How can I prevent duplicate keys when adding data to an object array?

Question

I have some code where I am looping through some records in a table, and whilst doing this I'm organising this data into a JavaScript object array.

For example the data below.

I need to ensure that if a record "Name" has already been recorded, then rather than create a new item it uses the existing item so that the end result will be an object or array like this:

{
    "bobjones": {
         "sessions": 7
    },
    "annfrank": {
         "sessions": 4
    },
    "fredsmith": {
         "sessions": 4
    }
}

Doing this in PHP is easy, but I'm struggling with this in JS due to the way keys are interpreted. Could somebody provide some direction?

An array would call for some brackets, where you have all braces. Was that a mistake in your example or is that a single array element? — isherwood, Jan 10 '19 at 15:49
Possible duplicate of [Access / process (nested) objects, arrays or JSON](https://stackoverflow.com/questions/11922383/access-process-nested-objects-arrays-or-json) — Liam, Jan 10 '19 at 15:49
Your object seems malformed. It's make much more sense to have something more like `[{name:'bobjones', sessions:'7'},...]`. *I'm organising this data into a JavaScript object array* I see an object, I don't see an array — Liam, Jan 10 '19 at 15:50

score 2 · Accepted Answer · answered Jan 10 '19 at 15:57

const inputData = [
  { name: 'Bob Jones', date: '----', session: 1 },
  { name: 'Ann Frank', date: '----', session: 1 },
  { name: 'Bob Jones', date: '----', session: 1 },
  { name: 'Fred Smith', date: '----', session: 1 },
]

var outputData = {}
for(let doc of inputData) {
  if(outputData[doc.name]) {
    outputData[doc.name].session = outputData[doc.name].session + doc.session
  } else {
    outputData[doc.name] = doc
  }
}

console.log(outputData)

Now outputData is:

{ 'Bob Jones': { name: 'Bob Jones', date: '----', session: 2 },
  'Ann Frank': { name: 'Ann Frank', date: '----', session: 1 },
  'Fred Smith': { name: 'Fred Smith', date: '----', session: 1 } }

score 0 · Answer 2 · answered Jan 10 '19 at 15:59

You can just test to see if the object property exists. If so, add to its sessions property, if not, set it.

if(obj[prop]) {
    obj[prop].sessions += sessions;
} else {
    obj[prop] = { sessions };
}

In the first if statement, if you're concerned that obj[prop] may exist but obj[prop].sessions may not, you can change it to if(obj[prop] && obj[prop].sessions) { ... }

score 0 · Answer 3 · answered Jan 10 '19 at 16:08

You could use the reduce method, assuming that you input data is an array of objects (since, that is generally how you get data when consulting a database).

const table = [
    {name: "Bob Jones", date: "12-12-2019", sessions: 3},
    {name: "Ann Frank", date: "12-9-2019", sessions: 4},
    {name: "Bob Jones", date: "12-9-2019", sessions: 2},
    {name: "Fred Smith", date: "12-9-2019", sessions: 4}
];

let res = table.reduce((res, {name, date, sessions}) =>
{
    if (res[[name]])
        res[[name]].sessions += sessions;
    else
        res[[name]] = {sessions: sessions};

    return res;
}, {});

console.log(res);

How can I prevent duplicate keys when adding data to an object array?

3 Answers3