Selecting Items in array - Javascript - Using Filter method

Question

Someone asked a question today about figuring out to select certain elements within an array from I to the end of the array and it made me wonder how to do that with the filter method.

One of the solutions that someone gave was to use slice and I understand that you're able to select from index to index, but how would you implement the filter method to do the same thing?

Example

let arr = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'];

let newArr = arr.filter(element => element >= element.indexOf(3));
    
console.log(newArr);

This is what I came up with, it doesn't work, but the idea is to select all strings that have an index of 3 or greater and return them into another array.

appreciate that

mph85
– mph85

2019-04-25 19:47:42 +00:00
Commented Apr 25, 2019 at 19:47 — mph85
– mph85, Commented Apr 25, 2019 at 19:47

Pointy · Accepted Answer · 2019-04-25 19:42:42Z

5

The runtime passes the index to the filter callback:

let newArr = arr.filter((element, index) => index >= 3);

Performance-wise you're still making a new array and copying values, so it's about the same as .slice().

edited Apr 25, 2019 at 19:42

answered Apr 25, 2019 at 19:41

Pointy

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2 Comments

Pranav C Balan Over a year ago

or index > 2.

mph85 Over a year ago

ahh gotcha, that was pretty simple, i appreciate that

jdagosta · Accepted Answer · 2019-04-25 19:44:39Z

1

You should create a function with the filter criteria:

let arr = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'];

function filterCriteria(item) {
  return item >= someValue;
}

function someFunction() {
  return arr.filter(filterCriteria);
}

"someFunction" will return the array filtered

answered Apr 25, 2019 at 19:44

jdagosta

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Comments

Nina Scholz · Accepted Answer · 2019-04-25 19:53:47Z

1

While you are visiting every item, you could use a counter and decrement it until the counter reaches zero. Then take this values.

const
    fromIndex = i => _ => !i || !i--,
    array = ['bug', 'cat', 'dog', 'flea', 'bat', 'hat', 'rat'],
    result = array.filter(fromIndex(3));

console.log(result);

edited Apr 25, 2019 at 19:53

answered Apr 25, 2019 at 19:48

Nina Scholz

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4 Comments

mph85 Over a year ago

ah once again Nina, your solutions are too elegant for my brain. What is going on in your fromIndex = i => _ => !i || !i--? what is that underscore? lodash? or something else?

Nina Scholz Over a year ago

lodash is just a not used variable. the underscore denoted that (as convention). maybe the more classical empty parameter list is better understandable, like: fromIndex = i => () => !i || !i--

mph85 Over a year ago

ah, but why do we need either _ or ()?

Nina Scholz Over a year ago

because of the closure and filter needs a function.

Saeed · Accepted Answer · 2022-09-09 14:30:48Z

1

the second argument in the filter callback is index. So You can do something like this

arr.filter((element,index) => index >= 3);

edited Sep 9, 2022 at 14:30

Saeed

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answered Apr 25, 2019 at 19:48

kesavi

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