Some details of JavaScript data types

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details javascript data types

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0 / JS Some details of the data type in

(1)JS Data types in
Basic data type

1、 string , You can use double quotes 、 Single quotation marks 、 The quotation marks

2、 number , such as : Value has 123/1.2/NaN/Infinity/-Infinity...

3、 boolean, The value is true/false

4、 null, The value is null

5、 undefined, The value is undefined

6、 bigint

7、 symbol, Used to create unique values

Reference data type

1、 object for example :

{} Common object

[] Array objects

③ The date is right for

④ Regular , such as :/^\\d+$/

⑤ ......

2、 function

① Ordinary function

② Constructors

③ Arrow function

④ Generator function


(2)number Some of the details of
number Values of type are :

1、 Positive numbers 、 negative 、 zero 、 decimal ......

2、NaN not a number Not a significant number , But it's number Type of

xxx, You're not alone . Not alone , It's all possible

NaN and NaN It's not equal in itself , It's not equal to any other value

isNaN(vlaue) Check the current value Is not a valid number , Not a valid number true; conversely , It's a valid number false, NaN) The result is true, It has a special treatment inside

3、Infinity infinity -Infinity Infinitesimal

console.log(typeof NaN); //=> 'number'
console.log(typeof Infinity); //=> 'number'
console.log(NaN == NaN); //=> false
console.log(NaN === NaN); //=> false
console.log(, NaN)); //=> true

△ NaN


Convert other data type values to number type :

1、 Explicit conversion :Number(vlaue) perhaps parseInt(value)/parseFloat(value) The rules they deal with at the bottom are different

2、 Implicit conversion ( Logic uses Number(value) Of )

① Mathematical operations

② be based on == When comparing


④ ....

(3) Some details of the string

string character string : Single quotation marks 、 Double quotes 、 The quotation marks , It's all strings

Other values are converted to strings :

1、 Explicit conversion :String(value) perhaps (vlaue).toString() It's about data type detection , Later on

2、 Implicit conversion : The plus sign is in addition to mathematical operations , It also produces string concatenation

+ Plus sign is slash youth
let n = '10',
m = 10;
console.log(10 + n);
let obj = {};
console.log(10 + obj);
console.log(10 + new Number(10));
console.log(10 + {id:'zhaoxiajingjing'});

△ What's the result ?

+ As a slash youth , My job is mathematical operators , The slash also serves as the string concatenation work , When does it switch roles ?

+ Only when one side has content : such as +n, Convert a value to a number ;++n/n++ It also converts values to numbers , And then we can move on In front of / Post self increment Arithmetic

+ When there's content on both sides

1、"+" There's a string on one side , Will become String splicing

2、"+" One side is the object , It may also become string concatenation :

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△ chart 1.1_"+" As a young man with a slash

among :①③ You get numbers 10, as a result of :{...} Not involved in the operation , The browser thinks it's a block of code , The calculation is +10

and :console.log({}+10) There is a bracket {}+10 Wrap it up. , It's going to think of it as a whole, and then it's going to do it

that , The underlying mechanism of objects in mathematical operations :

(1) Test the object's Symbol.toPrimitive 【primitive [ˈprɪmətɪv] n. The original 】 This property value , If so, the operation is based on this value , without , Take the next step

(2) Test the object's valueOf() This value 【 Original value / Base type value 】, If so, the operation is based on this value , If it's not the original value , Take the next step

(3) Get the object's toString() Turn it into a string => If it is " +" Handle , Then you see the string , To string concatenation

(4) If you end up with the number you want , Then convert the string into a number

let obj = {
[Symbol.toPrimitive]:function (){
return 10;
console.log(10 + obj); //=> 20

△ Objects get numbers

and , console.log(10 + new Number(10)) The result is numbers 20, Because new Number(10).valueOf() The raw value you get is a number 10

∴ The answer is :

let n = '10',
m = 10;
console.log(10 + n); //=> String splicing :'1010'
console.log(+n); //=> Convert a value to a number :10
console.log(++n); //=> Convert a value to a number , In front of the self increment :11
let obj = {};
console.log(10 + obj); //=> '10[object Object]'
console.log(10 + new Number(10)); //=> 20
console.log(10 + {id:'zhaoxiajingjing'}); //=> '10[object Object]'

△ + It's a slash youth

that , Excuse me, :i=i+1 i+=1; ++i/i++ Are these three the same ?

among :i=i+1 and i+=1 It's the same ;++i/i++ Most of the time it's the same as before .

