The Mysterious Array.reduce() Method in JavaScript

The array.reduce() method is famous for summing up an array of numbers.
var arr = [1,2,3];
var sum = arr.reduce((x,y)=>{
	return x + y
});
console.log(sum);
//6
It can be also used go conduct a series of substraction starting from the first number.
var arr = [100,50,10];
var remaining = arr.reduce((x,y)=>{
	return x - y
});
console.log(remaining);
//40
Or, the array.reduce() method can be used to execute a series of multiplication or division.
var arr1 = [1,2,3,4];
var fac = arr1.reduce((x,y)=>{
	return x * y
});
console.log(fac);
//24

var arr2 = [100,5,4];
var fac = arr2.reduce((x,y)=>{
	return x / y
});
console.log(fac);
//5

What Actually Happened Inside the Array.reduce() Method?

What happs inside the arrray.reduce() method is that it keeps the return value as a parameter for the next itiration of the calculation until all elements of the array are gone through. And the initial return value or element, since no calculation has been executed, will be the first element of the array. In the summation example above, in [1,2,3], the initail proxy return value is 1 which is kept as x in this statement "return x + y" where y is the next value 2. Then 1 + 2 is executed and returns 1 + 2 = 3, then 3 is kept as x, replacing the original value of 1. The itiration goes again for the next value in the array until all numbers are run through. The final return value will be given out. 
var arr1 = [1,2,3];
var sum = arr1.reduce((x,y)=>{
	console.log(x);
	return x + y;
});

//1 
//3

var arr2 = [1,2,3];
var sum = arr2.reduce((x,y,i)=>{
	console.log(i);
	return x + y;
});

//2 
//3

Other Parameters in the Array.reduce() Method

Beside x, the place the hold return value and y the next value to be applied in the calculation, we can call the index of the y element. Here, we won't see the index 0, since the first index of y is the second element in the array. The first element is already taken as the proxy return value in the first itiration.
var arr = [10,20,30,40,50];
var sum = arr.reduce((x,y,i)=>{
	console.log(i);
	return x + y;
});

//1
//2
//3
//4
We can also call the original array inside of the redece loop.
var arr = [10,20,30,40,50];
var sum = arr.reduce((x,y,i,a)=>{
	console.log(a);
	return x + y;
});

//[10,20,30,40,50]
//[10,20,30,40,50]
//[10,20,30,40,50]
//[10,20,30,40,50]
This is useful when we want to do something at the end of the itiration. For example, we can obtain the mean from an array of numbers.
var arr = [1,2,3,4,5];
var mean = arr.reduce((x,y,i,a)=>{
	var l = a.length;
	if(i == l-1){
		return (x+y)/l;
	};
	return x + y;
});
console.log(mean);
//3

Comments

Post a Comment

Popular posts from this blog

How to Make A Reusable Image Slideshow HTML Component With Vanilla JavaScript

Image
The image slide show I am making has no animation. The mechanism for displaying different slide is by using the display CSS property. The image with display none will not show, while the one with display block will. I will start build one that relys on HTML code then I will build one that relys on JavaScipt that renders the slide show onto the webpage. HTML Slideshow Setup A HTML image slide show can follow the following logic: An image is shown and the others are hidden, by defining the CSS display property to "block" to show and "none" to hide. An index is used to control the CSS display property of each slide with the initial index of 1. On initiation, all image tags for the slide show will be given the CSS property correspoding to whether it's shown or not. A left arrow and a right arrow are used to control the index with onclick event handlers. In order to position the left and right control...
Read more

HTML Tags and Inline CSS that Work In Plotly.js Title

When we create a plot object with Plotly.js, we throw in 3 basic parameters in the Plotly object which are the plot container id, data and layout, like this one: Plotly.newPlot('myDiv', data, layout); And in the layont object, there is a property, called title. The title property can be defined with an string, which will simply be just the title text that is going to display on the top of the plot.  I want to style my chart title when I use Plotly.js.; var layout = { title:`Plot Title` } Or in the text object within the title property. var layout = { title:{text:`Plot Title`} } Plot;y'js has some built-in title properties which we can use to style the title, sush as font color, family, size, padding, and position with x or y. var layout = { title: { text:'Plot Title', font: { family: 'Courier New, monospace', size: 24 }, xref: 'paper', x: 0.05, } }; Problems But I want to add a HTML tag...
Read more

How to Type Spaces In HTML Input And Display In the Browser

Image
When we type a space in an input and use it in JavaScript. The space will be recorded as text just like regular texts. Type In Multiple Spaces When we type some text with many spaces and displays the text in the browser, muiltple spaces will be parsed into one space, if they are connected. Type some text with spaces here: <input oninput="show_1.innerHTML = this.value" /><br /> <div id="show_1"></div> What does JavaScript Do to the Spaces? JavaScript does nothing to the spaces. It preservs all the spaces we type. The following code converts spaces into the letter "a". Space is represnted by \x20 in JavaScript. Type some text with spaces here: <input oninput="show_2.innerHTML = this.value.replace(/\x20/g,'a');" /><br /> <div id="show_2"></div> Why doesn't the Spaces Show in the Browser? Because the connected spaces are parsed into one space in HTML. ...
Read more