# If you have an array and want to check the presence of a value inside it, there’s a very fast way:
 
arr = ['foo', 'bar', 'baz']
is_present = 'foo' in arr #returns true
isnt_present = 'hello' in arr #returns false
 
# Compiles down to:
 
`var arr, is_present, isnt_present;
var __indexOf = Array.prototype.indexOf || function(item) {
for (var i = 0, l = this.length; i < l; i++) {
    if (this[i] === item) return i;
}
return -1;
};
arr = ['foo', 'bar', 'baz'];
is_present = __indexOf.call(arr, 'foo') >= 0;
isnt_present = __indexOf.call(arr, 'hello') >= 0;`