Front-end performance optimization methods

The scope of JavaScript determines the accessibility scope of variables, which are divided into global, function and block-level scope; the context determines the direction of this and depends on the function call method. 1. Scopes include global scope (accessible anywhere), function scope (only valid within the function), and block-level scope (let and const are valid within {}). 2. The execution context contains the variable object, scope chain and the values of this. This points to global or undefined in the ordinary function, the method call points to the call object, the constructor points to the new object, and can also be explicitly specified by call/apply/bind. 3. Closure refers to functions accessing and remembering external scope variables. They are often used for encapsulation and cache, but may cause

To get the value of the HTML input box, the core is to find the corresponding element through the DOM operation and read the value attribute. 1. Use document.getElementById to be the most direct way. After adding an id to input, you can get the element and read the value through this method; 2. Use querySelector to be more flexible, and you can select elements based on attributes such as name, class, type, etc.; 3. You can add input or change event listeners to achieve interactive functions, such as obtaining input content in real time; 4. Pay attention to the script execution timing, spelling errors and null judgment, and ensure that the element exists before accessing the value.

There are two core methods to get the selected radio button value. 1. Use querySelector to directly obtain the selected item, and use the input[name="your-radio-name"]:checked selector to obtain the selected element and read its value attribute. It is suitable for modern browsers and has concise code; 2. Use document.getElementsByName to traverse and find the first checked radio through loop NodeList and get its value, which is suitable for scenarios that are compatible with old browsers or require manual control of the process; in addition, you need to pay attention to the spelling of the name attribute, handling unselected situations, and dynamic loading of content

To use JavaScript to create a secure sandbox iframe, first use the sandbox attribute of HTML to limit iframe behavior, such as prohibiting script execution, pop-up windows and form submission; secondly, by adding specific tokens such as allow-scripts to relax permissions as needed; then combine postMessage() to achieve secure cross-domain communication, while strictly verifying message sources and data; finally avoid common configuration errors, such as not verifying the source, not setting up CSP, etc., and perform security testing before going online.

When using Date objects in JavaScript, you need to pay attention to the following key points: 1. Create an instance to obtain the current time using newDate(), or specify the time through string, year, month and day parameters. Recommend ISO format to ensure compatibility; 2. Use getFullYear(), getMonth() and other methods to obtain date and time, and manually splice and format the string; 3. Use getUTC series methods to process UTC time to avoid local time zone interference; 4. Calculate the time interval by the time stamp difference, but pay attention to the deviations that may be caused by cross-time zones or daylight saving time.

CompositionAPI in Vue3 is more suitable for complex logic and type derivation, and OptionsAPI is suitable for simple scenarios and beginners; 1. OptionsAPI organizes code according to options such as data and methods, and has clear structure but complex components are fragmented; 2. CompositionAPI uses setup to concentrate related logic, which is conducive to maintenance and reuse; 3. CompositionAPI realizes conflict-free and parameterizable logical reuse through composable functions, which is better than mixin; 4. CompositionAPI has better support for TypeScript and more accurate type derivation; 5. There is no significant difference in the performance and packaging volume of the two; 6.

There is an essential difference between JavaScript's WebWorkers and JavaThreads in concurrent processing. 1. JavaScript adopts a single-thread model. WebWorkers is an independent thread provided by the browser. It is suitable for performing time-consuming tasks that do not block the UI, but cannot operate the DOM; 2. Java supports real multithreading from the language level, created through the Thread class, suitable for complex concurrent logic and server-side processing; 3. WebWorkers use postMessage() to communicate with the main thread, which is highly secure and isolated; Java threads can share memory, so synchronization issues need to be paid attention to; 4. WebWorkers are more suitable for front-end parallel computing, such as image processing, and

Debugging complex JavaScript applications requires systematic use tools. 1. Set breakpoints and conditional breakpoints to intercept suspicious processes, such as before function entry, loop, asynchronous callback and filter according to conditions; 2. Enable Blackboxing function to block third-party library interference; 3. Use debugger statements to control debug entry based on environmental judgment; 4. Trace the call link through CallStack, analyze the execution path and variable status, thereby efficiently locate the root cause of the problem.