var arr = ["x", "y", "z"];

var map = {};

for (var k=0; k < arr.length; ++k) {

  map[arr[k]] = true;

}

​

function is_in_map(key) {

  try {

    return map[key] === true;

  } catch (e) {

    return false;

  }

}

​

​

function print_check(key) {

  console.log(key + " exists? - " + (is_in_map(key) ? "yes" : "no"));

}

​

print_check("x");

print_check("a");

​