


The user uploads a photo, and the size of the photo is unknown; a preview needs to be generated. This preview image must be applied according to the area provided to the user for preview and centered; if the image is too large, it needs to be scaled proportionally. As shown below.
Having a quick look, centering can be achieved using text-align:center;. However, proportional scaling cannot be solved by using the width and height attributes of the default . Because the user picture may be very long or very wide. I thought about their relationship online and judged it based on the conditions:
if(actual width > preview maximum width) {
Zoom width = preview maximum width
}
if(actual height> preview maximum height) {
Scaled height = preview maximum height
}
But there will be problems, such as when the width and height are both scaled, if the scaling is different, The image will not be scaled proportionally and will become very ugly. In order to make it scale proportionally, various judgments need to be made. Then this goes against the principle of automating the program we want. Thinking about it again, although I don’t like mathematics, mathematics is still very useful, and there should be other ways. And since it is scaled proportionally, why not use the ratio between the actual image and the width of the preview area to calculate their relationship? hmmmm... It's really OK. In fact we only ever need to scale either width or height. Because the ratio is only for large and small species. Specifically, write a function to implement it:
/ **
* Image adaptive scaling
* @param img {Element} User uploaded image
* @param maxWidth {Number} Maximum width of the preview area
* @param maxHeight {Number} Preview The maximum height of the area
*/
var resizeImg = function(img, maxWidth, maxHeight){
var w = img.width,
h = img.height;
// When the picture does not make any changes than the preview area,
IF (w & lt; maxwidth && h & lt; maxheight) Return;
// When the actual picture ratio is greater than the width ratio of the preview area,
// Scaling picture width, otherwise the width of the zoom
w/h & gt; maxwidth/maxheight? Img.width = maxwidth: img.height = maxHeight;

JavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.

JavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.


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