The principle of JSONP

JSONP is a cross-domain data interaction technique that bypasses browser same-origin policy restrictions by leveraging the fact that script tags are not subject to same-origin rules. It dynamically creates script tags to fetch data from different domains: the frontend defines a callback function, and the server wraps the data in this callback function before returning it. The browser then executes the function to receive the data. JSONP only supports GET requests, lacks standardized error handling, and poses security risks such as XSS and CSRF. Modern alternatives include CORS and WebSocket. While JSONP offers good compatibility with older browsers, more secure cross-domain solutions should be prioritized.

Basics of Ajax

Ajax is a technology for creating interactive web applications that exchanges data with the server asynchronously without reloading the entire page. The core principle involves using the XMLHttpRequest or Fetch API to perform background data exchange. The XMLHttpRequest object is the foundation of Ajax technology, supporting various HTTP request methods such as GET and POST. In modern development, JSON has become the primary data format, replacing XML. The Fetch API provides a more concise alternative. Error handling and timeout mechanisms are crucial for a robust Ajax implementation. Cross-origin requests are resolved through CORS, requiring the server to set appropriate response headers. Practical applications include fetching data and rendering pages, while performance optimization involves request caching, cancellation, and batch processing. Modern front-end frameworks like React also offer their own Ajax implementation methods.

Performance optimization tips

Front-end performance optimization involves multiple key aspects. First, identify performance bottlenecks such as excessive DOM operations or memory leaks. Reducing DOM operations can be achieved through batch modifications and using document fragments. Event delegation optimizes event handling for large numbers of elements. Debouncing and throttling techniques control the triggering of frequent events. Optimizing loops requires reducing computations and caching lengths. Memory management involves avoiding leaks, such as clearing timers and event listeners. Web Workers handle CPU-intensive tasks. Caching reduces repetitive calculations. For animation optimization, requestAnimationFrame is recommended. Code splitting and lazy loading improve initial load speed. Reducing repaints and reflows involves batching style modifications. Efficient selectors enhance query performance. Avoid synchronous layouts by batching read-write operations. Choose appropriate data structures, such as Sets for faster lookups. Minimize scope chain lookups by caching global variables. Bitwise operations accelerate intensive computations. Avoid `with` and `eval`. Optimize network requests by combining APIs. Modern JavaScript features make code more concise and efficient.

Design patterns implement the translation of this sentence into English.

Design patterns are reusable solutions to common problems in software development. In JavaScript, design patterns help developers build more flexible and maintainable code structures. This article explores several common design patterns and their implementations, including the Singleton pattern, which ensures a class has only one instance and provides a global access point; the Observer pattern, which defines a one-to-many dependency between objects; the Factory pattern, which provides an interface for creating objects; the Strategy pattern, which defines a family of algorithms and encapsulates each one; the Decorator pattern, which dynamically adds additional responsibilities; the Adapter pattern, which converts interfaces to allow incompatible classes to work together; the Proxy pattern, which controls access to an object; the State pattern, which allows an object to alter its behavior when its internal state changes; and the Composite pattern, which composes objects into tree structures to represent part-whole hierarchies. These patterns provide efficient solutions for JavaScript development.

Basics of Functional Programming

Functional programming is a programming paradigm that emphasizes pure functions, immutable data, and function composition. It avoids state mutation and side effects, making code easier to test and maintain. In JavaScript, features like higher-order functions, closures, and currying are widely used. Pure functions ensure the same input always returns the same output without side effects. Immutable data prevents unintended modifications. Higher-order functions like `map`, `filter`, and `reduce` can accept or return functions. Function composition combines multiple functions using `compose` or `pipe`. Currying transforms multi-argument functions into single-argument ones for reusability. Recursion replaces loops, and lazy evaluation delays computation. Functors and monads handle side effects and asynchronous operations. Pattern matching selects logic based on data structure. Persistent data structures retain old versions when modified. Type systems like TypeScript help catch errors. Functional programming may introduce performance overhead, requiring trade-offs. JavaScript libraries like Ramda support functional programming. Promises and `async/await` can integrate functional approaches for async tasks. Pure functions and immutable data simplify testing and debugging. Functional Reactive Programming (FRP) combines reactive programming to model event streams. Algebraic data types express business logic.

