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Generate Key Number Javascript Map Array

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  1. Javascript Filter
  2. Javascript Map Array Keys
  3. Generate Key Number Javascript Map Array Examples
  4. Javascript Map Function

Now we’ve learned about the following complex data structures:

  • Objects for storing keyed collections.
  • Arrays for storing ordered collections.

But that’s not enough for real life. That’s why Map and Set also exist.

Map

Map is a collection of keyed data items, just like an Object. But the main difference is that Map allows keys of any type.

Methods and properties are:

  • new Map() – creates the map.
  • map.set(key, value) – stores the value by the key.
  • map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
  • map.has(key) – returns true if the key exists, false otherwise.
  • map.delete(key) – removes the value by the key.
  • map.clear() – removes everything from the map.
  • map.size – returns the current element count.

For instance:

As we can see, unlike objects, keys are not converted to strings. Any type of key is possible.

Although map[key] also works, e.g. we can set map[key] = 2, this is treating map as a plain JavaScript object, so it implies all corresponding limitations (no object keys and so on).

So we should use map methods: set, get and so on.

Map can also use objects as keys.

For instance:

Using objects as keys is one of most notable and important Map features. For string keys, Object can be fine, but not for object keys.

Let’s try:

As visitsCountObj is an object, it converts all keys, such as john to strings, so we’ve got the string key '[object Object]'. Definitely not what we want.

To test keys for equivalence, Map uses the algorithm SameValueZero. It is roughly the same as strict equality , but the difference is that NaN is considered equal to NaN. So NaN can be used as the key as well.

This algorithm can’t be changed or customized.

Every map.set call returns the map itself, so we can “chain” the calls:

Iteration over Map

For looping over a map, there are 3 methods:

  • map.keys() – returns an iterable for keys,
  • map.values() – returns an iterable for values,
  • map.entries() – returns an iterable for entries [key, value], it’s used by default in for.of.

For instance:

The iteration goes in the same order as the values were inserted. Map preserves this order, unlike a regular Object.

Besides that, Map has a built-in forEach method, similar to Array:

Object.entries: Map from Object

When a Map is created, we can pass an array (or another iterable) with key/value pairs for initialization, like this:

If we have a plain object, and we’d like to create a Map from it, then we can use built-in method Object.entries(obj) that returns an array of key/value pairs for an object exactly in that format.

So we can create a map from an object like this:

Here, Object.entries returns the array of key/value pairs: [ ['name','John'], ['age', 30] ]. That’s what Map needs.

Object.fromEntries: Object from Map

We’ve just seen how to create Map from a plain object with Object.entries(obj).

There’s Object.fromEntries method that does the reverse: given an array of [key, value] pairs, it creates an object from them:

We can use Object.fromEntries to get an plain object from Map.

E.g. we store the data in a Map, but we need to pass it to a 3rd-party code that expects a plain object.

Here we go:

A call to map.entries() returns an array of key/value pairs, exactly in the right format for Object.fromEntries.

We could also make line (*) shorter:

That’s the same, because Object.fromEntries expects an iterable object as the argument. Not necessarily an array. And the standard iteration for map returns same key/value pairs as map.entries(). So we get a plain object with same key/values as the map.

Set

A Set is a special type collection – “set of values” (without keys), where each value may occur only once.

Its main methods are:

  • new Set(iterable) – creates the set, and if an iterable object is provided (usually an array), copies values from it into the set.
  • set.add(value) – adds a value, returns the set itself.
  • set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
  • set.has(value) – returns true if the value exists in the set, otherwise false.
  • set.clear() – removes everything from the set.
  • set.size – is the elements count.

The main feature is that repeated calls of set.add(value) with the same value don’t do anything. That’s the reason why each value appears in a Set only once.

For example, we have visitors coming, and we’d like to remember everyone. But repeated visits should not lead to duplicates. A visitor must be “counted” only once.

Set is just the right thing for that:

The alternative to Set could be an array of users, and the code to check for duplicates on every insertion using arr.find. But the performance would be much worse, because this method walks through the whole array checking every element. Set is much better optimized internally for uniqueness checks.

Iteration over Set

We can loop over a set either with for.of or using forEach:

Note the funny thing. The callback function passed in forEach has 3 arguments: a value, then the same valuevalueAgain, and then the target object. Indeed, the same value appears in the arguments twice.

That’s for compatibility with Map where the callback passed forEach has three arguments. Looks a bit strange, for sure. But may help to replace Map with Set in certain cases with ease, and vice versa.

The same methods Map has for iterators are also supported:

  • set.keys() – returns an iterable object for values,
  • set.values() – same as set.keys(), for compatibility with Map,
  • set.entries() – returns an iterable object for entries [value, value], exists for compatibility with Map.

Summary

Map – is a collection of keyed values.

Methods and properties:

  • new Map([iterable]) – creates the map, with optional iterable (e.g. array) of [key,value] pairs for initialization.
  • map.set(key, value) – stores the value by the key.
  • map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
  • map.has(key) – returns true if the key exists, false otherwise.
  • map.delete(key) – removes the value by the key.
  • map.clear() – removes everything from the map.
  • map.size – returns the current element count.

