Native JavaScript Learning JS variable Comprehensive understanding of the basics

1. Name of variable

Name of method (hump naming method)

All lowercase: All underscores between words and words (my_namespace)

Mixed case: First letter lowercase other words in uppercase.

Rules

First character English letter or underline

Make up an English alphanumeric underline

Taboo JavaScript keyword reserved word

2. Declaration of variables　

Display declaration: var keyword

Bad habits: No type duplicate declaration an implicit declaration does not declare a direct assignment

Positive solution: First declare after read and write first assignment after operation

3. Variable type　

Value type

A The Occupy space is fixed in the stack

B The value itself is saved and assigned

C) using typeof to detect the type of data

D) Basic type data is a value type

Reference type

A the occupied space is not fixed and stored in the heap

B to save and copy is a pointer to the object

C) using instanceof to detect the type of data

D the object constructed using the new () method is a reference type

4. Variable Scope　 　

Global variables that contain variables defined outside the function body define no var in the function body not recommended (any location can be invoked)

A local variable contains a parameter variable of a variable function declared using var within the body of a function (current function body part)

Scope Chain: Inner function can access outer function local variable

Outer function cannot access inner-layer function local variable

Lifecycle: Global variables exist unless they are displayed or deleted

local variable from declaration to function completed or displayed delete

Recycle mechanism tag Clear reference count

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