Count occurences of values in an array of objects

I'm new to Javascript and doing some exercises I found online. Since I want to really understand how things work and why some approaches are better than others, I would appreciate some feedback why my approaches are bad (or good).

I have an XML and process it with the SAX parser to get "trending keywords". As an overall result, my function should return the 5 most popular ones. The result of my parsing itself is an array with 20 objects in the format

[{
  attributes: {}
  //children is just one big entry, separated by ';'
  children: ["Comey, James B;Federal Bureau of Investigation;Trump, Donald J;United States Politics and Government"]
  IsSelfClosing: false,
  name: 'ADX_KEYWORDS',
  parent: null,
  __proto__: {}
}]

Now my idea was, to pull out all those children values and store it in a new array

let flattenArr = keywords.reduce( (acc, cur) => acc.concat(cur.children), []);

As result I get an array (also length 20) in the format

[["Comey, James B;Federal Bureau of Investigation;Trump, Donald J;United States Politics and Government"],
["Trump, Donald J;Comey, James B;Federal Bureau of Investigation;United States Politics and Government"]]

To process the single values, I split the array

let mapArr = flattenArr
  .map( arr => arr.split(';') )
  .reduce( (acc, cur) => acc.concat(cur), [] );

and as a result, the new array of length 115 contains every single keyword separated. And here is the point where I don't know how to proceed. I tried the example from MDN(Array.Reduce) but didn't really know if (and how) I would be able to process these object.

//Result from MDN example, {value: quantity}
{ 'Alice': 2, 'Bob': 1, 'Tiff': 1, 'Bruce': 1 }

So I had the idea of creating an object for every keyword, with a property for its quantity

{
  key: keyword,
  count: n
}

and tried to accomplish this with

let countKeywords =  mapArrreduce( (allKeywords, keyword) => {
  if (allKeywords[0] != null && keyword in allKeywords[keyword]) {
    allKeywords[keyword].count++
  } else {
    allKeywords.push({
      keyword: keyword,
      count: 1
    })
  }
  return allKeywords;
}, []);   

but my code doesn't work, and even after two days I still struggle to make it work, which made me post this question (which is kinda new for me, since I always found my answers on stackoverflow and therefore never had to ask a question myself)

via Tobias