Convert an array to a map in JavaScript

Sometimes it's useful to convert a array to a map for convenience or performance reasons. But how can we achieve that so that the resulting code will be easy to understand?

I'm using the term map here to mean a data structure where a value can be accessed using an unique key. In JavaScript, objects can be used as maps but there also exists a special Map type which has some advantages and disandvantages when compared to using objects. We won't be covering Map in this article.

Let's say we have a array of projects we want to group by completion month. We might need to do this if we frequently access the projects of a certain month and don't want to search them from the array every time, or maybe we are rendering the projects into month components using React/Vue/Svelte.

There's a couple of ways we can achieve this. First we will look at how to do this using the Array.reduce() method and then how to simplify things using for..of.

Here is the data we are going to be using in all the examples. It's a simple array of objects that have the attributes name and completed.

const data = [
  {
    name: "Project 1",
    completed: "01-2021"
  },
  {
    name: "Project 2",
    completed: "02-2021"
  },
  {
    name: "Project 3",
    completed: "02-2021"
  },
  {
    name: "Project 4",
    completed: "02-2021"
  }
];

Using Array.reduce()

Array.reduce takes two parameters, a function which is called for each element in the array and an initial value for the return value of the operation.

The function given to Array.reduce() should have the following signature (accumulator, currentValue, index, array) => {...}.

accumulator is a value which is carried over from the previous calls to the function, currentValue is the value in the array we are currently at, index is the index we are currently at, and array is the array reduce was called on. You can omit index and array if you have no use for them.

The basic idea of reduce is that on every call, we use the value from currentValue to shape accumulator how we want until we have looped through all the values in the array. The return value of the function is set as the new value of the accumulator for the next iteration.

After the function is called on the last element in the array, the value of accumulator is returned as the return value of reduce.

Here's how we could filter group our data using reduce

const projectsByMonth = data.reduce((result, project) => {
  const existingProjects = result[project.completed] || [];
  return {
    ...result,
    [project.completed]: [...existingProjects, project]
  }
}, [])

/*
{
  '01-2021': [ { name: 'Project 1', completed: '01-2021' } ],
  '02-2021': [
    { name: 'Project 2', completed: '02-2021' },
    { name: 'Project 3', completed: '02-2021' },
    { name: 'Project 4', completed: '02-2021' }
  ]
}
*/

Array.reduce gets the job done just fine, but the code is not the easiest to understand. While this example is a simple one, I almost always struggle to wrap my head around complex code which uses reduce. Could we make this better?

Using for..of

for..of is a way to loop over any iterable in JavaScript. We can use for..or to solve the same problem by creating the object beforehand and looping through the data.

let projectsByMonth = {};
for (const project of data) {
  const existingProjects = projectsByMonth[project.completed] || [];
  projectsByMonth = {
    ...projectsByMonth,
    [project.completed]: [...existingProjects, project]
  }
}

/*
{
  '01-2021': [ { name: 'Project 1', completed: '01-2021' } ],
  '02-2021': [
    { name: 'Project 2', completed: '02-2021' },
    { name: 'Project 3', completed: '02-2021' },
    { name: 'Project 4', completed: '02-2021' }
  ]
}
*/

While the code for adding the projects to the object is the same as in the previous example, the resulting code is a bit easier to understand because we are using a using a simple loop.

If you're concerded with performance you can also replace the object/array spread operators with Array.push.

let projectsByMonth = {};
for (const project of data) {
  const key = project.completed;
  if (!(key in projectsByMonth))
    projectsByMonth[key] = []

  projectsByMonth[key].push(project)
}

Whether you find for..of easier to understand, is mostly a matter of taste and familiarity with reduce. However, thinking about clarity of the code we write is an important part of software engineering. Clear, explicit code will help other programmers (or ourselves in a couple of months 😜) understand the code and the reasoning behind it.

I'll leave you with this quote from Martin Fowlers excellent book Refactoring