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Js

Eric Stanley
30 Dec, 2019

JavaScript: Array Manipulation

JavaScript offers different ways to manipulate arrays. However, there are methods that changes the original array and other methods that don’t. The methods that changes original array is called MUTATION. Let’s take a look at most common methods to manipulate arrays in JavaScript Now, that we know that there are methods that mutates the original array, it’s considered best practice to assign the array to const, if you know that the array that you use is not going to get mutated. It’s wise to use let otherwise. It is not a mandate that you need to use const to declare an array that will not be mutated, however, it’s easy for your colleague to understand that the array which is declared as const is not going to get changed anywhere in the code; which is why it’s considered best practice As usual, if you wanna try the examples in this post, I recommend codepen.io, but you are free to make your own choice. The most common interactions that we usually have with arrays areAdd item to array Remove item from array Update and item in the arrayAll the above mentioned actions can be done by both mutating and non-mutating methods. Instead of explaining each of the methods subjectively, I felt it would be better understood with a table. Here we go! testArray: [a, b, c, d, e, f, g, h, i];Code Original Array Processed Array Returned Val Is Mutated Action MethodtestArray.push('j') [a,b,c,d,e,f,g,h,i] [a,b,c,d,e,f,g,h,i,j] 10 true add push()testArray.unshift('z') [a,b,c,d,e,f,g,h,i,j] [z,a,b,c,d,e,f,g,h,i,j] 11 true add unshift()testArray.concat('k') [z,a,b,c,d,e,f,g,h,i,j] [z,a,b,c,d,e,f,g,h,i,j] [z,a,b,c,d,e,f,g,h,i,j,k] false add concat()['y', ...testArray, 'l'] [z,a,b,c,d,e,f,g,h,i,j] [z,a,b,c,d,e,f,g,h,i,j] [y,z,a,b,c,d,e,f,g,h,i,j,l] false add ...testArray.pop() [z,a,b,c,d,e,f,g,h,i,j] [z,a,b,c,d,e,f,g,h,i] j true remove pop()testArray.shift() [z,a,b,c,d,e,f,g,h,i] [a,b,c,d,e,f,g,h,i] z true remove shift()testArray.splice(0, 2) [a,b,c,d,e,f,g,h,i] [c,d,e,f,g,h,i] [a,b] true remove splice()testArray.filter(a => a!== 'c') [c,d,e,f,g,h,i] [c,d,e,f,g,h,i] [d,e,f,g,h,i] false remove filter()testArray.slice(1, 6) [c,d,e,f,g,h,i] [c,d,e,f,g,h,i] [d,e,f,g,h] false remove slice()testArray.slice(2) [c,d,e,f,g,h,i] [c,d,e,f,g,h,i] [e,f,g,h,i] false remove slice()testArray.splice(2, 1,30, 31) [c,d,e,f,g,h,i] [c,d,30,31,f,g,h,i] [e] true update splice()testArray.map(x => x ==='d' ? 29 : x) [c,d,30,31,f,g,h,i] [c,d,30,31,f,g,h,i] [c,29,30,31,f,g,h,i] false update map()I hope the above table is self-explanatory, however if you need to check the values of the testArray with real code, feel free to visit programmatic output where the exact same table is derived programmatically. Codepen pin link

