Question

I made some tests to check this but is quite not expected: http://jsperf.com/value-in-array-or-object

You can run those tests too..

Answer 1

It’s not faster. It appeared faster in your tests because you were assigning rather than comparing in the tests for 50 and 100 items:

if(a100[x] = 'item50'){
    break;
}

The test with 10 items was correct:

if(a10[x] === 'item5'){
    break;
}

See updated test cases here.