Javascript array of lists

JavaScript Arrays

An array is a special variable, which can hold more than one value:

Why Use Arrays?

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

An array can hold many values under a single name, and you can access the values by referring to an index number.

Creating an Array

Using an array literal is the easiest way to create a JavaScript Array.

It is a common practice to declare arrays with the const keyword.

Learn more about const with arrays in the chapter: JS Array Const.

Example

Spaces and line breaks are not important. A declaration can span multiple lines:

Example

You can also create an array, and then provide the elements:

Example

Using the JavaScript Keyword new

The following example also creates an Array, and assigns values to it:

Example

The two examples above do exactly the same.

There is no need to use new Array() .

For simplicity, readability and execution speed, use the array literal method.

Accessing Array Elements

You access an array element by referring to the index number:

Note: Array indexes start with 0.

[0] is the first element. [1] is the second element.

Changing an Array Element

This statement changes the value of the first element in cars :

Example

Converting an Array to a String

The JavaScript method toString() converts an array to a string of (comma separated) array values.

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];
document.getElementById(«demo»).innerHTML = fruits.toString();

Access the Full Array

With JavaScript, the full array can be accessed by referring to the array name:

Example

Arrays are Objects

Arrays are a special type of objects. The typeof operator in JavaScript returns «object» for arrays.

But, JavaScript arrays are best described as arrays.

Arrays use numbers to access its «elements». In this example, person[0] returns John:

Array:

Objects use names to access its «members». In this example, person.firstName returns John:

Object:

Array Elements Can Be Objects

JavaScript variables can be objects. Arrays are special kinds of objects.

Because of this, you can have variables of different types in the same Array.

You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:

Array Properties and Methods

The real strength of JavaScript arrays are the built-in array properties and methods:

Array methods are covered in the next chapters.

The length Property

The length property of an array returns the length of an array (the number of array elements).

Example

The length property is always one more than the highest array index.

Accessing the First Array Element

Example

Accessing the Last Array Element

Example

Looping Array Elements

One way to loop through an array, is using a for loop:

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];
let fLen = fruits.length;

You can also use the Array.forEach() function:

Example

const fruits = [«Banana», «Orange», «Apple», «Mango»];

Adding Array Elements

The easiest way to add a new element to an array is using the push() method:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits.push(«Lemon»); // Adds a new element (Lemon) to fruits

New element can also be added to an array using the length property:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits[fruits.length] = «Lemon»; // Adds «Lemon» to fruits

Adding elements with high indexes can create undefined «holes» in an array:

Example

const fruits = [«Banana», «Orange», «Apple»];
fruits[6] = «Lemon»; // Creates undefined «holes» in fruits

Associative Arrays

Many programming languages support arrays with named indexes.

Arrays with named indexes are called associative arrays (or hashes).

JavaScript does not support arrays with named indexes.

In JavaScript, arrays always use numbered indexes.

Example

const person = [];
person[0] = «John»;
person[1] = «Doe»;
person[2] = 46;
person.length; // Will return 3
person[0]; // Will return «John»

WARNING !!
If you use named indexes, JavaScript will redefine the array to an object.

After that, some array methods and properties will produce incorrect results.

Example:

const person = [];
person[«firstName»] = «John»;
person[«lastName»] = «Doe»;
person[«age»] = 46;
person.length; // Will return 0
person[0]; // Will return undefined

The Difference Between Arrays and Objects

In JavaScript, arrays use numbered indexes.

In JavaScript, objects use named indexes.

Arrays are a special kind of objects, with numbered indexes.

When to Use Arrays. When to use Objects.

• JavaScript does not support associative arrays.
• You should use objects when you want the element names to be strings (text).
• You should use arrays when you want the element names to be numbers.

JavaScript new Array()

JavaScript has a built-in array constructor new Array() .

But you can safely use [] instead.

These two different statements both create a new empty array named points:

These two different statements both create a new array containing 6 numbers:

The new keyword can produce some unexpected results:

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How do I create a javascript array of lists of objects?

