check if an element is repeated in list javascript code example

Example 1: javascript check if array has duplicates

function hasDuplicates(array) {
    return (new Set(array)).size !== array.length;
}

Example 2: check for duplicates in array javascript

[1, 2, 3].every((e, i, a) => a.indexOf(e) === i) // true

[1, 2, 1].every((e, i, a) => a.indexOf(e) === i) // false