A comprehensive understanding of js variables in native javascript learning

  • 2021-07-02 23:35:56
  • OfStack

1. Naming variables

Nomenclature of methods (hump nomenclature)

All lowercase: All underscores between words (my_namespace)

Mixed case: The first word starts with lowercase and other words start with uppercase.

Rules

First character English letter or underscore

Form an English alphanumeric underline

Taboo JavaScript Keyword Reserved Word

2. Declaration of variables

Display declaration: var keyword

Bad habit: No type duplicate declaration Implicit declaration No direct assignment declaration

Positive solution: first declaration, then read and write, first assignment, then operation

3. Variable types

Value type

A) The occupied space is fixed in the stack

B) What holds and assigns a value is the value itself

C) Use typeof to detect the type of data

D) Primitive type data is a value type

Reference type

A) Occupied space is not fixed in the heap

B) Save and copy a pointer to an object

C) Use instanceof to detect the type of data

D) Objects constructed using the new () method are reference types

4. Variable scope

Global variables contain variables defined outside the function. Variables defined inside the function are not var. Not recommended (can be called anywhere)

Local variables contain the parameter variables of the variable function declared using var inside the function body (inside the current function body)

Scope chain: inner function can access local variables of outer function

Outer function cannot access local variables of inner function

Lifecycle: The global variable 1 exists unless it is displayed and deleted

Local variables are declared until the function runs or is displayed and deleted

Reclaim mechanism flag clear reference count


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