let numbers = [2, 3, 5, 7, 11, 13, 17];

let contain2 = numbers.indexOf(2) !== -1;
//contain2 is true

let contain3 = 3 in numbers;
//contain3 is true

let index5 = numbers.indexOf(5);
//index5 is 2

//ECMAScript 7 feature
let contain5 = numbers.includes(5);
//contain5 is true

console.log(`contain2 is ${contain2}`);
console.log(`contain3 is ${contain3}`);
console.log(`contain5 is ${contain5}`);
console.log(`index5 is ${index5}`);