The differences from a regular Object:

  • Any keys, objects can be keys.
  • Additional convenient methods, the size property.

Set – is a collection of unique values.

Methods and properties:

  • new Set([iterable]) – creates the set, with optional iterable (e.g. array) of values for initialization.
  • set.add(value) – adds a value (does nothing if value exists), returns the set itself.
  • set.delete(value) – removes the value, returns true if value existed at the moment of the call, otherwise false.
  • set.has(value) – returns true if the value exists in the set, otherwise false.
  • set.clear() – removes everything from the set.
  • set.size – is the elements count.

Iteration over Map and Set is always in the insertion order, so we can’t say that these collections are unordered, but we can’t reorder elements or directly get an element by its number.

JavaScript arrays are used to store multiple values in a single variable.

Example

Try it Yourself »

What is an Array?

An array is a special variable, which can hold more than one value at a time.

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

var car1 = 'Saab';
var car2 = 'Volvo';
var car3 = 'BMW';

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?

The solution is an array!

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.

Syntax:

Example

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

Example

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Using the JavaScript Keyword new

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

Example

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The two examples above do exactly the same. There is no need to use new Array().
For simplicity, readability and execution speed, use the first one (the array literal method).

Access the Elements of an Array

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

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

Example

var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »

Note: Array indexes start with 0. Asteps to take after generating a new key.

[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

var cars = ['Saab', 'Volvo', 'BMW'];
cars[0] = 'Opel';
document.getElementById('demo').innerHTML = cars[0];
Try it Yourself »

Access the Full Array

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

Example

var cars = ['Saab', 'Volvo', 'BMW'];
document.getElementById('demo').innerHTML = cars;
Try it Yourself »

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:

Try it Yourself »

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

Object:

var person = {firstName:'John', lastName:'Doe', age:46};
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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:

myArray[0] = Date.now;
myArray[1] = myFunction;
myArray[2] = myCars;

Array Properties and Methods

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

Examples

var x = cars.length; // The length property returns the number of elements
var y = cars.sort(); // The sort() method sorts arrays

Array methods are covered in the next chapters.

Javascript Filter

The length Property

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

Example

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.length; // the length of fruits is 4
Try it Yourself »

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

Accessing the First Array Element

Example

fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var first = fruits[0];
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Accessing the Last Array Element

Example

fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
var last = fruits[fruits.length - 1];
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Looping Array Elements

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

Example

var fruits, text, fLen, i;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fLen = fruits.length;
text = '<ul>';
for (i = 0; i < fLen; i++) {
text += '<li>' + fruits[i] + '</li>';
}
text += '</ul>';
Try it Yourself »

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

Example

var fruits, text;
fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
text = '<ul>';
fruits.forEach(myFunction);
text += '</ul>';
function myFunction(value) {
text += '<li>' + value + '</li>';
}
Try it Yourself »

Adding Array Elements

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

Javascript Map Array Keys

Example

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits.push('Lemon'); // adds a new element (Lemon) to fruits
Try it Yourself »

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

Example

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[fruits.length] = 'Lemon'; // adds a new element (Lemon) to fruits
Try it Yourself »

WARNING !

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

Example

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits[6] = 'Lemon'; // adds a new element (Lemon) to fruits
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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

var person = [];
person[0] = 'John';
person[1] = 'Doe';
person[2] = 46;
var x = person.length; // person.length will return 3
var y = person[0]; // person[0] will return 'John'
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WARNING !!
If you use named indexes, JavaScript will redefine the array to a standard object.
After that, some array methods and properties will produce incorrect results.

Example:

var person = [];
person['firstName'] = 'John';
person['lastName'] = 'Doe';
person['age'] = 46;
var x = person.length; // person.length will return 0
var y = person[0]; // person[0] will return undefined
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The Difference Between Arrays and Objects

Generate Key Number Javascript Map Array Examples

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.

Avoid new Array()

There is no need to use the JavaScript's built-in array constructor new Array().

Use [] instead.

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

var points = new Array(); // Bad
var points = []; // Good

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

var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
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The new keyword only complicates the code. It can also produce some unexpected results:

var points = new Array(40, 100); // Creates an array with two elements (40 and 100)

What if I remove one of the elements?

var points = new Array(40); // Creates an array with 40 undefined elements !!!!!

Javascript Map Function

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How to Recognize an Array

A common question is: How do I know if a variable is an array?

The problem is that the JavaScript operator typeof returns 'object':

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
typeof fruits; // returns object
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The typeof operator returns object because a JavaScript array is an object.

Solution 1:

To solve this problem ECMAScript 5 defines a new method Array.isArray():

The problem with this solution is that ECMAScript 5 is not supported in older browsers.

Solution 2:

To solve this problem you can create your own isArray() function:

function isArray(x) {
return x.constructor.toString().indexOf('Array') > -1;
}
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The function above always returns true if the argument is an array.

Or more precisely: it returns true if the object prototype contains the word 'Array'.

Solution 3:

The instanceof operator returns true if an object is created by a given constructor:

var fruits = ['Banana', 'Orange', 'Apple', 'Mango'];
fruits instanceof Array; // returns true
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