Constructors and the new operator

A constructor is a special function in JavaScript used to create objects in conjunction with the `new` operator. When called with `new`, it automatically creates a new object and binds `this` to it. The `new` operator's execution process includes creating an empty object, setting the prototype, binding `this`, executing the constructor code, and returning the new object. The constructor's return value rules are: if an object is returned, it overrides the default object; primitive values are ignored. Constructors are typically named with an initial capital letter and should be called with `new`. They are used to initialize an object's state, and you can check `this` to determine if `new` was used. ES6's `class` is syntactic sugar for constructors. Manually implementing the `new` operator demonstrates its working principle. Constructors are closely related to the prototype chain and enable inheritance. Compared to factory functions, constructors have distinct characteristics. When using them, pay attention to `this` binding, performance considerations regarding method definition placement, and their applicability in design patterns like singletons, private variables, and object pools.

Function invocation methods and the this binding

In JavaScript, the way a function is invoked determines the `this` binding rules, which primarily include default binding, implicit binding, explicit binding, and `new` binding. Under default binding, in non-strict mode, `this` points to the global object, while in strict mode, it is `undefined`. Implicit binding occurs during method invocation, where `this` refers to the calling object, but implicit loss can occur. Explicit binding forces `this` to a specific value using `call`, `apply`, or `bind`. `new` binding causes `this` to point to the newly created instance. Arrow functions do not bind `this` but instead inherit it from the enclosing scope. In DOM event handlers, `this` defaults to the triggering element. Class methods require attention to `this` binding. In callbacks, `this` can be managed via closures, arrow functions, or `bind`. The priority of these rules, from highest to lowest, is: `new` binding, explicit binding, implicit binding, and default binding. Special scenarios, such as array methods, module scope, and asynchronous functions, also exhibit unique `this` behaviors. Understanding these rules is crucial for writing reliable code.

Function parameters and the arguments object

The relationship between JavaScript function parameters and the arguments object is a core aspect of function execution mechanics. Function parameters are variables declared at definition, passed by value (with object types passing a copy of the reference). ES6 introduced default parameters and rest parameters, enhancing flexibility. The arguments object is an array-like object containing all passed arguments and can be converted to a real array. In non-strict mode, named parameters and arguments influence each other, while strict mode severs this connection. Arrow functions do not have their own arguments but can access the arguments of their enclosing function. In practice, arguments is often used for dynamic parameter handling and forwarding, though modern development recommends using rest parameters instead. However, understanding arguments remains essential when maintaining legacy code.

Scope and Closures

JavaScript scope determines the accessibility range of variables, functions, and objects, adopting lexical scope, which is determined during the code-writing phase, including global scope, function scope, and block scope. `var` exhibits variable hoisting, while `let` and `const` have a temporal dead zone. A closure is a function that can access variables from an outer function even after the outer function has executed, preventing those variables from being garbage-collected. The scope chain is determined when a function is defined, searching for variables level by level. Improper use of closures may lead to memory leaks, such as accidental global variables or uncleared DOM references. Closures preserve context state in asynchronous programming, and block scope resolves the issue of `var` variable leakage. The `this` binding in closures requires attention, as arrow functions inherit the outer `this` value. Excessive use of closures may impact performance. Modern module systems leverage closures to achieve encapsulation.

Function Basics

JavaScript functions are the core unit of code organization, supporting multiple definition methods including function declarations and function expressions. Function declarations are hoisted, while function expressions are not. ES6 introduced arrow functions to simplify syntax. Function parameters support default values, and values are returned via `return`; if not explicitly returned, the default is `undefined`. Function scope limits variable access, and closures allow access to outer scopes. Higher-order functions can accept or return other functions, commonly seen in array methods. IIFEs execute immediately upon definition and are used to create isolated scopes. Recursive functions call themselves but require attention to termination conditions. Functions, as objects, have methods like `call`, `apply`, and `bind`. ES2017 introduced `async/await` to simplify asynchronous operations.

