Get number w/o pair in the array of numbers

getUniqueNumberInArray.js

function getUniqueNumberInArray(arr) {
    if (!arr || !arr.length) {
        return null;
    }

    let result = arr[0];
    for (let i=1; i<arr.length; i++) {
        result ^= arr[i]; // XOR each element in array
    }
    return result;
}

const arr = [ 0, 4, -1, 5, -1, 4, 0 ];
console.info(getUniqueNumberInArray(arr));

Maybe for-loop could be changed to:
arr.reduce( (result, element) => result ^ element )

Borzilov, good point.
But I try don't any cool functions from the "box" of language. Here it will be syntactic sugar and will not affect the algorithm itself.
The decision should be understood even by those who do not know the language