What is the scope of variables in JavaScript?

What is the scope of variables in javascript? Do they have the same scope inside as opposed to outside a function? Or does it even matter? Also, where are the variables stored if they are defined globally?

 

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TLDR  太长不看版

JavaScript has lexical (also called static) scoping and closures. This means you can tell the scope of an identifier by looking at the source code.

The four scopes are:

1. Global - visible by everything
2. Function - visible within a function (and its sub-functions and blocks)
3. Block - visible within a block (and its sub-blocks)
4. Module - visible within a module

Outside of the special cases of global and module scope, variables are declared using var (function scope), let (block scope), and const (block scope). Most other forms of identifier declaration have block scope in strict mode.

 

Overview

Scope is the region of the codebase over which an identifier is valid.

A lexical environment is a mapping between identifier names and the values associated with them.

Scope is formed of a linked nesting of lexical environments, with each level in the nesting corresponding to a lexical environment of an ancestor execution context.

These linked lexical environments form a scope "chain". Identifier resolution is the process of searching along this chain for a matching identifier.

Identifier resolution only occurs in one direction: outwards. In this way, outer lexical environments cannot "see" into inner lexical environments.

There are three pertinent相关的 factors in deciding the scope of an identifier in JavaScript:

1. How an identifier was declared
2. Where an identifier was declared
3. Whether you are in strict mode or non-strict mode

Some of the ways identifiers can be declared:   变量如何被声明的

1. var, let and const
2. Function parameters
3. Catch block parameter
4. Function declarations
5. Named function expressions
6. Implicitly defined properties on the global object (i.e., missing out var in non-strict mode)
7. import statements
8. eval

Some of the locations identifiers can be declared:   变量在什么位置被声明的

1. Global context
2. Function body
3. Ordinary block
4. The top of a control structure (e.g., loop, if, while, etc.)
5. Control structure body
6. Modules