Under the hood, the JavaScript Object is implemented with a hash table.
So, your Key:Value pair would be (your integer):true

A constant-time lookup function could be implemented as:

var hash = {

  1:true,

  2:true,

  7:true

  //etc...

};



var checkValue = function(value){

  return hash[value] === true;

};





checkValue(7); // => true

checkValue(3); // => false