Button3","date":"1597001062445"},{"name":"Button1","date":"1597002599045"},{...}

I am looking to use the JMESPath JavaScript library to retrieve only the last 3 entries for each distinct name value. For example:

[{"name":"Button3","date":"1597001014845"},{"name":"Button2","date":"1597001021644"},{"name":"Button2","date":"1597001049030"},{"name":"Button3","date":"1597001054139"},{"name":"Button2","date":"1597001060340"},{"name":"Button3","date":"1597001062445"},{"name":"Button1","date":"1597002694136"},{"name":"Button1","date":"1597002696349"},{"name":"Button1","date":"1597002699243"}]

This retrieves the last 3 occurrences per distinct button name.

I came across a solution on Stack Overflow using JQ: 

map_values(delpaths(keys_unsorted[:-2] | map([.]))

Get last N elements for each item of a JSON object

Is there another way to achieve this or using a different JavaScript module?

javascriptjsonjmespath

Answer №1

If maintaining the order of elements in your final array is not a concern, you can achieve this using a pure JavaScript approach:

const getLastNForEveryName = (arr, n) => {
  const lastNOfEach = arr.reduce((acc, curr) => {
    if(acc[curr.name] == null) { // Create a new key with current item if it doesn't exist
      acc[curr.name] = [curr]; 
    } else {
      if(acc[curr.name].length >= n) // Remove the first element if the array size reaches limit
        acc[curr.name].shift();

      acc[curr.name].push(curr); // Add current item to the array
    }
    return acc;
  }, {})
  
  return Object.values(lastNOfEach).flatMap(l => l); // Get values and flatten to avoid nested arrays
}

// Testing
const values = [{"name":"Button1","date":"1596959802144"},{"name":"Button2","date":"1596959804238"},{"name":"Button3","date":"1596959809334"},{"name":"Button1","date":"1597000878135"},{"name":"Button2","date":"1597000896335"},{"name":"Button3","date":"1597000901536"},{"name":"Button2","date":"1597000904437"},{"name":"Button3","date":"1597000909535"},{"name":"Button1","date":"1597000912250"},{"name":"Button2","date":"1597000939937"},{"name":"Button3","date":"1597000957940"},{"name":"Button2","date":"1597000964640"},{"name":"Button1","date":"1597001005141"},{"name":"Button2","date":"1597001010240"},{"name":"Button3","date":"1597001014845"},{"name":"Button2","date":"1597001021644"},{"name":"Button1","date":"1597001025738"},{"name":"Button2","date":"1597001049030"},{"name":"Button3","date":"1597001054139...

console.log(getLastNForEveryName(values, 3));

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