I am working with an array that looks like this -
["a", "a", "b", "c", "d", "e"]
My goal is to filter this array in a way that removes only the first occurrence of each element.
Based on the example array, the expected output would be - ["a"]
Can anyone provide me with a solution to achieve this using either JavaScript or Lodash?
If you're looking to find the first occurrence of duplicate items in an array, one approach is to utilize the Array#lastIndexOf method in conjunction with index checking.
const
data = ["a", "a", "b", "c", "d", "e"],
result = data.filter((v, i, a) => i !== a.lastIndexOf(v));
console.log(result);If you're looking to efficiently keep track of whether an item has already been found, one neat trick is to use an empty object as a map. You can then utilize Array#filter to eliminate undesired elements.
var list = ["a", "a", "b", "c", "d", "e"];
var occurrences = {};
var filteredList = list.filter(function(item) {
if (item in occurrences) return true; // include if already seen
occurrences[item] = 1; // mark as seen
return false; // exclude from new list
});
console.log(filteredList);Here's a more concise version:
let list = ["a", "a", "b", "c", "d", "e"], occurrences = {};
list = list.filter(item => item in occurrences ? 1 : occurrences[item] = 1 && 0);
console.log(list);To easily remove the first element of an array, you can utilize the shift method. More information on how to use it can be found here.
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