Document.onbeforescriptexecute - Web APIs
syntax document.onbeforescriptexecute = funcref; funcref is a function reference, called when the event is fired.

Document - Web APIs
document.onbeforescriptexecute represents the event handling code for the beforescriptexecute event.

Index - Web APIs
940 document.onbeforescriptexecute api, dom, non-standard, property, reference, element.onbeforescriptexecute fired when the code in a <script> element declared in an html document is about to start executing.