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 In the past, when we wanted to assign a value to a variable, such as an array type and an object type, to assign a value to a variable, we could only specify the value directly. It is very troublesome to write in this way, but under the ES6 syntax specification, it is allowed to directly extract the required values from arrays and objects according to a certain pattern, and directly assign variables to variables. This method is called Destructuring, which is simple to understand. That is, the left and right sides of the equal sign are equal.

 Due to browser same-origin policy restrictions. The Same Origin Policy is a convention. This is the main and essential security feature of the browser. Without the same-origin policy, normal browser functionality may be affected. The web is built on the same-origin policy, and browsers are just one implementation of the same-origin policy. The Same Origin Policy prevents JavaScript scripts from one domain from interacting with content from another domain. The so-called same origin (that is, the same domain) means that two pages have the same protocol, host and port.

 The main difference between them is: one exists locally, the other is a temporary session, and the storage time is different. The data saved in localStorage exists for a long time. When we visit the website next time, the webpage can directly read the data saved before; the data saved in sessionStorage is used for a session of the browser. When the session ends, such as closing the browser After the window, the data will be cleared. Aside from the difference in shelf life, the two objects are otherwise identical. The data they store is in the form of "key-value pairs". That is, each item of data has a key name and corresponding value. All data is stored in the form of strings. Due to cross-domain restrictions, they are only used on the client side and do not communicate with the server side.The size of the stored data is generally 5M.

 The reduce method contains two parameters, the first parameter is a callback function, which contains four parameters, the second parameter is an optional parameter, as the first parameter of the callback function. It executes the callback function for each element in the array in turn, excluding elements that are deleted or never assigned in the array, and accepts four parameters: the initial value (or the return value of the last callback function), the current element value, the current Index, the array on which reduce is called.

 In daily coding, date data is often formatted, and Date objects are used to process dates and times. This article first introduces the Date object, and then deepens its understanding and practical application through a small case of formatting a date, including function use cases such as year, month, time, etc., friends who need it can read and refer to it.

 What is a dom tree? In short, DOM is the Document Object Model, which provides a structured representation for documents and defines how to access the document structure through scripts. DOM is composed of nodes. After the HTML is loaded, the rendering engine will generate a DOM tree in memory based on the HTML document. This article uses a small case to traverse the dom tree recursively. The core of the method is to determine whether the incoming data is an array, and then traverse the root node. Note that there must be an end condition when using recursion.

 Deep copy is to re-open a new reference address for the variable, which is completely separated from the original, which is equivalent to dividing a new space for itself. Although the content in it is the same, even if one of them is changed, the other is not. will be affected. Shallow copy means that the reference address of the reference type in the stack cannot be changed. The two variables share the same reference address and change the things in the same place. Therefore, the shallow copy causes the change of one variable to cause the same change of the other variable. .

In JS, pseudo-arrays are very common, also called array-like. So it is very important to understand and understand good pseudo-arrays. This article will explain in detail what a pseudo-array is and how to convert a pseudo-array into a real array in ES5 and ES6 respectively. What is a pseudo-array? The main characteristic of a pseudo-array: it is an object, and that object has a length property.

 The role of call, apply, and bind is to change the context of function execution, and when they are used, they will change the this pointing to when the function is running. call, apply, and bind are all methods under Function.prototype, which are used to change the runtime context of the function. The final return value is the return value of the method you called. If the method has no return value, it returns undefined.

 Functions and methods are an important knowledge point in programming. The key point of a function is how to call and execute. The way it is called has a huge impact on how the code inside the function is executed, especially in the creation of the this parameter. In addition to the most basic calling method such as function name (), what other methods are there? The following briefly introduces the three most basic (excluding, bind, apply, call).

 Arrays are frequently used in development, and processing data in arrays is one of the more common and important operations. Therefore, processing data in arrays during development is an important skill. Every developer Everyone should master the operations of arrays, especially for junior developers who have just entered the industry, so be sure to master the relevant skills. This article mainly shares some operations to filter the data in the array, about the use of the filter() method.

 In javascript, for in is a standard in ES5, used to traverse keys. for of is a standard in ES6, used to traverse values. This article mainly explains for...in and for...of difference.

 A Set is a data structure called a collection, and a Map is a data structure called a dictionary.Both sets and dictionaries can store unique values, which are convenient for deduplication. Sets store elements in the form of [value, value], and dictionaries store elements in the form of [key, value].A Set can hold any value of any data type. Let's first briefly understand the properties and methods of the collection, and then study the specific use of this collection.

 What is AJAX? AJAX stands for "Asynchronous Javascript And XML" (Asynchronous JavaScript and XML), which refers to a web development technology for creating interactive web applications for data interaction between browsers and servers. AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, which allows web pages to request small amounts of information from the server instead of the entire page. AJAX describes a web application architecture that mainly uses scripts to manipulate HTTP. The main feature of AJAX applications is to use scripts to manipulate HTTP and web servers for data exchange without causing page reloads. Simply put, without reloading the entire web page, AJAX loads data in the background and displays it on the web page, which can partially refresh the page. The user has a better experience using the web page, but cannot go back.

 What is an iterator?An iterator is an interface that provides a unified access mechanism for various data structures. Any data structure can complete the traversal operation as long as the iterator interface is deployed.ES6 created a new traversal command for...of loop, which natively has a data structure with the iterator interface (which can be traversed with for...of). Contains Array, Arguments, Set, Map, String, TypedArray, NodeList. Of course, you can also implement this interface manually, which is convenient for practical use.

 In ES6, in addition to the new features of let and const, arrow functions are the most frequently used new features. But this feature is not unique to ES6. Arrow functions, as the name suggests, are functions defined using arrows (=>) and belong to a class of anonymous functions. It allows us to write smaller function syntaxes. The code of arrow functions is simpler and more flexible to write.

 Why does js introduce template strings?