268-missing-number.js

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function(nums) {
    let n = nums.length;
    let expectedSum = (n * (n + 1)) / 2;
    let actualSum = nums.reduce((acc, curr) => acc + curr, 0);
    return expectedSum - actualSum;
};