day06 interview questions

1. Webpack packaging principle

Webpack is a modern packaging tool. Its main function is to package and convert multiple modules according to specific rules, and finally generate static resources for the browser. The following is a brief packaging principle of Webpack:

1. Entry Point:
   Specifies the entry file for Webpack packaging, which can be a single file or multiple files. Webpack will analyze dependencies starting from the entry file.

2. Module Resolution:
   Webpack parses the path of the entry file and its related dependencies to determine the location of each module. Supports parsing different module types, such as JavaScript, CSS, images, etc.

3. Dependency Graph:
   Webpack builds a complete dependency graph by recursively traversing the dependencies of the entry file to understand the dependencies between all modules.

4. Loaders:
   Webpack uses loaders to handle non-JavaScript files. Loaders can convert various types of files into modules so that they can be bundled by Webpack. The loader is applied during the module resolution phase.

5. Plugins:
   Webpack plug-ins are used to perform a wider range of tasks, such as optimizing packaging results, resource management, environment variable injection, etc. Plug-ins operate during the packaging process through a hook mechanism.

6. Packaging output (Output):
   Webpack determines the output method of packaging results according to the configuration. Multiple modules can be packaged into one or more files, or file names with hash values ​​can be generated to implement a caching mechanism.

7. Optimization and Minification:
   Webpack will perform a series of optimization steps during the packaging process, such as removing unused code, extracting common modules, etc. Code compression and obfuscation can also be enabled through configuration to reduce the packaged file size.

8. Hot Module Replacement:
   Webpack supports module hot replacement, that is, during the development phase, only the changed modules are updated without refreshing the entire page, improving development efficiency.

Webpack's packaging principle involves multiple stages and concepts, and it processes all modules and resources into final static output. By configuring different rules and plug-ins, Webpack can be highly customized to meet various project needs.

2. How to optimize webpack packaging speed

To optimize the packaging speed of Webpack, you can consider the following aspects:

1. Use the latest version: Make sure to use the latest version of Webpack as every new version comes with performance improvements.

2. Reduce the number of files: Reducing the number of entry files and dependent modules can reduce the number of files that Webpack needs to process, thereby increasing packaging speed. Merging multiple small files into one large file or using Dynamic Import can help achieve this goal.

3. Configure reasonable resolveoptions: By properly configuring Webpack resolveoptions, you can reduce the time spent by Webpack in the module parsing process. For example, set extensionsto clearly specify the suffix name of the module type, use it aliasto map common module paths, and avoid Webpack from recursively parsing.

4. Use Loaders rationally: Try to streamline and optimize custom Loaders to avoid unnecessary file conversion and processing operations to reduce packaging time.

5. Use HappyPack or thread-loader plug-ins: These plug-ins can process some tasks in parallel to take advantage of multi-core CPUs to increase packaging speed.

6. Use cache: Enable cachethe option to cache already built content to avoid repeated builds and reduce packaging time. Consider adopting a persistent caching strategy, such as using webpack-parallel-uglify-pluginplugins and caching options of babel-loader.

7. Code Splitting: Use Webpack's code splitting function to logically divide the code into multiple blocks to achieve on-demand loading and parallel loading. This can improve the initial load speed of the page.

8. modeUse Tree Shaking: By configuring Webpack production, enable Tree Shaking to eliminate unused code and reduce the packaging size.

9. Parallelized build: With the help of tools such as Webpack Parallelism Plugin, each task can be executed in parallel between multiple processes to increase build speed.

10. Reduce the use of plug-ins: Evaluate whether the configured plug-ins are really needed. Reducing the use of unnecessary plug-ins can reduce the burden of Webpack and increase the packaging speed.

Please note that optimizing Webpack's packaging speed is a comprehensive process, and different application scenarios may have different optimization strategies. It is recommended to analyze and adjust based on specific project needs and performance bottlenecks.

3. Talk about the common Loaders in webpack? What problem was solved?

Common Loaders in Webpack include:

1. Babel Loader: Convert ES6+ JavaScript code into ES5 code compatible with different browsers.

2. CSS Loader: Loads and parses CSS files and handles operations such as URL reference, conversion, and compression.

3. Style Loader: Injects CSS code into HTML pages

These Loaders mainly solve the following problems:

1. Support for non-JavaScript file types: Webpack is a module packaging tool, but it only processes JavaScript files by default. Using Loader allows Webpack to process and recognize other file types and introduce them into the application as modules.

2. Convert and process file content: Loader can perform various conversion and processing operations, such as converting ES6+ code to ES5 code, converting Sass/Less/Stylus to CSS, compressing and optimizing images, etc.

3. Handle dependencies between modules: Loader can parse dependencies between modules and package dependent modules into output files.

4. Improve development efficiency: Through Loader automated processing, the workload of manual settings and operations is reduced, and development efficiency is improved.

All in all, the Loader in Webpack implements modular processing and conversion of non-JavaScript resources, allowing developers to easily use various front-end resources and providing a more efficient development and construction process.

4. Talk about the common Plugins in webpack? What problem was solved?

Common Plugins in Webpack include:

1. HtmlWebpackPlugin: Generate HTML files and automatically inject packaged resources (such as CSS, JavaScript) into HTML.

