Flatten nested JSON into a keyed array of values using Underscore

Question

From my server rest API, I request a javascript object that has the following structure:

[
  {
    "msgID": 3,
    "admTag": {
      "tagName": "Employee"
    }
  },
  {
    "msgID": 3,
    "admTag": {
      "tagName": "Safety"
    }
  },
  {
    "msgID": 3,
    "admTag": {
      "tagName": "Performance"
    }
  },
  {
    "msgID": 7,
    "admTag": {
      "tagName": "Role Based Training"
    }
  },
  {
    "msgID": 7,
    "admTag": {
      "tagName": "Account Service Information"
    }
  },
  {
    "msgID": 6,
    "admTag": {
      "tagName": "Consumer Product Safety Improvement"
    }
  }
]

To use this object on the client, I need to transform the structure to group by the msgID property and assign to it an array containing the values of the associated tagName. Like so:

[{
    "3": ["Employee","Safety","Performance"],
    "7": ["Role Based Training","Account Service Information"],
    "6": ["Consumer Product Safety Improvement"]
}]

I can accomplish this using a nested for loop but I know it can be done more efficiently and with less code using underscore. Using groupBy I am able to get a grouping of the msgID's which gives me the keys I need:

_.groupBy(tags.tags, function(model){ 
  return model.msgID;
});

...but I'm unsure how to 'pluck' the tagNames from the resulting object and assign them to the proper msgID.

Greatly appreciate any help on this!

Nina Scholz · Accepted Answer · 2015-12-21 18:04:55Z

1

A solution in plain Javascript. Just four lines of code.

var array = [{ "msgID": 3, "admTag": { "tagName": "Employee" } }, { "msgID": 3, "admTag": { "tagName": "Safety" } }, { "msgID": 3, "admTag": { "tagName": "Performance" } }, { "msgID": 7, "admTag": { "tagName": "Role Based Training" } }, { "msgID": 7, "admTag": { "tagName": "Account Service Information" } }, { "msgID": 6, "admTag": { "tagName": "Consumer Product Safety Improvement" } }],
    result = {};

array.forEach(function (a) {
    result[a.msgID] = result[a.msgID] || [];
    result[a.msgID].push(a.admTag.tagName);
});

document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

answered Dec 21, 2015 at 18:04

Nina Scholz

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Comments

dc5 · Accepted Answer · 2015-12-21 18:53:14Z

0

Using .groupBy, .mapValues and _.map you could do something like this:

var output = _(input)
  .groupBy('msgID')                         // Group by msgId
  .mapValues(function(group) {              // Transform value of each group
    return _.map(group, 'admTag.tagName');  // Return array of tagNames
  })
  .value();

jsFiddle

edited Dec 21, 2015 at 18:53

answered Dec 21, 2015 at 18:45

dc5

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Comments

Andreas · Accepted Answer · 2015-12-21 18:19:54Z

0

Using _.reduce from underscore

var result = _.reduce(tags.tags, function(l, r) {
    if (typeof l[r.msgID] === "undefined") {
        l[r.msgID] = [];
    }
    l[r.msgID].push(r.admTag.tagName);
    return l;
}, {});

fiddle

The same can be achieved with the native Array.prototype.reduce with nearly the same code

var result = tags.tags.reduce(function(l, r) {
    if (typeof l[r.msgID] === "undefined") {
        l[r.msgID] = [];
    }
    l[r.msgID].push(r.admTag.tagName);
    return l;
}, {});

fiddle

answered Dec 21, 2015 at 18:19

Andreas

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