async

A boolean attribute indicating that the browser should, if possible, execute the script asynchronously. This attribute must not be used if the src attribute is absent (i.e. for inline scripts). If it is included in this case it will have no effect. 

Dynamically inserted scripts execute asynchronously by default, so to turn on synchronous execution (i.e. scripts execute in the order they were loaded) set async=false See Browser compatibility for notes on browser support. See also Async scripts for asm.js.