如何检查数组元素是否是数组中唯一的值？

Question

I've seen many similar questions and answers here but none that directly answered this question. For each array element I'm looking for a way (with JavaScript) to check if it's the only one of its kind in the array, or if there is at least one other of it. For example:

const arr = [1,2,2]

looking for something that will return

true, false, false

when looping through arr.

Answer 1

 const arr = [1, 2, 2]; console.log(arr.map(item => arr.indexOf(item) === arr.lastIndexOf(item)));

const arr = [1, 2, 2];
arr.map(item => arr.indexOf(item) === arr.lastIndexOf(item));

Answer 2

You can do it in two passes:

• build a Map containing the count of each element
• look up each element in that Map

Like so:

const getCounts = iterable => {
    const counts = new Map();

    for (const x of iterable) {
        counts.set(x, (counts.get(x) ?? 0) + 1);  // use || for ES6 compat
    }

    return counts;
};

const arrCounts = getCounts(arr);
arr.map(x => arrCounts.get(x) === 1)