If i The value of is a string :

i=i+1 i+=1 It will be processed as string concatenation

++i/i++ First turn the value into a number , And then we're going to go ahead / Post accumulation

(4)symbol The only value


Symbol(): Create unique values

Symbol() Function will return symbol Type value

new Symbol() Report errors :Uncaught TypeError: Symbol is not a constructor

△ chart 1.2_symbol type



Symbol.toPrimitive It's a built-in Symbol value , It exists as a function value property of an object , When an object is converted to the corresponding original value , Will call the secondary function

let obj = {
[Symbol.toPrimitive]:function (hint){
console.log(hint); // hint Value :"number/string/default"
return 10;
console.log(10 + obj); //=> 20 hint Output 'default'
Number(obj); //=> hint Output 'number' obj Original value 10
String(obj); //=> hint Output 'string' obj Original value '10'

△ Symbol.toPrimitive

(5)BigInt Large number


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△ chart 1.3_bigint

2 / Stack memory Stack & Heap memory Heap

(1) The question is very simple.
var a = 12;
var b = a;
b = 13;
var a = {n: 12};
var b = a;
b['n'] = 13;
var a = {n: 12};
var b = a;
b = {n: 13};

△ It's easy to do the problem , The key is to understand the mechanism

(2) The first question is

We build a new one index.html file , Introduce... Into it 1.js file , Then open the browser to access index.html When you file , The browser will render JS

that JS An executable environment :

① browser : Browser kernel =>JS Rendering engine

② Mobile Hybrid blend APP => webview be based on webkit kernel

③ Node

④ ……

The browser will open up a block of memory to execute JS Code => Stack memory , Execution environment stack ECStack(Execution Context Stack)

There will be an area for global code execution : Global execution context ,EC(G) (Execution Context Global)

The purpose of forming context : Distinguish code execution in different regions

such as : Global execution context 、 Function execution context ……

The execution context will enter the execution environment stack and run , When the browser is closed, the global execution context will be destroyed

△ chart 1.4_ A sketch of the first question

var a = 12;

First step : Create values

① The base values are stored directly in the stack memory

② A reference value is a separate new block of memory to store

The second step : Declare variables declare

The third step : Variables are associated with values , Definition defined

var c; Variable declaration only declare, There is no definition of undefined,console.log(c);//=>undefined

(3) The second question is

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△ chart 1.5_ The second one is a sketch

Create values

1、 The basic data types are stored directly in the stack

2、 Reference data type :

(1) Open up a separate piece of memory heap Heap memory

(2) Every heap memory has one 16 Base address JS Cannot get this address in ,16 Base with 0x start

(3) Store key value pairs in the heap separately

(4) hold 16 The base address is stored in the stack : To facilitate the association of later variables

Reference data type : Variables operate on the right Heap memory address References to


① b Based on address 0x000001 Find heap memory

② Name the attribute in heap memory as n The attribute value of 13

(4) Third question


△ chart 1.6_ The third one is a sketch

(5) Stack memory function

Stack memory or Heap memory , It's all the memory that the browser allocates from the computer , The more you open up , The slower the performance of the computer => performance optimization : Memory optimization

Stack memory function : Code execution and storage of basic type values

Heap memory functions : Store reference data types

3 / Homework

var a = {n: 1};
var b = a;
a.x = a = {n: 2};

△ The first question is

In general :a=b=xxx The order of operations is :



however , When a.x=b=xxx when ,a.x Medium " spot " It's member visits , Its operators have a high priority :

a.x = xxx

b = xxx

var x = [12, 23];
function fn(y) {
y[0] = 100;
y = [100];
y[1] = 200;

△ The second question is

- end -



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