js execution context variables, functions, this
This article mainly introduces about js execution context variables, functions, and this. It has certain reference value. Now I share it with you. Friends in need can refer to
Execution in JavaScript Context and call stack
ES6 variable scope and promotion: detailed explanation of the life cycle of variables
Variable promotion
The definition of variables is in the code When preparsing, define at the top of the scope
No var No variable promotion
console.log(a); // undefined，如果没有定义会直接报错 var a = 'aaa'; console.log(a); // aaa // 下面代码全等于上面代码 var a; // 变量提升，函数作用域范围内 console.log(a); // undefined a = 'aaa'; console.log(a); // aaa console.log(a); // 直接报错 a = 'aaa';
Function promotion
-
The definition of the function is defined at the top of the scope during code pre-parsing
The function assignment is at the top of the scope
console.log(f1); // f1() { console.info('函数'); }
var f1 = function() { console.info('变量'); }
console.log(f1); // ƒ () { console.info('变量'); }
function f1() { console.info('函数'); }
console.log(f1); // ƒ () { console.info('变量'); }
// 下面代码全等于上面代码
var f1; // 定义提升
function f1() { console.info('函数'); } // 函数顶部赋值
console.log(f1); // f1() { console.info('函数'); }
f1 = function() { console.info('变量'); }
console.log(f1); // ƒ () { console.info('变量'); }
console.log(f1); // ƒ () { console.info('变量'); }Function context Relationship
The context of a function is determined when it is defined
var scope = "global scope";
function checkscope() {
var scope = "local scope";
function f() { return scope; }
return f;
}
checkscope()(); // local scopethis Context
this The context relationship is determined at execution time
Normal function call, this points to the window
// 在 function 里
function test() {
var type = this === window;
return type;
}
test(); // truemethod call, this points to the calling object
// 在对象里
var obj = {
test: function() {
var type = this === obj;
return type;
}
};
obj.test(); // true
// 在 prototype 对象的方法中
function obj() {
}
obj.prototype.test = function() {
return this;
}
var o = new obj();
o.test() === o; // trueconstructor function Call, this points to the object generated by new
// 调用 new 构造对象时
function obj() {
this.test = function() {
return this;
}
}
var o = new obj();
o.test() === o; // trueapply / call Call
function test() {
return this;
}
var o = {};
// apply
test.apply(o) === o; // true
// call
test.call(o) === o; // truedom’s event attribute
// 点击后输出 true
<input id="a" type="text" onclick="console.info(this === document.getElementById('a'))" />
// 点击后输出 true
<input id="a" type="text" />
<script type="text/javascript">
document.getElementById('a').addEventListener("click", function(）{
console.info(this === document.getElementById('a'));
});
</script>
// 点击后输出 true
<input id="a" type="text" />
<script type="text/javascript">
document.getElementById('a').onclick = function(）{
console.info(this === document.getElementById('a'));
});
</script>The above is the entire content of this article, I hope it will help everyone learn Helpful, please pay attention to the PHP Chinese website for more related content!
Related recommendations:
JS and JQ realize focus chart carousel effect
js realizes editing by clicking the button
About JS
Introduction to inheritance
The above is the detailed content of js execution context variables, functions, this. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
What should I do if I encounter garbled code printing for front-end thermal paper receipts?
Apr 04, 2025 pm 02:42 PM
Frequently Asked Questions and Solutions for Front-end Thermal Paper Ticket Printing In Front-end Development, Ticket Printing is a common requirement. However, many developers are implementing...
Demystifying JavaScript: What It Does and Why It Matters
Apr 09, 2025 am 12:07 AM
JavaScript is the cornerstone of modern web development, and its main functions include event-driven programming, dynamic content generation and asynchronous programming. 1) Event-driven programming allows web pages to change dynamically according to user operations. 2) Dynamic content generation allows page content to be adjusted according to conditions. 3) Asynchronous programming ensures that the user interface is not blocked. JavaScript is widely used in web interaction, single-page application and server-side development, greatly improving the flexibility of user experience and cross-platform development.
Who gets paid more Python or JavaScript?
Apr 04, 2025 am 12:09 AM
There is no absolute salary for Python and JavaScript developers, depending on skills and industry needs. 1. Python may be paid more in data science and machine learning. 2. JavaScript has great demand in front-end and full-stack development, and its salary is also considerable. 3. Influencing factors include experience, geographical location, company size and specific skills.
How to merge array elements with the same ID into one object using JavaScript?
Apr 04, 2025 pm 05:09 PM
How to merge array elements with the same ID into one object in JavaScript? When processing data, we often encounter the need to have the same ID...
Is JavaScript hard to learn?
Apr 03, 2025 am 12:20 AM
Learning JavaScript is not difficult, but it is challenging. 1) Understand basic concepts such as variables, data types, functions, etc. 2) Master asynchronous programming and implement it through event loops. 3) Use DOM operations and Promise to handle asynchronous requests. 4) Avoid common mistakes and use debugging techniques. 5) Optimize performance and follow best practices.
How to achieve parallax scrolling and element animation effects, like Shiseido's official website?
or:
How can we achieve the animation effect accompanied by page scrolling like Shiseido's official website?
Apr 04, 2025 pm 05:36 PM
Discussion on the realization of parallax scrolling and element animation effects in this article will explore how to achieve similar to Shiseido official website (https://www.shiseido.co.jp/sb/wonderland/)...
The Evolution of JavaScript: Current Trends and Future Prospects
Apr 10, 2025 am 09:33 AM
The latest trends in JavaScript include the rise of TypeScript, the popularity of modern frameworks and libraries, and the application of WebAssembly. Future prospects cover more powerful type systems, the development of server-side JavaScript, the expansion of artificial intelligence and machine learning, and the potential of IoT and edge computing.
The difference in console.log output result: Why are the two calls different?
Apr 04, 2025 pm 05:12 PM
In-depth discussion of the root causes of the difference in console.log output. This article will analyze the differences in the output results of console.log function in a piece of code and explain the reasons behind it. �...