2. MiniCssExtractPlugin: Extracts CSS code from packaged JavaScript files and generates independent CSS files.

3. DefinePlugin: Define global variables, which can be used in code to achieve configuration management in different environments.

4. CleanWebpackPlugin: Clean the output directory before each build to prevent old files from remaining.

5. CopyWebpackPlugin: Copy static resource files to the output directory.

6. HotModuleReplacementPlugin: Enables the hot module replacement function, which eliminates the need to refresh the entire page when updating modules.

7. ProvidePlugin: Automatically loads the variables required in the module, reducing the work of manual introduction.

8. UglifyJsPlugin: Compresses and obfuscates JavaScript code to improve execution efficiency and loading speed.

9. OptimizeCSSAssetsPlugin: Optimize and compress CSS code to reduce file size.

10. BundleAnalyzerPlugin: Visually displays the size and composition of packaged files to help optimize packaging configuration.

These Plugins mainly solve the following problems:

1. Processing output files: Through plug-ins such as HtmlWebpackPlugin and MiniCssExtractPlugin, you can easily handle the generation and optimization of output files, including HTML file generation, CSS file extraction, etc.

2. Resource management and optimization: Through plug-ins such as CopyWebpackPlugin and UglifyJsPlugin, static resource copying and optimization can be realized, including copying static files, compressing JavaScript, etc.

3. Enhanced development experience: Through the HotModuleReplacementPlugin plug-in, hot module replacement can be achieved, and modules can be updated without refreshing the entire page, improving development efficiency.

4. Environment configuration management: Through the DefinePlugin plug-in, you can define global variables and use different configurations in the code according to the environment to facilitate development and deployment in different environments.

5. Performance optimization and analysis: Through plug-ins such as OptimizeCSSAssetsPlugin and BundleAnalyzerPlugin, CSS and packaged files can be optimized and analyzed to improve performance and maintainability.

In short, Webpack's Plugin has expanded its functions and capabilities to solve problems such as output file generation and optimization, resource management and optimization, enhanced development experience, environment configuration management, and performance optimization and analysis, helping developers better build and optimize Front-end application.

5. Tell me about your understanding of promises

Promise is a JavaScript object used to handle asynchronous operations. It represents the final completion (or failure) of an asynchronous operation and its result. Promise has the following characteristics:

1. Status: Promise objects have three statuses: pending, fulfilled, and rejected. The initial state is pending. When the asynchronous operation is executed successfully, the Promise enters the completed state and returns the result; when the asynchronous operation fails, the Promise enters the rejected state and returns an error message. Once in one of these states, a Promise will never change state again.

2. Chained calls of asynchronous operations: Through the then() method of Promise, multiple asynchronous operations can be called in a chained manner. Each time the then() method is called, a new Promise instance will be returned, allowing multiple asynchronous operations to be executed in sequence and the results passed to subsequent operations. This chained call method can avoid callback hell and improve code readability and maintainability.

3. Error handling: Promise provides the catch() method to handle errors that occur in the Promise chain. When an error occurs in any Promise in the chain, the error will be passed to the nearest catch() method for processing. Without explicit error handling, errors are passed down the chain until they are caught or the end of the Promise chain is reached.

4. Execution sequence control: Through some special methods of Promise, such as Promise.all() and Promise.race(), multiple Promises can be controlled centrally. Promise.all() receives a Promise array as a parameter. When all Promises are completed, it returns a new Promise, containing the results of all Promise; Promise.race() receives a Promise array as a parameter. When any of them When a Promise completes, it immediately returns a new Promise, using the result of the first completed Promise.

By using Promise, you can handle asynchronous operations more elegantly, avoid callback hell, and improve the readability and maintainability of your code.

6.What is the async function and what does it do?

The async function is a special function used to define asynchronous operations. It can be defined by preceding the function declaration with asyncthe keyword. Using async functions can make writing asynchronous code more concise and readable.

The role of async function is mainly reflected in the following aspects:

1. Asynchronous operation: The keyword can be used inside the async function awaitto wait for an expression that returns a Promise to complete. This makes it more convenient to perform asynchronous operations in async functions. When awaita keyword is encountered, the async function pauses execution and waits for the awaited Promise object to enter the completion state and return the result. At the same time, the event loop can continue to perform other tasks to avoid blocking.

2. Error handling: Inside the async function, you can use the try-catch statement to capture and handle possible errors. By catching exceptions, we can handle errors at the appropriate location to avoid errors that cause the entire program to crash or generate unhandled exceptions.

3. Return Promise object: async functions always return a Promise object. The state of the Promise object is determined based on the execution results of the async function. If a Promise object is not explicitly returned within the async function, the return value will be automatically wrapped into a Promise object in the fulfilled state, and the Promise object will be returned.

4. Simplification of Promise chain: Promise chain can be written with concise syntax using async function. Inside an async function, you can write asynchronous operations in a linear fashion and awaitwait for each operation to complete via the keyword. This avoids deeply nested callback functions and makes the code more readable and maintainable.

All in all, the async function provides a more concise and clear coding method when handling asynchronous operations, making it easier to write and read asynchronous code. It also provides error handling capabilities, combining asynchronous operations with synchronous code more closely and elegantly.