Solution 1: Starting from: You only need create a new array object: and then, insert all arrays that you want into the new array: With push() method the array object is added at end of the new array. Question: I’m trying to make a list of arrays in JAVASCRIPT but my program doesn’t do what I expect.

How do I create a javascript array of lists of objects?

I need to create a dictionary of lists. Is that possible in Javascript? I am looking for something that will let me add objects for a feature/subfeature pairs and also iterate the feature/subfeature collections. My feature/subfeature data is a series of integer pairs:

where the 1st number is the feature index, and the second number is the subfeature index. Each of these pairs can have a list of objects. I want to iterate the list by feature index and them by objects. Something like

forEach( item where feature index == someIndex, function(foo) < forEach (item[someindex, foo.index] , function(bar) < display bar.prop1, bar.prop2, . 

I will making a database call and add the results as items to this structure.

This structure is emulating something I put together in .Net using a dictionary that used a tuple as a key and list of objects as the value. The declaration was :

A simple solution would be simply nested arrays, so something like

So each time you push to the array, you just add a new array as the entry

Then I would keep a separate array to store the actual features/subfeatures and access them in directly using the number. So something like:

Hope that puts you in the right direction!

This example may be a bit more than you need. But perhaps you will find benefits.

//Object representing a Feature function Feature(featureID, subFeatureID, list) < this.featureID = featureID; this.subFeatureID = subFeatureID; this.list = list; >//Object to hold features function FeatureCollection() < this._collection = new Array(); >//Augment the FeatureCollection prototype FeatureCollection.prototype.add = function(feature) < this._collection.push(feature); >; FeatureCollection.prototype.foreach = function(someIndex, listFunction) < //For each feature set, loop within the collection //until the someIndex is found for(var i=0,length=this._collection.length;ibreak; > > > //Create a feature collection var featureCollection = new FeatureCollection(); //Create a new feature var testFeature = new Feature(1,2,["hello","world"]) //Add the feature to the collection featureCollection.add(testFeature) //Iterate the collection invoking the provided anonymous //function if the index passed matches featureCollection.foreach(1, function(listItem) < console.log("FeatureID: " + this.featureID + " ListItemValue:" + listItem) >); 

If you don't need anything fancy, and you know the limits of both arrays , there is a trick you can do.

Some might consider it hacky, others would consider it elegant.

Instead of using an array, you could use an object and hashes. Turn the two indexes into a string value to use as the hash key.

var myVals = <>; myVals["1,4"] = "Hi"; myVals["9,5"] = "There"; for (var i = 0; i < 10; i++) < for (j = 0; j < 10; j++) < var key = i + "," + j; var val = myValsJavascript array of lists; if (val) < // do something >> 

How to create list of objects in javascript, var employee = < Column1: null, Column2: null, create: function () < var obj = new Object(); obj.Column1 = "";

Sorting a list of lists in javascript

I'm trying to sort a list of lists in ascending order based on the second element in javascript. I'm following this answer javascript sort list of lists by sublist second entry but my list remains the same.

var a = [ [[1,2,3],10], [[5,6,7],0] ] a.sort(function(x,y) y[1];>); 

Instead of comparing them with > , you should subtract one from the other using - to sort in ascending order:

var a = [ [[1,2,3],10], [[5,6,7],0] ] a.sort(function(x,y)); 

var a = [ [[1,2,3],10], [[5,6,7],0] ] a.sort(function(x,y)); 

Javascript sorted lists, 1 Answer. No, there isn't, all you have is Array#sort which you've ruled out using repeatedly (and with good reason!). You'll have to use an insertion sort approach. However, it also states that it's "efficient for data sets that are already substantially sorted".