Timer and delay device

JavaScript provides two timing mechanisms, `setTimeout` and `setInterval`, for delaying and repeating code execution. `setTimeout` executes a callback function once after a specified delay, while `setInterval` repeats execution at fixed intervals. Both return a unique ID for clearing the timer. The article details their usage, parameter passing, clearing methods, and practical applications such as delayed search, polling server status, and simple animations. Advanced techniques like dynamically adjusting intervals, batch operations, and precise timing are also covered. Performance considerations highlight that long-running timers may impact page performance, especially on mobile devices, where browsers throttle timers in background tabs. For animations, `requestAnimationFrame` is recommended as the preferred choice. The article explains the relationship between timers and the event loop, as well as the execution order of microtasks and macrotasks. Finally, common pitfalls like `this` binding issues, closure problems, and memory leaks are listed, along with solutions and debugging tips.

The history object controls

The `history` object is a JavaScript interface provided by the browser for manipulating the browser's session history. It allows developers to programmatically control forward and backward navigation without refreshing the page, as well as modify the current page's URL. This object plays a crucial role in modern single-page application (SPA) development, enabling pages to update the URL without reloading while maintaining compatibility with browser navigation buttons. The `history` object offers various properties and methods, including the `length` property (which returns the number of history entries), the `state` property (which returns the current state object), and navigation methods like `back`, `forward`, and `go`. The `pushState` and `replaceState` methods allow modifying the URL without reloading the page. Handling the `popstate` event is essential for implementing SPA routing. Practical use cases include SPA routing, infinite scroll, and page state management. When using it, considerations such as browser compatibility, security restrictions, and performance optimization are necessary. Compared to traditional hash-based routing, the History API provides cleaner URLs and better SEO support. Common issues include state loss after page refreshes, which requires server-side configuration. Advanced usage patterns can enable features like undo/redo functionality.

Information about the screen object

The `screen` object in JavaScript provides key information about the user's screen, including properties such as width, height, available space, and color depth. This information is crucial for responsive design and device adaptation. Developers can leverage these properties to adjust layouts or load resources of different resolutions based on varying screen sizes. Modern browsers also support extended properties like screen orientation and multi-monitor position detection. The article further covers methods for handling screen orientation changes, applications in multi-screen environments, and performance optimization tips such as caching results and using CSS media queries as an alternative to certain detection methods. Lastly, it mentions browser compatibility issues and advanced use cases like full-screen image galleries.

navigator object properties

The `navigator` object is a global object provided by the browser to retrieve browser-related information, containing numerous properties and methods for detecting browser type, version, operating system, etc. - `navigator.userAgent` returns the user agent string, which can determine the browser type and operating system. - `navigator.platform` returns the operating system platform. - `navigator.language` returns the browser's preferred language setting. - `navigator.cookieEnabled` indicates whether cookies are enabled. - `navigator.onLine` detects network status. - `navigator.geolocation` retrieves geographic location (requires user permission). - `navigator.clipboard` provides clipboard operations (must be used in a secure context). - `navigator.mediaDevices` accesses media devices like cameras and microphones. - `navigator.hardwareConcurrency` returns the number of processor cores. - `navigator.storage` queries storage space. - `navigator.deviceMemory` returns the device's memory size. - `navigator.vendor` returns browser vendor information. - `navigator.doNotTrack` detects the user's "Do Not Track" setting. - `navigator.connection` provides network connection information. - `navigator.permissions` queries permission status. - `navigator.getBattery` retrieves battery status. - `navigator.plugins` returns installed plugins. - `navigator.mimeTypes` returns supported MIME types. - `navigator.javaEnabled` checks if Java is enabled. - `navigator.sendBeacon` asynchronously sends small amounts of data, suitable for use during page unloading.

Event monitoring mode

The event listener pattern is the core mechanism for handling user interactions in JavaScript. Using the `addEventListener` method, event handlers can be registered. Event propagation consists of three phases: capture, target, and bubbling. Event delegation leverages the bubbling mechanism to handle child element events on a parent element. Custom events can be created and triggered. Asynchronous event handling uses `AbortController` to enable cancellable listeners. Performance optimization techniques include debouncing and throttling. Cross-browser compatibility requires consideration of legacy IE differences. The event object contains rich information such as coordinates and the triggering element. Modern frameworks like React and Vue encapsulate native events. Mobile support includes touch gesture events. Keyboard events enable precise input control. HTML5 provides native drag-and-drop functionality. Page lifecycle events monitor loading and unloading states. Network events detect connection changes. Media queries enable responsive design.