- Generate Key When I Map Array Javascript Code
- Generate Key When I Map Array Javascript Code
- Javascript Map Array Keys
- Generate Key When I Map Array Javascript In Excel
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The
length
property of Array
objects is one that many who are relatively new to JavaScript do not understand. Many mistakenly believe that the length tells you exactly how many entries there are in an array whereas this is only true of some arrays. Some beginners do not even realize that length
is a writable property of arrays. To clarify just exactly how the length
property works, let’s take a look at what happens when we either change its value ourselves or run something that updates the array that also results in the length changing.May 23, 2018 JavaScript has always allowed you to work with key/value pairs through objects. The Maps collection made available with ES6 improves on that. In this tutorial we cover Maps. Jun 15, 2018 It simply loops through the array (or any iterable, really) from 0 to length and creates a new index key in the enclosing array with the value returned from the spreading array at the current index. Since JavaScript returns undefined from our spreading array at each of its indices (remember, it does this by default because it doesn’t have the. Apr 10, 2020 Visit Get Started with Google Maps Platform for full instructions or click the button below for guided setup. Get Started To use the Maps JavaScript API you must have an API key. The API key is a unique identifier that is used to authenticate requests associated with your project for usage and billing purposes. Get the API key.
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. The map method creates a new array with the results of calling a function for every array element. The map method calls the provided function once for each element in an array, in order. Note: map does not execute the function for array elements without values. Note: this method does not change the original array.
Let’s start at the beginning. A JavaScript array has a property called
length
and optionally has numbered properties with names between 0 and 4294967294 inclusive. It also has a number of methods for manipulating the properties some of which we will look at as a part of our examination of how the length property works. Note that JavaScript does not support associative arrays and so while you can add named properties to an array, they do not form a part of the array and will be ignored by all the array methods. They also will not affect the length.To make it easier to show exactly what happens to the array properties as we process various statements, we will run the following function after each piece of code. This will log the length of the array and all of the numbered properties to the browser’s console.
Creating an Array
We will begin by looking at different ways to create an array in JavaScript. The first two of these examples create arrays where only the length is set and there are no numbered entries at all. The second two create numbered entries from 0 to one less than the length.
An array where the length is greater than the amount of numbered properties is known as a sparse array while one with the length equal to the number of numbered properties is a dense array.
Note that the array literal notation (where you define a new array using just empty brackets) is preferred when creating new arrays.
The array methods that process the numbered properties (
forEach
in our case) will only process those that exist. If you instead process the array using a for
or while
loop then the loop will also attempt to process those properties that don’t exist and the array will identify those entries that don’t exist as being undefined
. Your code would then be unable to distinguish between the last of the above examples and the first two. You should always use the array methods for processing an array where you are not certain that you are dealing with a dense array.Changing the Length
The following examples look at what happens if we set a new length for the array that is less than the current length.
Note that when creating an array using
[]
notation each entry consists of a value followed by a comma. Where the value is omitted then no property is created for that position. The last comma may only be omitted if there is a value supplied for that property as otherwise the length will be reduced by one.Removing Entries
JavaScript provides three methods pop, shift and splice that can remove entries from the array and which therefore reduce the length of the array. In each case the value (or values) removed are returned by the call.
Adding Entries
Generate Key When I Map Array Javascript Code
We can add a new entry into an array simply by specifying a position in the array for which a numbered property does not yet exist. We can also use one of the three methods JavaScript provides (push, unshift and splice) for inserting new entries and, where necessary, moving the old ones.
Replacing Entries
Where we assign a new value to an entry that already exists, then that entry simply gets a new value and the rest of the array is unaffected. Also by combining the variants of the
splice()
method that we have already looked at we can replace existing entries or fill gaps in the array.Conclusion
The above examples should have given you a better idea of how the
length
property of an array works. This can be greater or equal to the number of entries in the array. Where it is equal we have a dense array and where it is greater we have a sparse array. Exactly what a particular array method does can depend on whether there is actually a property corresponding to a given position in a sparse array. If we change the length of an array it removes any numbered properties in the array that are in positions that are greater than the new length. If the length was equal to the amount of numbered properties and we increase the length then we convert a dense array to a sparse one. The array methods for deleting and adding properties in the array will move the existing entries around where necessary and will also retain and move any gaps between the properties.by Alex Permyakov
When you read about Array.reduce and how cool it is, the first and sometimes the only example you find is the sum of numbers. This is not our definition of ‘useful’. ?
