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function getMissingElement(array) {
    for (var i = 1; i < array.length; i++) {
        if (array[i] - array[i-1] !=1 ) {
            return array[i]-1;
        } 
    }   
    
    return null;
}

console.log(getMissingElement([1,2,3,4,5,6,7]));// => null
console.log(getMissingElement([6,7,8,10,11,12,13,14,15]));// => 9
console.log(getMissingElement([-4,-3,-2,0,1,2,3,4]));// => -1