Map.entries( ) In JavaScript

What is a map in JavaScript?

  • Map is a data structure in JavaScript which allows storing of [key, value] pairs where any value can be either used as a key or value.
  • The keys and values in the map collection may be of any type and if a value is added to the map collection using a key which already exists in the collection, then the new value replaces the old value.
  • The iteration of elements in a map object is done in the insertion order and a “for…” loop returns an array of all [key, value] pairs for each iteration.

Differences between Objects and Maps in JavaScript
Both of these data structures are similar in many ways such as both store values using keys, allow retrieval of those values using keys, deletion of keys and verify whether a key holds any value or not. However, there are quite significant differences between Objects and Maps in JavaScript which make the usage of maps a better and more preferable option in many cases.

  • The keys used in maps can be any type of values such as functions, objects etc whereas the keys in objects are limited to symbols and strings.
  • The size of a map can be known easily by using the size property but while dealing with objects, the size has to be determined manually.
  • A Map should be preferred in cases where the requirement involves frequent addition and removal of [key, value] pairs because a map is an iterative data type and can be directly iterated whereas iterating an object requires obtaining its keys in a specific manner.

Map.entries() Method in JavaScript
The Map.entries() method in JavaScript is used for returning an iterator object which contains all the [key, value] pairs of each element of the map. It returns the [key, value] pairs of all the elements of a map in the order of their insertion.
The Map.entries() method does not require any argument to be passed and returns an iterator object of the map.
Applications:
Whenever we want to get all the [key, value] pairs of each element of a map using an iterator object, we use the Map.entries() method.
Syntax:



mapObj.entries()

Parameters Used:

  • It does not require any parameters to be passed.

Return Value:

  • The Map.entries() method returns the [key, value] pairs of all the elements of a map in the order of their insertion.

Examples of the above function are provided below.

Example:

Input : var myMap = new Map();
        myMap.set(0, 'geeksforgeeks');
        myMap.set(1, 'is an online portal');
        myMap.set(2, 'for geeks');
        var iterator_obj=myMap.entries();
        document.write(iterator_obj.next().value,"<br>");
        document.write(iterator_obj.next().value,"<br>");
        document.write(iterator_obj.next().value,"<br>");


Output : Array [0, "geeksforgeeks"]
         Array [1, "is an online portal"]
         Array [2, "for geeks"]

Explanation: In this example, a map object “myMap” has been created with a three [key, value] pairs and an iterator object “iterator_obj” method is created which uses Map.entries() method to return the [key, value] pairs of all the elements of a map in the order of their insertion.

Code 1:

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<script>
    // creating a map object
    var myMap = new Map();
  
// Adding [key, value] pair to the map
myMap.set(0, 'geeksforgeeks');
myMap.set(1, 'is an online portal');
myMap.set(2, 'for geeks');
  
// creating an iterator object using Map.entries() method
var iterator_obj = myMap.entries();
  
// displaying the [key, value] pairs of all the elements of the map
document.write(iterator_obj.next().value,"</br>");
document.write(iterator_obj.next().value,"</br>");
document.write(iterator_obj.next().value,"</br>");
  
</script>                    

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Output :

     Array [0, "geeksforgeeks"]
     Array [1, "is an online portal"]
     Array [2, "for geeks"]

Code 2:

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<script>
    // creating a map object
    var myMap = new Map();
  
// Adding [key, value] pair to the map
myMap.set(0, 'Maps');
myMap.set(1, 'in JavaScript');
  
// creating an iterator object using Map.entries() method
var iterator_obj = myMap.entries();
  
// displaying the [key, value] pairs of all the elements of the map
document.write(iterator_obj.next().value,"</br>");
document.write(iterator_obj.next().value,"</br>");
  
</script>                    

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Output :

     Array [0, "Maps"]
     Array [1, "in JavaScript"]

Exceptions :

  • If the variable is not of the Map type then the Map.entries() operation throws a TypeError.
  • If the “iterator_obj.next().value” is used more number of times as compared to [key, value] pairs of a map, the Map.entries() function returns undefined for all those cases.

Reference :https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/entries



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