The XMLHttpRequest.timeout property represents a number of milliseconds a request can take before automatically being terminated. The default value is 0, which means there is no timeout. An important note the timeout shouldn't be used for synchronous XMLHttpRequests requests, used in a document environment or it will throw an InvalidAccessError exception. You may not use a timeout for synchronous requests with an owning window.
IE10 and 11 do not support synchronous requests, with support being phased out in other browsers too. This is due to detrimental effects resulting from making them.
More info can be found here.