Moreover, I’ve never seen it in a real codebase. But, what I’ve seen a lot is 7–8 line for-loop statements for solving a regular task where Array.reduce could do it in one line.
Recently I rewrote a few modules using these great functions. It surprised me how simplified the codebase became. So, below is a list of goodies.
If you have a good example of using a map or reduce method — post it in the comments section. ?
Let’s get started!
1. Remove duplicates from an array of numbers/strings
Well, this is the only one not about map/reduce/filter, but it’s so compact that it was hard not to put it in the list. Plus we’ll use it in a few examples too.
2. A simple search (case-sensitive)
The filter() method creates a new array with all elements that pass the test implemented by the provided function.
3. A simple search (case-insensitive)
4. Check if any of the users have admin rights
The some() method tests whether at least one element in the array passes the test implemented by the provided function.
5. Flattening an array of arrays
The result of the first iteration is equal to : […[], …[1, 2, 3]] means it transforms to [1, 2, 3] — this value we provide as an ‘acc’ on the second iteration and so on.
We can slightly improve this code by omitting an empty array
[]
as the second argument for reduce(). Then the first value of the nested will be used as the initial acc value. Thanks to Vladimir Efanov.Note that using the spread operator inside a reduce is not great for performance. This example is a case when measuring performance makes sense for your use-case. ☝️
Thanks to Paweł Wolak, here is a shorter way without Array.reduce:
Also Array.flat is coming, but it’s still an experimental feature.
6. Create an object that contains the frequency of the specified key
Let’s group and count the ‘age’ property for each item in the array:
Thanks to sai krishna for suggesting this one!
7. Indexing an array of objects (lookup table)
Instead of processing the whole array for finding a user by id, we can construct an object where the user’s id represents a key (with constant searching time).
It’s useful when you have to access your data by id like
uTable[85].name
a lot.8. Extract the unique values for the given key of each item in the array
Let’s create a list of existing users’ groups. The map() method creates a new array with the results of calling a provided function on every element in the calling array.
9. Object key-value map reversal
This one-liner looks quite tricky. We use the comma operator here, and it means we return the last value in parenthesis —
acc
. Let’s rewrite this example in a more production-ready and performant way:Here we don’t use spread operator — it creates a new array on each reduce() call, which leads to a big performance penalty: O(n²). Instead the old good push() method.
10. Create an array of Fahrenheit values from an array of Celsius values
Think of it as processing each element with a given formula ?
11. Encode an object into a query string
12. Print a table of users as a readable string only with specified keys
Sometimes you want to print your array of objects with selected keys as a string, but you realize that JSON.stringify is not that great ?
JSON.stringify can make the string output more readable, but not as a table:
13. Find and replace a key-value pair in an array of objects
Let’s say we want to change John’s age. If you know the index, you can write this line:
users[1].age = 29
. However, let’s take a look at another way of doing it:Here instead of changing the single item in our array, we create a new one with only one element different. Now we can compare our arrays just by reference like
updatedUsers users
which is super quick! React.js uses this approach to speed up the reconciliation process. Here is an explanation.14. Union (A ∪ B) of arrays
![Array Array](/uploads/1/2/6/1/126195695/354783104.png)
Less code than importing and calling the lodash method union.
15. Intersection (A ∩ B) of arrays
The last one!
As an exercise try to implement difference (A B) of the arrays. Hint: use an exclamation mark.
Generate Key When I Map Array Javascript Code
Thanks to Asmor and incarnatethegreat for their comments about #9.
That’s it!
If you have any questions or feedback, let me know in the comments down below or ping me on Twitter.
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Generate Key When I Map Array Javascript In Excel
Thanks for reading ❤️