Jun 15, 2018 This explains why our callback is never called and nothing happens when we call the map function on the array — there are no index keys! As you now know, what we need is an array whose internal object representation contains a key for each number from 0 to length. The best way to do this is to spread the array out into an empty array.

1. Javascript Filter
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3. Create Array Of Numbers Javascript
4. Generate Key Number Javascript Map Array Example

1. This will be O(n) where n is the number of objects in array and m is the number of unique values. There is no faster way than O(n) because you must inspect each value at least once. The previous version of this used an object, and for in. These were minor in nature, and have since been minorly updated above.
2. Create Key Value Pair Array Using Javascript, Our today's article is simple but demanding. Most of the new or experience java-script developer required below code. In this code snippet I have define java-script array values are in the form of key and value.

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:

Javascript Filter

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

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.

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.

If this was useful, please click the clap ? button down below a few times to show your support! ⬇⬇ ??

Here are more articles I’ve written:

Production ready Node.js REST APIs Setup using TypeScript, PostgreSQL and Redis.
A month ago I was given a task to build a simple Search API. All It had to do is to grab some data from 3rd party…

Thanks for reading ❤️

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:

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• 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.

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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.

Generate Key Number Javascript Map Array Example

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.