How does array.filter((item, index) => array.indexOf(item) === index) work?

Question

I'm aware the that filter method iterates over all the items in the array, but how does this method work when you add in index as a parameter too? Does it then iterate over entire set of pairs of (item, index)?

I'm trying to find the unique element in an array in which every element is duplicated apart from one. e.g. [1,1,2], [1,2,3,2,1], etc

Did you read the documentation for filter? developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — epascarello
– epascarello, Commented Aug 4, 2022 at 17:44
The index won't help much for determining unique elements. Walk through the input array, keeping track of numbers you've seen by adding them to a set. For each number, push it to the output if it's not in the set or ===1. Then add it to the set — danh
– danh, Commented Aug 4, 2022 at 18:16

Filinto Delgado · Accepted Answer · 2022-08-04 17:53:01Z

1

When you add an index the index parameter will be the index of the element that the iteration is happening on, for example:

const myArray = [a, b, c]
myArray.filter((item, index) => {console.log(`The item ${item} is on index ${index} on the array`)}

That will print

$ The item a is on index 0 on the array
$ The item b is on index 1 on the array
$ The item c is on index 2 on the array

See more infromatiom on MDN page for filter

answered Aug 4, 2022 at 17:53

Filinto Delgado

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7 Comments

mathsnerd22 Over a year ago

So with array.filter((item, index) => array.indexOf(item) === index), if we let array = [1, 1, 2], the inputs for (item, index) are (1, 0), (1,1), (2,2)?

Filinto Delgado Over a year ago

In this case you are not printing anything if you wanted that result you would have to make a console.log(item, index)

Filinto Delgado Over a year ago

Array.filter make a new array if the item pass the test in this case all of them would have passed the test, if you stored the returned value in a variable you would ended up with the same array

mathsnerd22 Over a year ago

how does all of them pass the test? the only pair that works is (1,1)?

Filinto Delgado Over a year ago

Because array.indexOf(item) === index returns true so it pass in the test

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Mohamed Mahdi · Accepted Answer · 2022-08-04 17:53:19Z

0

Try this code:

['a','b','c'].filter((item, index) => console.log(item, index))

It basically iterates through each item and its index.

edited Aug 4, 2022 at 17:53

answered Aug 4, 2022 at 17:51

Mohamed Mahdi

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2 Comments

mathsnerd22 Over a year ago

so like (a,0) then (b, 1) then (c, 2)?

Mohamed Mahdi Over a year ago

Exactly, you can try it your own in a console. jsconsole.com

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