Merge duplicate objects in array of objects

Question

I have below array of objects,

var data = [     {         label: "Book1",         data: "US edition"     },     {         label: "Book1",         data: "UK edition"     },     {         label: "Book2",         data: "CAN edition"     } ]; 

I want to merge the duplicate objects based on attribute 'label' so that Final output will look like below,

var data = [     {         label: "Book1",         data: ["US edition", "UK edition"] //data attribute is merged     },     {         label: "Book2",         data: "CAN edition"     } ]; 

Can someone help me identify the approach?

Answer 1

I would probably loop through with filter, keeping track of a map of objects I'd seen before, along these lines (edited to reflect your agreeing that yes, it makes sense to make (entry).data always an array):

var seen = {}; data = data.filter(function(entry) {     var previous;      // Have we seen this label before?     if (seen.hasOwnProperty(entry.label)) {         // Yes, grab it and add this data to it         previous = seen[entry.label];         previous.data.push(entry.data);          // Don't keep this entry, we've merged it into the previous one         return false;     }      // entry.data probably isn't an array; make it one for consistency     if (!Array.isArray(entry.data)) {         entry.data = [entry.data];     }      // Remember that we've seen it     seen[entry.label] = entry;      // Keep this one, we'll merge any others that match into it     return true; }); 

In an ES6 environment, I'd use seen = new Map() rather than seen = {}.

Note: Array.isArray was defined by ES5, so some quite older browsers like IE8 won't have it. It can easily be shimmed/polyfilled, though:

if (!Array.isArray) {     Array.isArray = (function() {         var toString = Object.prototype.toString;         return function(a) {             return toString.call(a) === "[object Array]";         };     })(); } 

Side note: I'd probably also always make entry.data an array, even if I didn't see two values for it, because consistent data structures are easier to deal with. I didn't do that above because your end result showed data being just a string when there was only one matching entry. (We've done that above now.)

Live example (ES5 version):

var data = [      {          label: "Book1",          data: "US edition"      },      {          label: "Book1",          data: "UK edition"      },      {          label: "Book2",          data: "CAN edition"      }  ];  snippet.log("Before:");  snippet.log(JSON.stringify(data, null, 2), "pre");  var seen = {};  data = data.filter(function(entry) {      var previous;        // Have we seen this label before?      if (seen.hasOwnProperty(entry.label)) {          // Yes, grab it and add this data to it          previous = seen[entry.label];          previous.data.push(entry.data);            // Don't keep this entry, we've merged it into the previous one          return false;      }        // entry.data probably isn't an array; make it one for consistency      if (!Array.isArray(entry.data)) {          entry.data = [entry.data];      }        // Remember that we've seen it      seen[entry.label] = entry;        // Keep this one, we'll merge any others that match into it      return true;  });  snippet.log("After:");  snippet.log(JSON.stringify(data, null, 2), "pre");

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