What are the application scenarios of closures?

Shallow copy only copies the top-level properties of the object, nested reference types share the same memory address, and modifying the copy will affect the original object; deep copy recursively copies all levels to ensure complete independence. For example, using Object.assign or extension operators is a shallow copy, modifying nested objects will reflect the original object; while JSON.parse(JSON.stringify(obj)) or Lodash's _.cloneDeep() can achieve deep copy to avoid mutual influence. In addition, deep copying requires attention to special types of processing such as functions, circular references, and Date. When communicating across languages, the impact of Java serialization on data structures must also be considered.

Selecting HTML elements in JavaScript can be achieved in three ways: 1. Use getElementsByClassName to select by class name, return dynamic HTMLCollection, you need to write the class name, such as 'box', and access the first element through boxes[0]; 2. Use getElementsByTagName to select by tag name, and also return dynamic HTMLCollection, suitable for batch operations such as modifying all li styles; 3. Use querySelectorAll to select by CSS selector, support more complex selection logic, return static NodeList, if only the first match is required, you can use querySe

JavaScript's date processing requires attention to creating, getting set values, formatting and time zone issues. 1. Create a Date object by using no parameters, timestamps, date strings or components; note that the month starts at 0 and the browser parses the strings may be inconsistent. 2. Use getDate, setDate and other methods to get/set the value, and use getTime() to compare the date. 3. Intl.DateTimeFormat can be used to ensure consistency, and third-party libraries can also be used. 4. UTC method should be used to deal with time zones to avoid confusion. The sending server recommends using UTC and converting it to the backend.

Debouncingdelaysfunctionexecutionuntilafterapauseinactivity,idealforsearchinputsandformvalidation;2.Throttlinglimitsexecutiontoonceperfixedinterval,perfectforscrollandmousemoveevents;bothreduceperformanceloadwhilepreservingUX.

JavaScriptProxies and ReflectAPI can be used to enhance the interoperability of Java and JavaScript in Nashorn or GraalVM environments. 1. Use Proxy to intercept access to Java objects, such as recording attribute access logs or dynamically modifying the return value, and implementing it through get traps and Reflect.get; 2. You can proxie the Java class itself and control constructor calls through construct traps; 3. Use apply traps to automatically convert parameter types before calling Java methods to improve compatibility between JS and Java; 4. Reflect provides more consistent operation methods, such as Reflect.has

The method of setting up JavaScript security headers in Node.js applications is to use Helmet middleware to implement it through the backend. 1. Install and introduce the Helmet package and add it to the Express middleware chain to enable the default security headers; 2. Common security headers include CSP to prevent XSS attacks, X-Content-Type-Options to avoid MIME type guessing, X-Frame-Options to prevent click hijacking, HSTS to force HTTPS access, etc.; 3. CSP policies can be customized to meet external resource loading needs, but 'unsafe-inline' should be avoided, and Nonce or Hash is recommended instead; 4. Enable

Readable and writable streams in JavaScript achieve efficient data transmission through chunking processing. 1. ReadableStream is used to read data in asynchronously in blocks, suitable for large file reading, HTTP response and other scenarios; 2. WritableStream receives data and processes it, such as writing to files or recording logs; 3. Use the .pipe() method to automatically process back pressure, connecting readable and writable streams; 4. Transform stream can be converted during data flow; 5. In actual use, pay attention to error processing, back pressure control and correct ending of streams. Mastering these key points will help improve big data processing capabilities and performance.

Variable and function enhancement is the mechanism by which the JavaScript engine collects declarations into memory during the compilation phase, and does not really move the code. When a variable is declared using var, it will be promoted to the top of the scope, but the assignment remains in place; let and const will also be promoted, but will not be initialized, and it will enter a "temporary dead zone", and an error will be reported when access is reported. Function declarations will be fully promoted, including the function body, so they can be called before declaration; while the function expression only has the variable name promoted, and the function body is still in place. In actual development, suggestions are: 1. Declare variables at the top of the scope; 2. Priority is given to let and const; 3. Avoid calling function expressions or arrow functions before the function; 4. Do not repetitively declare functions and variables with the same name. Understanding the lifting mechanism helps