Classification parameter stack of JS common built-in object data types

Classification of objects

• JS The objects in are divided into ： Custom object 、 Built-in objects 、 Browser object
• The first two objects are JS Basic content , Belong to ECMAscript, The third browser object is JS alone possess Of
• Built-in objects yes JS Some self-contained objects , For developers , Provide common or basic properties and methods
• The biggest advantage of built-in objects is that they help us develop quickly
• JS Provide more to built-in objects ：Math、Date、Array、String etc.

MDN Use of documents

1. Check the function of this method
2. Check the meaning and type of parameters inside
3. Check the meaning and type of the return value
4. adopt demo To test

Math object

Not a constructor , So no need new To call , Instead, you can directly use the properties and methods inside

• Math.PI                               // PI
• Math.floor（）                   // Rounding down
• Math.ceil（）                   // Rounding up
• Math.round （）                   // Round the page Round to the nearest     -3.5 The result is -3
• Math.abs（）                   // The absolute value
• Math.max（） Math.min（）            // Maximum and minimum

Math Object random number method

• Math.random()  Returns a random decimal , The scope is [ 0 , 1 ) Between , from 0 Start （ Include 0） To 1（ barring 1）
• There are no parameters in this method

Date Date object

It's a constructor , Must use new Call create date object

• If there are no parameters , Returns the current time of the current system
• Parameters are commonly written   Digital 2019,10,01 （ The return is 11 Month is not 10 month ） String type ‘2019 - 10 - 1 8：8：8’
• Used to handle dates and times

Date formatting

getMonth（） The returned month is small 1 Months , need +1

getDay（） The return on Sunday is 0

Get the total number of milliseconds of the date

• valueOf（）
• getTime（）
• Simple writing     var date1 = +newDate（）        + newDate  The total number of milliseconds is returned   （ The most commonly used ）
• console.log（Date.now（））   H5 newly added

Time stamp , Never repeat

Countdown case analysis

1. Subtract the present time from the future time
2. Use time stamps to do , The total milliseconds of user input time minus the total milliseconds of current time
3. Convert the remaining total milliseconds into days 、 when 、 branch 、 second

Array Array objects

Two ways ： Literal [ ]        new keyword

var arr1 = newArray（2）     //  2  Represents the length of an array  , There are two empty array elements 

var arr1 = newArray（2,3） amount to var arr1 = [2,3]  Indicates that there are two array elements  2  Follow  3

Check if it's an array

1. instanceOf  Operator It can be used to detect whether it is an array
2. Array.isArray( Parameters )

Add a method to delete array elements

Method name	explain	Return value
push（ Parameters 1）	Add one or more elements at the end , Pay attention to modifying the original array	And returns the new length
pop（）		Returns the value of the element it deleted
unshift（ Parameters 1）	Add one or more elements to the beginning of an array , Pay attention to modifying the original array	And returns the new length
shift（）	Deletes the first element of the array , Array length minus 1 No parameter 、 Modify the original array	And returns the value of the first element

push

1. push Yes, you can add new elements to the array
2. push（） Parameters write array elements directly
3. push After the completion of , The return result is the length of the new array
4. The original array changes

unshift

1. unshift('blue');    // Add... At the front , The returned number is the group leader
2. The original array changes

pop

1. pop（）    Delete the last element     What is returned is the deleted element
2. （） There are no parameters in it

shift

1. shift（）  Delete first element     What is returned is the deleted element
2. （） There are no parameters in it

Get array element index indexOf            str.indexOf（‘ Characters to find ’,[ Starting position ]）

Be careful ： It is often used to judge whether an element is in an array ！！ return -1 Indicates that it is not in the array

Array conversion string join   toString（）

Flip array ：arr.reverse（）

Bubble sort ：arr.sort（）

String object

Basic packaging type ： Is to put simple data types , Packaging becomes a complex data type

var str = ('andy')
 amount to 
（1）  Wrap simple data types into complex data types 
var temp = newString('andy')
（2）  Give the value of the temporary variable to  str
str = temp 
（3）  Destroy this temporary variable 
temp = null

The immutability of string

Means that the value inside is immutable , Although it looks different , In fact, the address has changed , Created a new memory space

Don't go ahead A lot of splicing character string , It's easy to cause computer jam

String method

The string itself is not modified , When the operation is completed, a new string is returned

• Return position according to character

indexOf( Characters to find , Starting position )： return Specify content Position in the meta string , Can't find return -1 , The starting position is index Reference no.             indexOf（ To find the character [ The starting position ]）

lastindexOf（）    From back to front look for , Only look for The first match Of   

• Returns the character according to the position

Method name	explain	Use
charAt（index）	Returns the specified position character （index String index ）	str.charAt（0）
charCodeAt（index）	Gets the character of the specified position ASCLL code （ Reference no. ）	str.charCodeAt（index）
str [index]	Gets the character at the specified location	H5,IE8+ Support

• Operation method of string

Method name	explain
concat（str1,str2,str3...）	concat（） Method to connect two or more strings . String concatenation , Equivalent to +
substr（start,length）	from start Position start （ Reference no. ）,length Take the number of
slice（start,end）	from start Position start , Intercept to end Location ,end Can't get （ Both are index numbers ）
substring（start,end）	from start Start to intercept to end,end Can't get , Basic and slice Agreement , Negative values are not acceptable

• Replace character   replace（‘ Replaced characters ’,‘ The replaced character ’） It will only replace the first character
• Character to array split（‘ Separator ’）
• Convert a capital toUpperCase（）
• Convert lowercase toLowerCase（） 

JS Simple data types and complex data types

Simple data type is also called basic data type or Value type , Complex data types are also called Reference type

• Value type ： The value stored in the variable during storage is the value itself

string、number、boolean、undefined、null A special （ Returns an empty object ）

If you want to store a variable as an object in the future , I haven't figured out what to put , And save it as null

• Reference type ： When storing variables, only the address is stored in the variables （ quote ）, adopt new Keyword created objects （ System object 、 Custom object ） Such as ：object、Array、Date

Heap and stack

Differences in stack space allocation ：

1. Stack （ operating system ）： The operating system automatically assigns the parameter value of the storage function 、 The value of a local variable . The operation mode is similar to the stack in the data structure           Simple data types are stored on the stack
2. Pile up （ operating system ）： Store complex types （ object ）, Release is assigned by the programmer , If programmers don't release , Recycling by garbage collection mechanism               Complex data types are stored in the heap

Be careful ：JS There is no concept of stack , By stack , It is easier to understand how the code is executed

• Simple data type Open directly in the stack A space storage value
• Complex data types are stored in the stack first Hexadecimal address , Then this address Point to the heap The data of , Values of truly complex data types All stored in the pile Inside

Parameter transfer of data type

Parameters of simple data types

Function parameter can be regarded as a variable , When one Value type variables are passed as parameters to the formal parameters of the function when , In fact, it is the value of the variable in the stack space Copy A formal parameter to the function , Then any modification of formal parameters inside the function is Does not affect external value variables

Parameters of complex data types

A function parameter can also be regarded as a variable , When passing a reference type variable to a formal parameter , In fact, it is the heap address of the variable in the stack space Copy the One for the formal parameters , Formal parameters and actual parameters The same address is saved , therefore Operate on the same object