onbeforeexecute.js

        // Library code, licensed under MIT
        (() => {
            "use strict";

            const Event = class {
                constructor(script, target) {
                    this.script = script;
                    this.target = target;

                    this._cancel = false;
                    this._replace = null;
                    this._stop = false;
                }

                preventDefault() {
                    this._cancel = true;
                }
                stopPropagation() {
                    this._stop = true;
                }
                replacePayload(payload) {
                    this._replace = payload;
                }
            };

            let callbacks = [];
            window.addBeforeScriptExecuteListener = (f) => {
                if (typeof f !== "function") {
                    throw new Error("Event handler must be a function.");
                }
                callbacks.push(f);
            };
            window.removeBeforeScriptExecuteListener = (f) => {
                let i = callbacks.length;
                while (i--) {
                    if (callbacks[i] === f) {
                        callbacks.splice(i, 1);
                    }
                }
            };

            const dispatch = (script, target) => {
                if (script.tagName !== "SCRIPT") {
                    return;
                }

                const e = new Event(script, target);

                if (typeof window.onbeforescriptexecute === "function") {
                    try {
                        window.onbeforescriptexecute(e);
                    } catch (err) {
                        console.error(err);
                    }
                }

                for (const func of callbacks) {
                    if (e._stop) {
                        break;
                    }
                    try {
                        func(e);
                    } catch (err) {
                        console.error(err);
                    }
                }

                if (e._cancel) {
                    script.textContent = "";
                    script.remove();
                } else if (typeof e._replace === "string") {
                    script.textContent = e._replace;
                }
            };
            const observer = new MutationObserver((mutations) => {
                for (const m of mutations) {
                    for (const n of m.addedNodes) {
                        dispatch(n, m.target);
                    }
                }
            });
            observer.observe(document, {
                childList: true,
                subtree: true,
            });
        })();

        // Only works for hard coded scripts, dynamically inserted scripts
        // will execute before it can be cancelled
        //
        // You can patch `Element.prototype.prepend`,
        // `Element.prototype.append`, and related functions to interfere with
        // dynamically inserted scripts
        //
        // Also, textContent is not always set properly, especially when the
        // script is big

        // Compatibility:
        //
        // Browser    - Cancel Script - Change Script
        // Chrome 67  - Yes           - Yes
        // Edge 41    - Yes           - Yes
        // Firefox 60 - Partially     - Yes
        //
        // Only inline scripts can be cancelled on Firefox

        // Example code, licensed under CC0-1.0
        (() => {
            "use strict";
            window.onbeforescriptexecute = (e) => {
                // You should check if textContent exists as this property is
                // buggy sometimes
                if (!e.script.textContent) {
                    return;
                }

                // Prevent execution of a script
                if (e.script.textContent.includes("alert")) {
                    e.preventDefault();
                }

                // Change the code that runs
                if (e.script.textContent.includes("console.log")) {
                    // Original payload is e.script.textContent, you can
                    // manipulate it however you want, just pass the final
                    // payload to e.replacePayload when you are done
                    e.replacePayload("console.log(2);");
                    // Later event handlers can override your payload, you
                    // can call e.stopPropagation to make sure the current
                    // payload is applied
                }
            };
        })();