Javascript - In the Map and filter functions the iteration variable impilicitly defined or we can explicitly define a variable to iterate through an array in js

Akhil
September 9, 2024
198 views
0 votes
2 Answers

In the map() and filter functions(), is the iteration variable implicitly defined in it? or we can also explicitly define a variable to iterate through the array?

const numberArray = [1, 2, 3, 4, 5];

 const res = numberArray
.map(number => number * 2)
.filter(number => number > 5);

console.log(res);

Regarding declaration of iteration variable in Maps and Filters, how we can define it?

Tags: arrays javascript

Answers

- guriev
- September 9, 2024 at 9:10 am
- 0 votes
0
The map and filter methods have the same parameters: element, index and reference to the source array. In your case you use only the first argument. You can give any name to the arguments, the main thing is to keep their order. Accordingly, you define «variable» names when you declare your arrow function inside map or filter. The variable represents the current element of the array at each iteration.

Example with other names:
```
const res = numberArray
  .map(num => num * 2) // You can use 'num', or any other name
  .filter(val => val > 5); // Similarly, 'val' is just a placeholder
```
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- huanfeng
- September 9, 2024 at 9:27 am
- 0 votes
0
First i would recommend you to read the doc first:

map, filter

The callback function implicitly defines the iteration variable number in the map and filter functions.
The callback function has 3 params:

element
The current element being processed in the array.

index
The index of the current element being processed in the array.

array
The array map() was called upon.

number here is the first param,

Maybe below is more obvious to you:
```
const numberArray = [1, 2, 3, 4, 5]

const res = numberArray.map(function(number) {
  return number * 2
}).filter(function(number) {
  return number > 5
})

console.log(res)
```
Regarding how to implement the map by yourself:

map

const numberArray = [1, 2, 3, 4, 5]
```
const res = []
for (let i = 0; i < numberArray.length; i++) {
  const number = numberArray[i] * 2
  if (number > 5) {
    res.push(number)
  }
}

console.log(res)
```
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