Memory leak issue

The essence of memory leaks is the failure to correctly release allocated memory in a program, leading to a gradual reduction in available memory. Although JavaScript has an automatic garbage collection mechanism, developers still need to pay attention to memory management. Common scenarios include uncleared timers, lingering DOM references, closure misuse, etc. Undeclared global variables are mounted to the global object, causing leaks. Uncleared timers persistently reference external objects. Lingering DOM references keep removed elements in memory. Closures capture scope variables, potentially retaining data unintentionally. Poor cache management can lead to infinite growth. Third-party libraries may also cause hidden leaks. Detection methods include Chrome's heap snapshot comparison, while Node.js can use the heapdump module. WebWorker memory requires separate cleanup. WeakMap and WeakSet allow objects to be garbage-collected. Design patterns like pub-sub cleanup and object pooling can prevent leaks. Frameworks like React and Vue also offer specific solutions.

The garbage collection mechanism

JavaScript's garbage collection mechanism alleviates developer burdens by automatically managing memory and releasing unused object memory. It primarily employs the mark-and-sweep algorithm to traverse reachable objects and clear unreachable ones, while the reference-counting algorithm is limited due to its inability to handle circular references. The V8 engine divides the heap into the new generation and the old generation, applying different recycling strategies for each. Common memory leaks include accidental global variables, uncleared timers, DOM references, and improper use of closures. WeakMap and WeakSet use weak references that do not prevent garbage collection. Performance optimization recommendations include avoiding unnecessary objects, promptly dereferencing, and using object pools. Browsers and Node.js provide memory analysis tools to help monitor and optimize memory usage. Node.js also requires special attention to Buffer and event listener management.

Execution context and variable object

In JavaScript, execution context and variable object are the core mechanisms of code execution. Execution contexts are divided into three types: global, function, and eval. Their lifecycle includes two phases: creation and execution. During the creation phase, the variable object, scope chain, and `this` binding are processed. The variable object stores variable and function declarations. In the global context, it is the `window` object, while in the function context, it is called the activation object and includes `arguments`. Variable hoisting is the result of processing declarations during the creation phase. The scope chain determines the lookup path for variables. `let` and `const` have block-level scope, and closures can access the variable object of outer functions. The execution context stack manages the order of execution. Function declarations take precedence over variable declarations. Strict mode restricts assignments to undeclared variables. The variable object affects memory management, and `this` provides dynamic scoping. ES6 extends the concept of the variable object. Debugging tools can observe execution contexts. Execution contexts are closely related to the event loop mechanism, where different tasks enter different queues, influencing execution order.

The binding rule of 'this' translates this sentence into English.

In JavaScript, the binding rules of `this` determine the reference of `this` during function execution, primarily including default binding, implicit binding, explicit binding, and `new` binding. Under default binding, a standalone function call has `this` pointing to the global object (or `undefined` in strict mode). In implicit binding, when a function is called as an object method, `this` points to the calling object, but implicit loss may occur due to assignments. Explicit binding forces `this` to a specific value using `call`, `apply`, or `bind`. Arrow functions inherit `this` from their outer scope. `new` binding makes `this` point to the newly created instance. In terms of priority, `new` binding has the highest precedence, while default binding has the lowest. Special scenarios include `this` in DOM events, timers, and higher-order functions, each with unique characteristics. Common issues involve incorrect `this` references in nested functions and callbacks, which can be resolved using arrow functions or explicit binding. Advanced patterns involve soft binding and `this` rules in prototype chains. Understanding these rules is crucial for writing reliable code.

Application scenarios of closures

Closures are a powerful and commonly used feature in JavaScript that allow functions to access and manipulate external variables even after the outer function has finished executing. They are widely applied in various scenarios such as data encapsulation, function factories, event handling, timers, module patterns, memoization, higher-order functions, asynchronous programming, debouncing, throttling, function currying, state management, iterators, generators, and lazy initialization. Through closures, it is possible to create private variables, generate dynamic functions, preserve event states, cache computation results, manage asynchronous operations, control function execution frequency, transform multi-parameter functions, manage application state, implement custom iterators, and delay resource initialization, among other functionalities. These applications demonstrate the flexibility and importance of closures in JavaScript programming.