CODEWITHSUNDEEP
javascriptarrays
jason
jason

Reputation: 4801

How does this snippet for removing duplicates from an array work

Regarding this post (Remove Duplicates from JavaScript Array) on creating a new array of unique values from another array.

Code in question:

uniqueArray = myArray.filter(function(elem, pos) {
    return myArray.indexOf(elem) == pos;
})

Using this as the test data:

var names = ["Mike","Matt","Nancy","Adam","Jenny","Nancy","Carl"];

Desired result is an array with only unique values:

var unique_names = ["Mike","Matt","Nancy","Adam","Jenny","Carl"];

Where I'm at:

I understand that filter will run a function on each member of the array, and that elem is the element being reviewed, and that pos is its index. If something causes that function to return false, then that element will not be included in the new array. So walking through it, this happens:

  1. Is myArray.indexOf("Mike") the same as 0? Yes, so add "Mike" to the new array.
  2. Is myArray.indexOf("Matt") the same as 1? Yes, so add "Matt" to the new array.
  3. Is myArray.indexOf("Nancy") the same as 2? Yes, so add "Nancy" to the new array.

[repeat for all elements. All pass.]

Basically I don't get why the 2nd Nancy would evaluate to false.

Upvotes: 0

Views: 106

Answers (2)

Matt Burland
Matt Burland

Reputation: 45155

6) Is myArray.indexOf("Nancy") the same as 5? No (it's 2, just like it step 3), so skip the duplicated "Nancy".

indexOf gives you the first occurrence of the item.

Upvotes: 1

Iftah
Iftah

Reputation: 9572

The indexof is the index of the first appearance of the element, so the second Nancy would get the index of the first Nancy, and would be filtered out.

Upvotes: 1

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