Decode your json in the the proper datastructure and recreate your array of objects using the proper algorithm for the matching.

Something like:

1. take the first object
2. save the Field A
3. iterate other objects and check for the field A
4. if you find another Field A add the new "Variable Field B" in the new object structure with the found value or the new "Variable Field A" with the found value.
5. delete the found items in the original array (in order to prevent duplications)
6. next iteration

If you're using javascript for that check .hasOwnProperty("propertyName") method that check if a field exists in the object. It should be usefull for checking if you have to get "Variable Field A" or "Variable Field B" from the object.

This is a great application for the Array reducer function. Try this:

const data = [] // your JSON data
data.reduce((acc,item)=>({
  ...acc,
  [item['Field A']]: {...acc[item['Field A']],...item}
}),{})

The result:

{
  ABC: {
    'Field A': 'ABC',
    'Variable Field A': '66',
    'Variable Field B': '-729'
  },
  DEF: {
    'Field A': 'DEF',
    'Variable Field A': '70',
    'Variable Field B': '334'
  },
  GHI: {
    'Field A': 'GHI',
    'Variable Field A': '135',
    'Variable Field B': '962'
  },
  JKL: {
    'Field A': 'JKL',
    'Variable Field A': '19',
    'Variable Field B': '241'
  }
}

you can do something like this

const group = (data, field) => Object.values(data.reduce((res, item) => {
  const existing = res[item[field]] || {}
  
  return {
    ...res,
    [item[field]]: {...existing, ...item}
  }

}, {}))


const data = [{
    "Field A": "ABC",
    "Variable Field A": "66"
},
{
    "Field A": "DEF",
    "Variable Field A": "70"
},
{
    "Field A": "GHI",
    "Variable Field A": "135"
},
{
    "Field A": "JKL",
    "Variable Field A": "19"
},
{
    "Field A": "ABC",
    "Variable Field B": "-729"
},
{
    "Field A": "GHI",
    "Variable Field B": "962"
},
{
    "Field A": "DEF",
    "Variable Field B": "334"
},
{
    "Field A": "JKL",
    "Variable Field B":"241"
}]

const result = group(data, 'Field A')

console.log(result)