Higher order functions

I recently started learning JavaScript and I tumbled upon a concept called Higher Order Functions (HOF). There are quite a lot of documents in the internet that explains what an HOF is, and here is my thought. Before explaining what an HOF is, let’s see how a function looks like in JavaScript. Functions in JavaScript is not different from functions in any other programming languages, which means, I assume that you kinda know how a function generally looks like. Below is a function snippet in JavaScript. function myTestFunction() { alert("Thanks for clicking") }Now, that we know how a function looks like in JavaScript, let’s continue on knowing what HOF is and how is it different from a normal function. HOF is a concept in JavaScript where a function (say x) is passed as an argument to another function (say y) OR the calling function (say z) returns a function (say x) after execution Let’s take the above function for example (#1), and pass it into another function document.addEventListener("click", myTestFunction) function myTestFunction() { alert("Thanks for clicking") }Now, document.addEventListener is called whenever an event in the first parameter is triggered in the document, and the second parameter is a function that gets executed whenever the event is triggered In other words, the above function can also be written as document.addEventListener("click", function myTestFunction() { alert("Thanks for clicking") })And this works perfectly fine. So, if this works perfectly fine, why would I need to write another function and call it within the addEventListener instead of writing directly within the listener. The answer is, the function can be reused now, if it’s outside the listener. Kinda makes sense? Let’s take another step and look at another example (#2) which is a little more complex. function createMultiplier(multiplier) { return function(x) { return x * multiplier } }let doubleMe = createMultiplier(2) let tripleMe = createMultiplier(3) let quadrupleMe = createMultiplier(4)document.write(doubleMe(45) + " ") document.write(tripleMe(45) + " ") document.write(quadrupleMe(45) + " ")As you can see, the createMultiplier function returns a function here which can be used in different places. Let’s take a look at how the code looks without HOF function multiply(x, multiplier) { return x * multiplier }let doubleMe = multiply(45, 2) let tripleMe = multiply(45, 3) let quadrupleMe = multiply(45, 4)document.write(doubleMe + " ") document.write(tripleMe + " ") document.write(quadrupleMe + " ")Looks like, the code without HOF is still less number of lines and effective too! Until now, even I was not sure about the use of HOF. But let’s take 1 more step from here. Let’s parameterize the multiplier (#3) With HOF function createMultiplier(multiplier) { return function(x) { return x * multiplier } }let nums = [41, 42, 43, 44, 45, 46]let doubleMe = createMultiplier(2) let tripleMe = createMultiplier(3) let quadrupleMe = createMultiplier(4)for (i = 0; i < nums.length; i++) { document.write(doubleMe(nums[i]) + " ") document.write(tripleMe(nums[i]) + " ") document.write(quadrupleMe(nums[i]) + " ") }Without HOF function multiply(x, multiplier) { return x * multiplier }let nums = [41, 42, 43, 44, 45, 46]for (i = 0; i < nums.length; i++) { let doubleMe = multiply(nums[i], 2) let tripleMe = multiply(nums[i], 3) let quadrupleMe = multiply(nums[i], 4) document.write(doubleMe + " ") document.write(tripleMe + " ") document.write(quadrupleMe + " ") }As you can see, the numbers 2, 3 and 4 are constants and we are not making any changes to those parameters within the loop. However, we cannot move them outside of the for loop since the resultant value is dependent on this constant. Whereas, in the code with HOF, we have moved the constants successfully outside of the loop. Now, why do we do that? Does it impact the performance? The answer in this case is, the code looks a little bit neat and NO it does not affect the performance. Here’s another example of HOF in JavaScript let myColors = ["red", "orange", "yellow", "green"] myColors.forEach(saySomethingNice) function saySomethingNice(color) { document.write("The color " + color + " is a great color. ") }The forEach method in the array object automatically loops thro’ every element in the array without using a loop, which means less code to do the same work. The code in example #3 looks like this now! function createMultiplier(multiplier) { return function(x) { return x * multiplier } }let nums = [41, 42, 43, 44, 45, 46]let doubleMe = createMultiplier(2) let tripleMe = createMultiplier(3) let quadrupleMe = createMultiplier(4)nums.forEach(num => { document.write(doubleMe(num) + " ") document.write(tripleMe(num) + " ") document.write(quadrupleMe(num) + " ") })In my personal terms, if you are a novice JS developer and if you copy and paste a code in JavaScript which you feel that it’s hard to understand, but still works the way you wanted it to, you are probably looking into a HOF. That’s my personal view of HOF though! Ending Note: The latest JavaScript ES6 standard, has tackled a lot of limitations that the ES5 standard had, and HOF is one of those.