What is the scope of variables in JavaScript? 還沒看完

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  太長不看版

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相關的 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:   變量如何被聲明的

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:   變量在什麼位置被聲明的

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

 

Declaration Styles

var

Identifiers declared using var have function scope, apart from when they are declared directly in the global context, in which case they are added as properties on the global object and have global scope. There are separate rules for their use in eval functions.

var聲明的變量，有function scope（除了那些全局定義的）

let and const

Identifiers declared using let and const have block scope, apart from when they are declared directly in the global context, in which case they have global scope.

Note: let, const and var are all hoisted. This means that their logical position of definition is the top of their enclosing scope (block or function). However, variables declared using let and const cannot be read or assigned to until control has passed the point of declaration in the source code. The interim臨時的 period is known as the temporal dead zone.

function f() {
    function g() {
        console.log(x)
    }
    let x = 1
    g()
}
f() // 1 because x is hoisted even though declared with `let`!