Let’s take the above example and use reduce on it: let arrNum = Įverything is the same as the map() and filter() methods – but what’s important to understand is how the reduce method works under the hood. The reduce() method reduces an array to a single value by performing the desired operation on the elements collectively. Now, you could use a for loop or the forEach() method, but reduce is built for this kind of task. Let’s say you are asked to find the sum of all elements of an array. Then it'll form a new array with the calculated values. So, the filter() method will pick each value of that array starting from the index and perform the operation on each value. On the right-hand side, I called the filter() method on the arrNum array. Then, I initialized another variable arrNewNum that will store the new array that the filter() method will create. It returns true or false for each time a new num is passed for the comparison, where the ‘num’ variable is fed the data by the filter() method. Next, I initialized a function divByFive with ‘num’ as an argument. Here, I have an array arrNum with 7 elements in it. Here's the code to do that: let arrNum = Now, you would like to see what numbers can be divided by 3 and make a separate array from them. Let’s take an example: Suppose you have an array arrName and that array stores a bunch of numbers. The filter() method also creates a new array. The name kind of gives it away, doesn't it? You use this method to filter the array based on conditions you provide. The map() method makes a new array whereas the ‘forEach()’ method mutates/changes the original array with the calculated array. That is because this property is what makes the map() method different from the ‘forEach()’ method. Important: Notice how I’m stressing not changing the original array. So, the map() method will pick each value of that array starting from the index and perform the desired calculation on each value. On the right-hand side, I called the map() method on the ‘arrOne’ array. Then, I initialized another variable arrTwo that will store the new array that the map() method will create. If you’re new to arrow functions but are familiar with regular functions, an arrow function is the same as this: function(num) This returns the product of num and 5, where the ‘num’ variable is fed the data by the map() method. Next, I initialized an arrow function multFive with ‘num’ as an argument. So what's going on here? Well, I have an arrOne array with 6 elements in it. Return num * 5 //'num' here, is the value of the array.Īnd here's the output: Here's the code to do that: let arrOne = Let’s say you want to multiply each element by 5 while not changing the original array. Map() takes a maximum of three arguments, which are value/element, index, and array. The map method will help you do this: let arrOne = This is where map() comes into the picture. But you also don’t want to mess with the original array. Suppose you have an array arrOne where you’ve stored some numbers, and you’d like to perform some calculations on each of them. It’ll be fun! How to Use the map() Method You'll also learn where you'll want to use them and how to use them, with the help of analogies and examples. In this tutorial, you'll see how each of these high-order array methods work. Map, filter, and reduce are three of the most useful and powerful high-order array methods. So this makes these methods a must-learn. Second, React builds on key concepts like the map(), filter(), and reduce() JavaScript methods (after all – React is a JavaScript library). So if that's what you're doing, first of all – great job! You have made a wise decision by not starting directly with React. If you want to learn React, it's important to get a fair understanding of some core JavaScript concepts first.
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