JavaScript Tutorial Series – Arrays

In JavaScript, arrays store a list of items. An item may be a primitive such as a string or number. An item may also be an object or another array. Indexing starts at 0.
Arrays are objects. To determine if a variable is an array, use

 Object.prototype.toString.call(theArray) === '[object Array]'

or the newer way of

 Array.isArray(theArray);

Some of the more commonly used actions that may be performed on an array include:

Initialize:

 let myArray = ['a', 'b', 'c'];

Arrays can be created by calling:

 let myArray2 = new Array('a', 'b', 'c');

The first way is much simpler, so there’s not really any reason to use this way.

To retrieve the number of items in the list, do the following:

 let count = myArray.length;

Individual items in an array may be accessed by it’s index. To access the first item, call

 let firstItem = myArray[0];

To access the last item, call

 let lastItem = myArray[myArray.length-1];

Add an item to the end of the list:

 myArray.push('d');

Add an item to the beginning of the list:

 myArray.unshift('d');

Remove last item:

 myArray.pop();

Remove first item:

 myArray.shift();

Get the number of items in an array:

 let count = myArray.length();

Get the index of an item in an array: Note: this returns -1 if the item is not found:

 let index = myArray.indexOf('b');

Return a certain range of items, based on index, from an array:

 let subset = myArray.slice(1, 2);

I tend to use objects more than arrays, but one common way to use arrays is when extracting pieces of a string. A string, such as an id, may contain several pieces of information separated by a delimiter. For example, the following string could represent a date with a dash as the delimiter:

01-20-2020

The month, date and year could be extracted as follows:

 let dateStr = '01-20-2020'; let datePieces = dateStr.split('-'); console.log('Month: ' + datePieces[0]); console.log('Day: ' + datePieces[1]); console.log('Year: ' + datePieces[2]);

An array may store another array, creating a multidimensional array. For example:

 let array2d = [[0,0], [3,5]];

Arrays can be searched using the Array.prototype.find function. A callback function which defines the search criteria is passed into the find function. The result is then returned.
Example:

 function search (item) { return item === 'b'; } let bElement = myArray.find(search);

Implementation:

Inside the javascript_tutorial.js file created in the JavaScript set up tutorial, add the following lines that are in bold:

// JavaScript Tutorial /* * This file will add JavaScript * to a website. */ (function() { let doc = document;  let defaultLocation = [-86.7816, 36.1627]; // longitude, latitude })();

The JavaScript tutorial series starts with this post.

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