The const keyword and constant declarations

The `const` keyword, introduced in ES6, is used to declare block-scoped constants. It must be initialized upon declaration and cannot be reassigned. For primitive values, the value is immutable, but for objects and arrays, their properties or elements can still be modified. `const` has the temporal dead zone (TDZ) feature, meaning accessing it before declaration will throw an error. In practical development, `const` should be prioritized, and `let` should only be used when reassignment is necessary. `const` works well with `for...of` and `for...in` loops, as it creates a new binding for each iteration. Combining it with `Object.freeze` can create truly immutable objects. Global `const` variables do not become properties of the `window` object. Most modern browsers support `const`, but older versions may require transpilation tools. In TypeScript, `const` has special type inference behavior—when declaring literals, the type is narrowed to its most specific form.

The let keyword and its block-level scope

The introduction of the `let` keyword in ECMAScript 6 changed the way variables are declared in JavaScript by introducing block-level scoping, where variables are only accessible within the block they are declared in. This addresses issues with `var`, such as hoisting and global pollution. Variables declared with `let` are not hoisted and are subject to the Temporal Dead Zone (TDZ). Block-level scoping is particularly useful in loops and conditional statements, preventing variable overwriting and leakage. When combined with closures, `let` enables more predictable code behavior by creating a new lexical environment for each loop iteration. ES6 also allows function declarations within block scopes, similar to `let` variables. `let` is well-suited for the module pattern, enabling private variables without the need for IIFEs. Both `let` and `const` provide block-level scoping, but `const` declares constants that cannot be reassigned. In practice, it is recommended to prefer `const` by default and use `let` only when reassignment is necessary, while avoiding `var`. Modern browsers support `let`, and tools like Babel can transpile it for older browsers. In asynchronous programming, `let` ensures callbacks capture the correct variable value due to block scoping. Theoretically, block-level scoping improves engine optimization by clearly defining variable lifetimes.

Copy all text with one click: navigator.clipboard.writeText(document.body.innerText);

This text provides a detailed explanation of the technical method for copying webpage body content to the clipboard using `navigator.clipboard.writeText` and `document.body.innerText`. It introduces the basic usage of the Clipboard API, including reading and writing text and data, compares the differences between `innerText`, `textContent`, and `innerHTML`, and lists practical application scenarios such as browser bookmark tools, extension development, and webpage button implementation. It also explains permission and security requirements, including HTTPS, user gestures, and the Permissions API, and offers error handling, compatibility solutions, and performance optimization suggestions, such as text preprocessing, chunked copying, and selective copying. Finally, it demonstrates user experience enhancement techniques and advanced application examples, such as Markdown conversion, source attribution copying, and multilingual support.

Turn the webpage into a game: document.body.innerHTML = '<canvas id="game"></canvas>';

This line of code quickly creates a canvas game canvas by replacing the body's innerHTML property, with the core mechanism being to clear the original DOM and insert new elements. Subsequently, it needs to obtain the canvas context, configure basic properties, and implement the game loop using the requestAnimationFrame method. Keyboard and mouse event listeners are added to enable interactive control. The article provides a bouncing ball game example demonstrating position updates and collision detection. It also discusses performance optimization techniques like off-screen rendering and object pooling, as well as advanced topics such as game state management, sound effects, and responsive design. Finally, it introduces debugging methods and large-scale game architecture, offering developers a complete guide from basics to advanced game development.

Block ads: setInterval(() => document.querySelectorAll('.ad').forEach(el => el.remove()), 1000);

This text provides a detailed explanation of various technical methods for implementing ad-blocking on web pages using JavaScript, ranging from basic techniques like timed removal of ad elements to advanced optimizations using MutationObserver for performance. It covers intercepting network requests and WebSocket connections to prevent ad loading, addressing multiple practical scenarios including news websites, video platforms, and mobile ad blocking, with complete code examples provided. The article also discusses legal considerations, impacts on website functionality, and precautions against anti-blocking technologies. Finally, it explores applications in browser extensions and methods for combining with CSS to block ads.