Creating list of objects in Javascript

Is it possible to do create a list of your own objects in Javascript ? This is the type of data I want to store :

Date : 12/1/2011 Reading : 3 ID : 20055 Date : 13/1/2011 Reading : 5 ID : 20053 Date : 14/1/2011 Reading : 6 ID : 45652 

dynamically build list of objects

var listOfObjects = []; var a = ["car", "bike", "scooter"]; a.forEach(function(entry) < var singleObj = <>; singleObj['type'] = 'vehicle'; singleObj['value'] = entry; listOfObjects.push(singleObj); >); 

here's a working example http://jsfiddle.net/b9f6Q/2/ see console for output

Maybe you can create an array like this:

var myList = new Array(); myList.push('Hello'); myList.push('bye'); for (var i = 0; i

Going off of tbradley22's answer, but using .map instead:

var a = ["car", "bike", "scooter"]; a.map(function(entry) < var singleObj = <>; singleObj['type'] = 'vehicle'; singleObj['value'] = entry; return singleObj; >); 

Javascript - Get list of items in one list that are not in, Convert the list to be checked, to an object, in liner time. Because, objects are technically hashtables, which offer faster lookups O(1)). Then, iterate over the first list and check if the current element is there in the object or not. If it is not there, add it to the result.

List of arrays in Javascript

I'm trying to make a list of arrays in JAVASCRIPT but my program doesn't do what I expect.

I want to make a list of arrays like this:

var myListofArrays; var firstArray = [1,2,3,4,5,6]; var secondArray = [6,5,4,3,2,1]; var thirdArray = [1,1,1]; 

I want to add those arrays in myListArray to finally get something like that:

myListofArrays[0] = [1,2,3,4,5,6]; myListofArrays[1] = [6,5,4,3,2,1]; myListofArrays[2] = [1,1,1]; 

Anyone knows how to do it?

var firstArray = [1,2,3,4,5,6]; var secondArray = [6,5,4,3,2,1]; var thirdArray = [1,1,1]; 

You only need create a new array object:

and then, insert all arrays that you want into the new array:

mylistOfArrays.push(firstArray); mylistOfArrays.push(secondArray); mylistOfArrays.push(thirdArray); 

With push() method the array object is added at end of the new array. This method also returns the new length.

You can visit this reference to get a detailed list of array methods: Array Methods

There are many ways to do this. Try:

var myListofArrays = []; var firstArray = [1,2,3,4,5,6]; var secondArray = [6,5,4,3,2,1]; var thirdArray = [1,1,1]; myListofArrays.push(firstArray); myListofArrays.push(secondArray); myListofArrays.push(thirdArray); 

myListofArrays.push(firstArray, secondArray, thirdArray); 

var firstArray = [1,2,3,4,5,6], secondArray = [6,5,4,3,2,1], thirdArray = [1,1,1], myListOfArrays = [firstArray, secondArray, thirdArray]; 

And there are still probably plenty other valid ways to do this.

Beside Andrew Clavin's answer you can also try this:

var myListofArrays =[]; var firstArray = [1,2,3,4,5,6]; var secondArray = [6,5,4,3,2,1]; var thirdArray = [1,1,1]; myListofArrays[0]=firstArray; myListofArrays[1]=secondArray; myListofArrays[2]=thirdArray; console.log(myListofArrays); 

Hi I think you are almost there.

You need to declare your variable as an array before you start dumping things in it.

If you want to shorten down your code, you could also try this:

var myListofArrays = []; var firstArray = [1,2,3,4,5,6]; var secondArray = [6,5,4,3,2,1]; var thirdArray = [1,1,1]; myListofArrays = [firstArray, secondArray, thirdArray]; 

This will push all of your declared arrays into your master array(myListofArrays) and because you declared it as an array at the top, the javascript will understand it is an array and push the rest of the arrays in accordingly.

Javascript - List of Lists in Immutable.js, @mhodges Totally agree with you on that. The fact that class is being pushed hard and taxonomy is getting into JS is concerning. TypeScript is also gaining a lot of ground which i don't think its a bad thing, but it concerns me to start seeing code written only in TypeScript because some people just cant live …

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