Author - Daniels Kenneth In category - Software development Publish time - 20 October 2022

Instead of pulling out the names of each task, let’s say I want to get a list of just the tasks that took me two hours or more to get done. That makes our code more declarative—it sayswhatto do, nothow to do it. You’ll appreciate how much more readable, maintainable, and, erm, debuggable this can make your code. While we’re using a for loop under the hood, wrapping it up into a function hides the details and lets us work with the abstraction instead. Instead, it’ll quietly hand back an array full of nothing.

  • JavaScript array map() function creates the new array populated with the results of calling the provided function on every item in the calling array.
  • Each step in the process needs to be done in order for the final outcome to occur.
  • We now need to create an array containing the total score of each Jedi.
  • The map() function takes the callback, only the array item is required.
  • You can use those operators to define custom aggregation expressions in JavaScript.
  • That’s what functional programming is in a nutshell.

A value to which previousValue is initialized the first time the callback is called. If initialValue is specified, that also causes currentValue to be initialized to the first value in the array. If initialValue is not specified, previousValue is initialized to the first value in the array, and currentValue is initialized to the second value in the array. We’ve gotten a list of task names frommap, and a filtered list of tasks that took a long time with… You do have to remember to include areturnstatement in your callback. If you don’t, you’ll get a new array filled with undefined.

Counting instances of values in an object

Say you have received an array containing multiple objects – each one representing a person. The thing you really need in the end, though, is an array containing only the id of each person. Find the difference in age between the oldest and youngest family members, and return their respective ages and the age difference. It is the value of the current element in an array.

How do you filter objects in JavaScript?

JavaScript’s Objects are not iterable like arrays or strings, so we can’t make use of the filter() method directly on an Object . filter() allows us to iterate through an array and returns only the items of that array that fit certain criteria, into a new array.

JS has no sum function , but you can make your own, using reduce. The result will have at most the same number of elements as before, but it could have as few as zero elements if none matched. The result will have the same number of elements as before. The value returned by reduce() in this case would be 95. The code below shows what happens if we call reduce() with an array and no initial value.

Array.reduce() syntax

The map() method does not change the length of the array so that it will return the same-sized array as an input array. Now, that output array will apply to reduce() method, and it will reduce it to one value.

javascript map reduce

ForEachreturnsundefined, so it doesn’t chain with other array returns an array, so youcan chain it with other array methods. Starting in MongoDB 4.4, mapReduce no longer supports the deprecated BSON type JavaScript code with scope for its functions. Themap, reduce, and finalize functions must be either BSON type String or BSON type JavaScript . To pass constant values which will be accessible in the map,reduce, and finalize functions, use the scope parameter. Count the occurrences of distinct elements in the given 2D array.


The accumulator can be pretty much anything (integer, string, object, etc.) and must be instantiated or passed when calling .reduce(). If you’re starting in JavaScript, maybe you haven’t heard of .map(), .reduce(), and .filter(). For me, it took a while as I had to support Internet Explorer 8 until a couple years ago.

Is JavaScript map faster than object?

According to my benchmarks, unless the keys are strings of small integers, Map is indeed more performant than Object on insertion, deletion and iteration speed, and it consumes less memory than an object of the same size.

What’s different here is that reduce passes the result of this callback from one array element to the other. The first is the function that determines what happens to the two values and the second sets the starting value. There are some scenarios where the order doesn’t matter as much. For example, the task of making instant coffee. It doesn’t matter in which order you put in the sugar and coffee powder — as long as it’s in the mug. That’s what functional programming is in a nutshell. For map-reduce operations that require custom functionality, you can use the $accumulator and $function aggregation operators, available starting in version 4.4.

Functional Programming for the Object-Oriented Developer — Chapter 0

But if you don’t need to be compatible with this very old browser, you have to become familiar with those methods. However, easy can quickly turn into a spaghetti jumble of complicated once you work with more than one for or while loop. Array methods eliminate the potential craziness that comes with long code, nested brackets and, lost semi-colons. JavaScript can be efficient — if you take the time to learn it properly.

The given input is an array, the elements of which are arrays of strings. The result is an object whose property names are the values from the arrays and their value is the number of their occurrences. It’s really hard to test your programming knowledge after you have completed a tutorial or a lecture. We have prepared some exercises to help out beginner devs to solidify their understanding of Map, Filter, Reduce, and other useful array methods.

The Quick Lowdown on Functional Programming

Aggregation pipelines provide better performance and usability than map-reduce. You could use ‘nested for loops’ to solve most of the problems you will encounter, but that leaves the code hard to read and understand. It is a value to be passed to the function as the initial value. It is the array object the current item belongs to.

  • Usually some action is performed on the value and then a new value is returned.
  • Using map, you don’t have to manage the state of the for loop yourself.
  • We pass it a callback, which accepts the previous value and current value as arguments, and returns the result of adding them together.
  • An even more succinct way of writing map in modern JavaScript is with arrow functions.
  • In a for loop this is messy, as you have to use the current item and keep track of the next item.
  • TypeError The array contains no elements and initialValue is not provided.

Every exercise has a brief description of the problem, starting code, links to relevant MDN docs, and expected results. Try to solve the problems without taking a peek at the solution.

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