Automatically fill out the form: document.querySelector('input').value = 'I am here to slack off';

Automatically filling out forms primarily involves manipulating DOM elements via JavaScript to set input field values. The basic method uses `querySelector` to select elements and modify their `value` property. In practice, handling different types of form elements—such as text boxes, checkboxes, radio buttons, dropdown menus—requires tailored approaches. Complex forms demand more precise selector strategies, including selecting by ID, name attributes, or hierarchical relationships. For dynamically generated forms or elements within Shadow DOM, special handling is needed, as merely setting the `value` property may not suffice; triggering events like `input` or `change` is often required to simulate real user input. Form validation and submission also require specific attention. In real-world applications, considerations include ensuring the form is fully loaded, handling dynamic loading scenarios, and preventing duplicate submissions. Browser extensions implementing this must account for content script isolation. Security and privacy best practices involve avoiding hard-coded sensitive information and retrieving data from secure storage instead. Cross-origin form operations are restricted by the same-origin policy. Modern frontend frameworks like React or Vue may require special methods to trigger state updates. A well-designed auto-fill system should enhance user experience by mimicking human input pacing, providing visual feedback, and defining field-mapping systems for complex form structures.

Modify webpage content: document.body.innerHTML = document.body.innerHTML.replace(/work/g, 'slack off');

The `innerHTML` property in JavaScript can dynamically retrieve or set page content. Using the `replace` method enables text substitution, such as replacing "work" with "slacking off." Directly modifying `innerHTML` replaces the entire DOM structure, which may lead to event listener loss or performance issues. In practical applications, batch text replacement on a page can be achieved via button clicks. Key considerations include event listener loss, performance concerns, and security risks. The extended section mentions replacing text in specific elements or combining regular expressions for advanced replacements. Compared to other DOM manipulation methods, `innerHTML` parses HTML tags, `textContent` displays plain text, and `innerText` considers CSS styling.

Code reuse principles

Code reuse is a core principle of software development that significantly enhances efficiency, reduces maintenance costs, and improves consistency. In JavaScript, efficient reuse is achieved through design patterns, modular solutions, and language features. Functions are the most basic unit of reuse, encapsulating repetitive logic, while higher-order functions elevate reusability. Object-oriented programming enables reuse through inheritance and composition, with composition favored over inheritance. The mixin pattern facilitates multiple inheritance. Modern JavaScript achieves large-scale reuse through module systems, such as ES6 modules and dynamic imports. Design patterns like factory and strategy patterns further enable reuse, and utility libraries help avoid reinventing the wheel. In React, custom hooks and higher-order components (HOCs) reuse logic. When reusing code, avoid over-abstraction, maintain appropriate granularity, consider use cases, document interfaces, and ensure test coverage. Balance performance and reuse carefully. TypeScript's type system enhances reuse reliability. In browser environments, custom events and Web Components enable reuse, while Node.js leverages middleware patterns. Future trends include Islands architecture, micro frontends, WebAssembly, and Server Components to advance reuse.

Secure coding standards

JavaScript secure coding standards emphasize reducing vulnerability risks through strict practices. Variable declarations should use `const` and `let` to avoid hoisting. External inputs require rigorous validation, with libraries like Joi or Yup recommended. DOM operations should escape dynamic content. Secure communication mandates HTTPS, and sensitive data should be stored in HttpOnly cookies. Cryptographic operations should use the Web Crypto API. To prevent XSS, avoid directly using `dangerouslySetInnerHTML`. CSRF protection requires implementing token mechanisms. Dependency management involves regular audits and version locking. Error handling must avoid exposing sensitive information. Memory safety requires attention to event listener leaks. Large file processing should use streaming APIs. Type safety can be enhanced with JSDoc. Browser storage should choose appropriate methods based on data type. Adhering to these standards significantly improves code quality and application security.

Performance optimization recommendations

JavaScript performance optimization involves multiple key dimensions: reduce DOM operations by using document fragments for batch processing to avoid frequent repaints and reflows; event delegation can minimize the number of listeners by handling child element events through a parent element; high-frequency events should use throttling and debouncing to control execution frequency; memory management requires timely resource release to avoid leaks caused by closures; algorithm optimization involves selecting appropriate data structures to reduce complexity; asynchronous code can leverage Web Workers for chunked processing to prevent blocking the main thread; code organization should adopt modularization and dynamic loading to enhance efficiency; performance monitoring utilizes tools like the Performance API for continuous analysis and optimization.