The understanding of prototype chain and scope chain in js and the problem of this pointing [Picture]

JS Detailed explanation of scope and scope chain: Recommend an article http://www.php.cn/js-tutorial-8031.html

It’s a difficult sentence to understand. Here's a look: When code is executed in an environment, a scope chain of variable objects is created. The purpose of the scope chain is to ensure ordered access to all variables and functions that the execution environment has access to

Scope: The scope of a variable is the one defined in the program source code variable area. (Note that the scope of a variable is determined when the variable is defined, not when a method is executed)

es5 has two scopes, function scope and global scope

Each piece of JavaScript code (global code or function) has a scope chain associated with it.

This scope chain is a list or linked list of objects. This group of objects defines the variables "in scope" in this code.

When js needs to find the value of variable x (this process is called variable resolution), it will start searching from the first object in the chain. If this object has an object named x attribute, the value of this attribute will be used directly. If there is no attribute named x in the first object, js will continue to search for the next object in the chain. If the second object still does not have an attribute named x, it will continue to look for the next one, and so on. If no object in the scope chain contains attribute x, then it is considered that x does not exist in the scope chain of this code, and eventually a ReferenceError exception is thrown.

Summary: The function of the scope chain is that for variables, you can find the value according to this chain. You can only search upwards. If you find the first one, you will no longer search. This scope and the scope chain are both It is determined when the variable is defined, regardless of the order of execution.

Some questions:

The execution result of this code is 3, because when function c is defined, it constitutes a scope, and its previous The scope is the global one, not the self-executing function (

The scope and scope chain are determined when the variable is defined, and have nothing to do with the order of execution.

)

The reason why i can be printed outside the for statement in this question is because the js statement does not constitute a scope, and its scope still belongs to the global world. Changing var to let will Report an error, try

#This question is relatively comprehensive. I have to understand it carefully many times before I understand it. It is worth studying, so I won’t write an explanation first. . think. . .

This question requires a good understanding of the prototype chain. The running result is

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