Differences Between Using Array.push() and Literal (Bracket) Notation in JavaScript

I am looking at this specific answer.

What is the reason behind Code Snippet 2 not producing the same result as Code Snippet 1?

Code Snippet 1:

var firstEvents = events.reduce(function(ar, e) {
  var id = e.getId();
  if (e.isRecurringEvent() && e.isAllDayEvent() && !ar.some(function(f) {return f.eventId == id})) {
    ar.push({eventTitle: e.getTitle(), eventId: id, startDate: e.getAllDayStartDate(), endDate: e.getAllDayEndDate()});
  }
  return ar;
}, []);

Code Snippet 2:

var firstEvents = events.reduce(function(ar, e) {
  var id = e.getId();
  if (e.isRecurringEvent() && e.isAllDayEvent() && !ar.some(function(f) {return f.eventId == id})) {
    ar['eventTitle'] = e.getTitle();
    ar['eventId'] = id;
    ar['startDate'] = e.getAllDayStartDate();
    ar['endDate'] = e.getAllDayEndDate();
  }
  return ar;
}, []);

EDIT:

Why does Code Snippet 3 work as expected (adding one object per event series) while Code Snippet 4 fails to do so?

Code Snippet 3:

var firstEvents = events.reduce(function(ar, e) {
  var id = e.getId();
  if (e.isRecurringEvent() && e.isAllDayEvent() && !ar.some(function(f) {return f.eventId == id})) {
    ar.push({eventTitle: e.getTitle(), eventId: id});
  }
  return ar;
}, []);

Code Snippet 4:

var firstEvents = events.reduce(function(ar, e) {
  var id = e.getId();
  if (e.isRecurringEvent() && e.isAllDayEvent() && !ar.some(function(f) {return f.eventId == id})) {
    ar.push({eventTitle: e.getTitle()});
  }
  return ar;
}, []);

Answer №1

ar.push({/*...*/}) creates a new object and adds it to the array.

ar['eventTitle'] = e.getTitle(); and similar lines of code are used to set properties on the array with those specific names, replacing any previous values. These actions do not insert entries into the arrays.

Using push is the correct approach if your intention is to insert objects into the array.

While it may seem unconventional to have custom properties on arrays, it's important to remember that arrays are essentially objects in JavaScript, allowing for the addition of properties.

If you wish to add elements to the array using direct assignment, the syntax would be:

ar[ar.length] = {eventTitle: e.getTitle(), eventId: id, startDate: e.getAllDayStartDate(), endDate: e.getAllDayEndDate()};

This method achieves the same objective as utilizing push.

Answer №2

The initial code snippet utilizes the push method to append an additional object to the array named ar.

On the other hand, the second example employs bracket notation to modify the existing values in the object ar.

Answer №3

The initial illustration showcases how the reduce function is appending data to the aggregated array.

ar.push({...})

In contrast, the aggregation assigns values to the array as keys, emphasizing that arrays === objects in JavaScript.

ar['key-name'] = whatever

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