Introduction to DSL metaprogramming in JavaScript_javascript skills
When I was looking at the JavaScript Template source code, I found a very interesting usage for generating functions. I thought, isn’t this metaprogramming?
JavaScript Metaprogramming
Metaprogramming refers to the writing of certain types of computer programs that write or manipulate other programs (or themselves) as their data, or complete some work at runtime that should be completed at compile time.
JavaScript eval
The eval() function evaluates a string and executes the JavaScript code within it.
It can be used as follows:
eval("x=10;y=20;document.write(x*y)")
Of course, this is only used to execute a certain function, but this product is very cost-effective and error-prone.
The eval function should be avoided whenever possible.
So a better way is to use New Function()
The big difference between using New Function() and eval() is that eval is not just a function,
eval() evaluates a string as a JavaScript expression within the current execution scope and can access local variables. New Function() parses a string stored in it into a function object that can then be called by JavaScript code. Because the code runs in a separate scope, local variables cannot be accessed.
In other words, eval() will interfere with the scope of the current function. .
JavaScript new Function()
The Function constructor creates a new Function object. In JavaScript, each function (function) is actually a Function object. The Function object generated using the Function constructor is parsed when the function is created. This is less efficient than if you declare a function and call it in your code, because functions declared using function statements are parsed along with other statements.
New Function() will be less efficient in comparison, and this is what we can predict under the current situation.
A simple example looks like this:
var add = new Function(['x', 'y'], 'return x y');
new Function() will parse the string into a function. . Then we can execute it through apply
Function.apply(null, args)
And this is what I see in the JavaScript Template:
new Function(
tmpl.arg ',tmpl',
"var _e=tmpl.encode" tmpl.helper ",_s='"
str.replace(tmpl.regexp, tmpl.func)
"';return _s;"
);
Of course we have other methods.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.

Understanding how JavaScript engine works internally is important to developers because it helps write more efficient code and understand performance bottlenecks and optimization strategies. 1) The engine's workflow includes three stages: parsing, compiling and execution; 2) During the execution process, the engine will perform dynamic optimization, such as inline cache and hidden classes; 3) Best practices include avoiding global variables, optimizing loops, using const and lets, and avoiding excessive use of closures.

Python is more suitable for beginners, with a smooth learning curve and concise syntax; JavaScript is suitable for front-end development, with a steep learning curve and flexible syntax. 1. Python syntax is intuitive and suitable for data science and back-end development. 2. JavaScript is flexible and widely used in front-end and server-side programming.

Python and JavaScript have their own advantages and disadvantages in terms of community, libraries and resources. 1) The Python community is friendly and suitable for beginners, but the front-end development resources are not as rich as JavaScript. 2) Python is powerful in data science and machine learning libraries, while JavaScript is better in front-end development libraries and frameworks. 3) Both have rich learning resources, but Python is suitable for starting with official documents, while JavaScript is better with MDNWebDocs. The choice should be based on project needs and personal interests.

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

Different JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.

JavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.


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