Find missing integer(s) in given an array.You know that every integer 1-N appears once in the array

Here is solutions

var numbers = [0,1,3,4,5,7,8]; // Missing 2,6
var missing = [];

// Find the missing array items
for ( var i = 0; i < numbers.length; i++ ) {

    if ( (numbers[i+1] - numbers[i]) > 1 ) {
        missing.push( numbers[i+1] - numbers[1] );   
    }
}

console.log( missing );