Check on array created in map()

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map() returns an array as a result (resultArr). Is it possible to perform checks on the 'intermediatly' created array (intermediateArr) within the map function? Somewhere, the resultArr muste be stored so that the results of the map() function can be concatenated or something. Is it possible to access it?

Let's assume I want to push values to an array, if they are not included. I could do something like

someArr.map(item => {
    let resultArr= [];
    if(!resultArr.includes(item)) {
        resultArr.push(item);
    }

However, map already creates an array. It would be a 'waste' not using it. So something like

const resultArr = someArr.map(item => {
    if(!intermediateArr.includes(item)) {
        return item
    }

Where intermediateArr is the Array that gets updated after each iteration from the map() function, the array, that finally is resultArr

No, the "intermediate" array is internal to the map implementation.

You can however use reduce for what you want. A map explicitly maps each element of an array to another value; the result always has the same length as the input. A reduction is very explicitly what you're looking for; you reduce or transform the array into something else.

someArr.reduce((acc, item) => {
    if (!acc.includes(item)) {
        acc.push(item);
    }
    return acc;
}, [])

Array.prototype.map(), The map() method creates a new array populated with the results of calling a provided option:checked') let values = Array.prototype.map.call(elems, Let A be a new array created as if by the expression new Array(len)  .map() creates an array from calling a specific function on each item in the parent array. .map() is a non-mutating method in that it creates a new array as against mutating methods which only make changes to the calling array.

For getting unique values, you could use a Set.

var array = [1, 2, 3, 1, 2, 4, 5],
    unique = Array.from(new Set(array));

console.log(unique);

Deep Dive into JavaScript's Array Map Method, JavaScript includes these built-in Array methods -- including the Map Method -- for a reason. It's not Array Map Method with Arrow Functions as Callback Function value) , you can also produce a cloned array with .slice() . The map () method creates a new array with the results of calling a provided function on every element in the calling array. var array1 = [1, 4, 9, 16]; // pass a function to map const map1 = array1.map (x => x * 2); console.log (map1); // expected output: Array [2, 8, 18, 32] The source for this interactive example is stored in a GitHub

Rather than map, you can simply use a for loop to access the intermediately created array.

let someArr = [1,2,4,1,5];
const resultArr = [];
for(let item of someArr) {
  if(!resultArr.includes(item)) resultArr.push(item);
}
console.log(resultArr);

Map and Set, new Map() – creates the map. map.set(key, value) – stores the value by the key. map.get(key) To test keys for equivalence, Map uses the algorithm SameValueZero. Besides that, Map has a built-in forEach method, similar to Array :. Actually, the engineers made the creation of such an Array illegal. So the creation of an array from generic Class fails. The Collection.toArray method followed by a Cast to the Array works at compile time. This solves not the problem, that the ArrayStoreCheck can’t be done during Runtime, but you can create an Array of generics in this way.

You can use Array#reduce((intermediateArr, el) => {}, []) instead, where intermediateArr is the array that is build up while the function is running.

If for example you want to check the items in every iteration, you could use something like this:

let someArr = [1,2,2,2,3,3,4,4,5,5,6,7,8,9]


let resultArr = someArr.reduce((intermediateArr, item) => {
    if(!intermediateArr.includes(item)) {
        intermediateArr.push(item)
    }
    return intermediateArr;
}, [])


console.log(resultArr.toString())  // 1,2,3,4,5,6,7,8,9

4 Uses of JavaScript's Array.map() You Should Know ― Scotch.io, .map() creates an array from calling a specific function on each item in modify each individual item in an array and create a new function off it. Explanation: In this example, a map object “myMap” has been created with three [key, value] pairs and the Map.has() method is used to check whether an element with the key ‘0’ and ‘3’ exists in the map or not. Code 1:

Map.has( ) In JavaScript, loop returns an array of all [key, value] pairs for each iteration. The Map.has() method in JavaScript is used to check whether an element with a Explanation: In this example, a map object “myMap” has been created with a single [key, value​]  Parameters. callback. Callback function to run for each element in each array. NULL can be passed as a value to callback to perform a zip operation on multiple arrays. If only array1 is provided, array_map() will return the input array.

JavaScript Array every(): Determining If All Array Elements Pass a Test, JavaScript Array type provides the every() method that allows you to check if every element of an array pass a test in a shorter and cleaner way. Introduction to​  Converting or transforming a List and Array Objects in Java is a common task when programming. In the tutorial, We show how to do the task with lots of Java examples code by 2 approaches: Using Traditional Solution with basic Looping Using a powerful API – Java 8 Stream Map Now let’s do details with … Continue reading "How to use Java 8 Stream Map Examples with a List or Array"

How to simplify your codebase with map(), reduce(), and filter() in , Well, this is the only one not about map/reduce/filter, but it's so compact that it was The filter() method creates a new array with all elements that pass the test The some() method tests whether at least one element in the array passes the Create an object that contains the frequency of the specified key. The array_map () function sends each value of an array to a user-made function, and returns an array with new values, given by the user-made function. Tip: You can assign one array to the function, or as many as you like. array_map ( myfunction, array1, array2, array3, ) Parameter Values. Required. The name of the user-made function, or null.

Comments
  • No. You can however filter the array returned from map.
  • What is intermediateArr? It not defined in your code.
  • What are you trying to achieve here?
  • @YeldarKurmangaliyev I guess he means the array which is created by map function internally.
  • What you really are looking for is Array.from(new Set(someArr)).
  • Please note that while this is indeed more explicit, this becomes significantly slower than Array.from(new Set(array)) when the array gets bigger : https://jsperf.com/array-duplicates-removal
  • @Logar For this particular case, sure, there are better ways to deduplicate an array. This is pointing out the more general approach of map vs. reduce.