Javascript Object Cloning in Higher Order Functions

Question

I'm very new to JavaScript and I have been working through the Eloquent JavaScript (2nd) exercises in order to learn more. One particular exercise (5.3) has been giving me a lot of trouble.

The goal of the exercise is to take an array of objects that contains the date of death for a variety of people and group them by century based on death date. This is what I have so far:

function groupBy(array, groupOf) {
  var groups = {};
  array.forEach(function(element) {
    var groupName = groupOf(element);
    if (groupName in groups)
      groups[groupName].push(element);
    else
      groups[groupName] = element;
  });
  return groups;
}

var byCentury = groupBy(ancestry, function(person) {
  return Math.ceil(person.died / 100);
});

I believe the problem can be narrowed down to this line: groups[groupName] = element; but I don't understand why this is wrong.

Thanks for the help. I apologize in advance if this is obvious.

thefourtheye · Accepted Answer · 2014-10-18 05:31:43Z

You need to create an array in this line

groups[groupName] = element;

instead you are just assigning the object. Since objects don't have push method, your code is failing. You can create an array, like this

groups[groupName] = [element];

Also, you can use the shorthand notation to write the if..else condition, like this

groups[groupName] = groups[groupName] || [];
groups[groupName].push(element);

Or, you can simply write an if condition, like this

if (groupName in groups === false) {
    groups[groupName] = [];
}
groups[groupName].push(element);

Both the if condition and the groups[groupName] || [] makes sure that, groups[groupName] has an empty array, if groupName does not exist in groups.

groups[groupName] = groups[groupName] || [];

Here, groups[groupName] will be evaluated to undefined if groupName is not found in groups, which is actually falsy. Since the first expression is falsy, the second expression [] will be the result of the right hand side. So, if the groupName doesn't exist, then an empty array will be assigned to groups[groupName].

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