How can some() be used to check for duplicates in JS?

Question

I'm wondering how the some() method and arrow functions can be used to simplify my duplicate check in JavaScript. This is what I have at the moment (being used in a guessing game):

function guessRepeatValidate(userGuess) {
    let passed = true;
    if (guessArchive.indexOf(userGuess) > -1) {
        passed = false;
    }
    return passed;
}

This has been puzzling me for a while, so it'd be great to see what you folks have in mind to make this work with some().

please add the array. your question is unclear. so you want to search for duplicates or just for a single existence? — Nina Scholz, Feb 07 '21 at 19:25
`some` is sort of like `includes`.. the difference is, `includes` takes in a value, checks through the array, if a value in the array equals the parameter value, return true.. how `some` works is that it takes in a function, it checks through the array(putting in the value of each index as it goes along) and if your function returns `true`.. the condition in the array is met — The Bomb Squad, Feb 07 '21 at 19:26
Unless this is some sort of homework assignment, it isn't very efficient to use `.some()` to check for duplicates. A classic way to check for duplicates is to use a Set as in [How to see if an array has 2 or more elements that are the same](https://stackoverflow.com/questions/65927471/how-to-see-if-an-array-has-2-or-more-elements-that-are-the-same/65927499#65927499). — jfriend00, Feb 07 '21 at 19:50
> please add the array. your question is unclear. so you want to search for duplicates or just for a single existence? Checking for duplicates by utilizing only the .some() function. I probably should've been clearer about that. The array is dynamic and is the input passed into the guessing game from a simple HTML webpage. — Colton Rushton, Feb 07 '21 at 20:11

score 2 · Accepted Answer · answered Feb 07 '21 at 19:28

You could take a closure over a Set and return the check if an item exist in the set.

const 
    guessRepeatValidate = userGuess => userGuess.some(
        (s => v => s.has(v) || !s.add(v))
        (new Set)
    );

console.log(guessRepeatValidate([1, 2, 3]));
console.log(guessRepeatValidate([1, 2, 3, 4, 2]));

score 1 · Answer 2 · answered Feb 07 '21 at 19:25

1

You can do it simply by using some with indexOf

const arr = ['a', 'b', 'c', 'a']
const containsDuplicate = arr.some((el, i) => arr.indexOf(el, i+1) > -1)

console.log(containsDuplicate)

answered Feb 07 '21 at 19:25

Dominik Matis

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How can some() be used to check for duplicates in JS?

2 Answers2