Lightweight beforescriptexecute / onbeforescriptexecute polyfill

beforescriptexecute-polyfill.js

(function(){
    if("onbeforescriptexecute" in document) return; // Already natively supported
    
    let scriptWatcher = new MutationObserver(mutations => {
        for(let mutation of mutations){
            for(let node of mutation.addedNodes){
                if(node.tagName === "SCRIPT"){
                    let syntheticEvent = new CustomEvent("beforescriptexecute", {
                        detail: node,
                        cancelable: true
                    })
                    // .dispatchEvent will execute the event synchrously,
                    // and return false if .preventDefault() is called
                    if(!document.dispatchEvent(syntheticEvent)){
                        node.remove();
                    }
                }
            }
        }
    })
    scriptWatcher.observe(document, {
        childList: true,
        subtree: true
    })
})()

example.js

document.addEventListener("beforescriptexecute", event => {
    // There's no efficient way to set event.target
    // Other polyfills try to accomplish this through hacky means like 
    // replacing the addEventListener prototype.
    // Instead, we pass the element through the sanctioned event.details property.
    //
    // So if the event came from our polyfill, our target will be in event.details.
    // If it's a native event, it will be in event.target
    // Nice, clean, simple.
    
    let script = event.details || event.target;
    
    if(script.textContent.contains("function dontExecuteThisFunction(please)")){
        event.preventDefault();
        // Now the script won't execute.
        // Use document.createElement("script") to inject replacement script here, if you want.
    }
});