Javascript map get by index

Javascript map get by index

Last updated: Dec 29, 2022
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# Get the First Element of a Map in JavaScript

Use destructuring assignment to get the first element of a Map , e.g. const [firstKey] = map.keys() and const [firstValue] = map.values() .

The keys() and values() methods return an iterator object that contains the Map’s keys and values.

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const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); const [firstKey] = map.keys(); console.log(firstKey); // 👉️ site const [firstValue] = map.values(); console.log(firstValue); // 👉️ bobbyhadz.com 

We called the keys() method on the Map to get an iterator that contains the keys of the elements in the Map .

Copied! 
const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); // 👇️ [Map Iterator] console.log(map.keys()); const [firstKey] = map.keys(); console.log(firstKey); // 👉️ site 

We used destructuring assignment to get the first key and assigned it to a variable.

You can use the Map.values() method to get an iterator of the Map’s values.

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const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); // 👇️ [Map Iterator] console.log(map.values()); const [firstValue] = map.values(); console.log(firstValue); // 👉️ bobbyhadz.com 

The last step is to use destructuring assignment to assign the first Map value to a variable.

An alternative approach is to convert the Map to an array.

# Get the First Element of a Map using spread syntax (. )

This is a two-step process:

1. Use the spread syntax (. ) to convert the Map object to an array.
2. Access the array at index 0 to get the first element of the Map .

Copied! 
const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); const first = [. map][0]; console.log(first); // 👉️ [ 'site', 'bobbyhadz.com' ] 

We used the spread syntax (. ) to convert the Map to an array and accessed the array element at index 0 .

Copied! 
const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); // 👇️ [ [ 'site', 'bobbyhadz.com' ], [ 'id', 1 ] ] console.log([. map]); // 👇️ [ 'site', 'bobbyhadz.com' ] console.log([. map][0]); 

Accessing the first array element returns an array containing the key and value of the first element in the Map .

Note that converting a Map with thousands of elements to an array would be slow and inefficient if you only have to access a single element.

Alternatively, we can use some of the methods on the Map object.

# Get the First Element of a Map using Map.entries()

This is a three-step process:

1. Use the Map.entries() method to get an iterable of the Map’s key-value pairs.
2. Use the next() method to get the first element of the iterable.
3. Assign the value to a variable.

Copied! 
const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); const first = map.entries().next().value; // 👇️ [ 'site', 'bobbyhadz.com' ] console.log(first); console.log(first[0]); // 👉️ site console.log(first[1]); // 👉️ bobbyhadz 

We used the Map.entries() method to get an iterable of the Map’s key-value pairs.

Copied! 
const map = new Map(); map.set('site', 'bobbyhadz.com'); map.set('id', 1); const iterable = map.entries(); // 👇️ [Map Entries] console.log(iterable); // 👇️ const firstIteration = iterable.next(); console.log(firstIteration); const first = firstIteration.value; console.log(first); // 👉️ [ 'site', 'bobbyhadz.com' ] 

We used the next() method to get an object containing the value of the first iteration.

Lastly, we accessed the value property on the object to get an array of the key and value of the first Map element.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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JavaScript map with index [SOLVED]

Higher-order functions take other functions as arguments and/or return a function value, and in JavaScript, we have built-in higher-order functions such as filter , reduce , and map which are available based on the array prototype.

However, our focus is on the map function which allows us to create a new array by applying a callback function on every element in the array it’s called on. In this article, we will discuss how map works and how to work with map with index.

How JavaScript Map Works

As stated, map creates or returns a new array after the effects of a callback function have been done on an array’s elements.

Let’s illustrate how map works by changing the elements of an array by either multiplying them or adding the string “added” if the element is a Number or String . In the illustration, the map function takes a single callback function (an arrow function), and this arrow function takes one argument (the element at every iterative instance) and as within its function body the conditional logic that applies the necessary action on the element depending on their type .

const arr = [12, 34, "one", "two", 56, "four"]; const arrOptimized = arr.map((x) => < if (typeof x === "string") < return x + "added"; >else if (typeof x === "number") < return x * 2; >>); console.log(arrOptimized); 

[ 24, 68, 'oneadded', 'twoadded', 112, 'fouradded' ]

Also, remember that the map function doesn’t change the content of the array it is applied on, it only returns a new array.

JavaScript Map With Index

Now, if we need to have access to the index of each element when using the map function, it is fairly easy to access it. For the example in the previous section, the callback function took only one argument — the element — which is required. We can take other arguments which are optional, and the index argument is one such.

Therefore, if we need the index , we can add a new argument to our callback function and make use of the argument. Using the same illustration as in the previous section, we can log an updated statement with the index position.

const arr = [12, 34, "one", "two", 56, "four"]; const arrOptimized = arr.map((x, y) => < if (typeof x === "string") < console.log(`Element "$" of the index $ has been optimized`); return x + "added"; > else if (typeof x === "number") < console.log(`Element "$" of the index $ has been optimized`); return x * 2; > >); console.log(arrOptimized); 

Element 12 of the index 0 has been optimized Element 34 of the index 1 has been optimized Element "one" of the index 2 has been optimized Element "two" of the index 3 has been optimized Element 56 of the index 4 has been optimized Element "four" of the index 5 has been optimized [ 24, 68, 'oneadded', 'twoadded', 112, 'fouradded' ]

The x and y bindings represent the element and the index of the said element at every iterative instance.

Summary

To work with map is an interesting approach, especially with composability, and if you need access to the index of the element within the map , you can make use of the second argument that’s optional for your callback function. With that, map with index is possible and easy to use.

References

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  How to Get the First Element of a Map in JavaScript (Easy Ways)

  

  In this article, we’ll be exploring some ways to quickly get the first element of a Map object in JavaScript.

  1. Call next() on Map entries()

  To get the first element of a Map , we can call the entries() on the Map to get an iterable object, then call the next() method on this iterable. For example:

  const map = new Map([ ['key1', 'value1'], ['key2', 'value2'], ['key3', 'value3'], ]); const firstElement = map.entries().next().value; console.log(firstElement); // [ 'key1', 'value1' ] 

  The Map entries() method returns an iterable of key-value pairs for all elements of the Map . The next() method returns the next element in the iterable sequence. Since it’s the first time we’re calling it on the iterable, it returns the first element in the sequence. We use the value property of the element to get the key-value pair representing the first element of the Map .

  2. Array.from()

  We can also use the Array.from() method to get the first element of the Map :

  const map = new Map([ ['key1', 'value1'], ['key2', 'value2'], ['key3', 'value3'], ]); const firstElement = Array.from(map)[0]; console.log(firstElement); // [ 'key1', 'value1' ] 

  Note

  On a Map with many elements, this method is significantly slower than the first, as it creates a new array from all the Map elements. We conducted a performance comparison between the two methods on a Map with 1 million elements, and these were the results on average:

  Iterable next(): 0.015ms Array from() 251.093ms

  11 Amazing New JavaScript Features in ES13

  This guide will bring you up to speed with all the latest features added in ECMAScript 13. These powerful new features will modernize your JavaScript with shorter and more expressive code.

  

  Ayibatari Ibaba is a software developer with years of experience building websites and apps. He has written extensively on a wide range of programming topics and has created dozens of apps and open-source libraries.

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