


Javascript Study Notes - Functions (4): arguments object_basic knowledge
Every Javascript function can access a special variable in its own scope - arguments. This variable contains a list of all arguments passed to the function.
The arguments object is not an array. Although syntactically it has the same features as an array, for example it has a length property. But it does not inherit from Array.prototype, in fact, it is an object.
Therefore, we cannot directly use some array methods such as push, pop or slice on arguments. So in order to use these methods, we need to convert it to a real array.
Convert to array
The following code will return an array containing all elements of the arguments object.
Array.prototype.slice.call(arguments);
Since the conversion is very slow, this is not recommended in performance-critical programs.
Pass parameters
The following is a recommended way to pass the arguments object from one function to another.
function foo() {
bar.apply(null, arguments);
}
function bar(a, b, c) {
// do stuff here
}
Another clever method is to use call and apply at the same time to quickly create an unbound outer method.
function Foo() {}
Foo.prototype.method = function(a, b, c) {
console.log(this, a, b, c);
};
// Create an unbound version of "method"
// It takes the parameters: this, arg1, arg2...argN
Foo.method = function() {
// Result: Foo.prototype.method.call(this, arg1, arg2... argN)
Function.call.apply(Foo.prototype.method, arguments);
};
The relationship between function parameters and arguments attribute
Thearguments object creates getter and setter methods for both its own properties and the formal parameters of its functions.
Therefore, modifying the formal parameters of a function will affect the property values of the corresponding arguments object, and vice versa.
function foo(a, b, c) {
arguments[0] = 2;
a; // 2
b = 4;
arguments[1]; // 4
var d = c;
d = 9;
c; // 3
}
foo(1, 2, 3);
Performance issues
arguments will not be created in only two situations, one is declared as a local variable inside the function, and the other is used as a formal parameter of the function. Otherwise, the arguments object is always created.
Since getter and setter methods are always created with the arguments object, using arguments itself has little impact on performance.
However, there is one situation that seriously affects the performance of Javascript, and that is the use of arguments.callee.
function foo() {
arguments.callee; // do something with this function object
arguments.callee.caller; // and the calling function object
}
function bigLoop() {
for(var i = 0; i foo(); // Would normally be inlined...
}
}
In the above code, the foo function is no longer a simple inline extension, because it needs to know both itself and its caller. This not only negates the performance gain brought by inline expansion, but also destroys the encapsulation of the function, because the function itself may need to depend on a specific calling context.
Therefore, it is recommended that you try not to use arguments.callee.
The above is all about the Javascript arguments object. Do you guys understand it thoroughly? Simply put
arguments refers to the parameter object of the function (referring to the actual parameters passed in)
arguments.length refers to the length of the parameter object of the function
arguments[i] refers to the value of the i-th parameter (the first one is 0)

The differences in performance and efficiency between Python and JavaScript are mainly reflected in: 1) As an interpreted language, Python runs slowly but has high development efficiency and is suitable for rapid prototype development; 2) JavaScript is limited to single thread in the browser, but multi-threading and asynchronous I/O can be used to improve performance in Node.js, and both have advantages in actual projects.

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.


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