This time I will bring you a basic JavaScript knowledge summary. There are a total of eleven knowledge points. Basic JavaScript knowledge summary (5) briefly introduces array or object typeof and Type conversionThe following is a practical case , let’s take a look.

Array

Array objects are used to store a series of values ??in separate variable names. The form is a square bracket, in which many things can be written, separated by commas. Each comma can separate two warehouses. Each warehouse can put things, such as Number, String, undefined, and any type of value can be placed. .

var arr = [1,2,undefined,"abc",["a",1,5],null]//Adding the array document.write(arr.push("Daniel Wu")) ;//Print arr-->[1,2,undefined,"abc",["a",1,5],null,"Daniel Wu"]//Delete array arr.splice(0,1)/ /The printout is [2,undefined,"abc",["a",1,5],null]//The modification of the array arr[0];//The printout is 0arr[0] = 100;//In The printout is 100;//View the array for(var i = 0;i

Other methods about arrays

For...In Statement
Use the for...in statement to loop through the elements in the output array.

Merge two arrays - concat()
How to use the concat() method to merge two arrays.

Use the elements of the array to form a string - join()
How to use the join() method to form a string from all the elements of the array.

Literal Array - sort()
How to use the sort() method to literally sort an array.

Number Array - sort()
How to use the sort() method to numerically sort an array.

Object

JavaScript object is data that has properties and methods. Everything in JavaScript is an object: strings, numbers, arrays, dates, etc. In JavaScript, an object is data that has properties and methods.

var obj = {    key : value,
    建 : 值,
    屬性: 屬性值; 
}var car = {    type:"Fiat", 
    model:500, 
    color:"white"
    money:undefined,    newCar:false,
}//增加對象屬性car.width = "1.6m";//刪除對象屬性delete.car.width;//修改對象屬性car.width = "1.5m";

The difference in programming forms

Procedure-oriented

Object-oriented

The difference is too complicated, I don’t accept it, I’ll look for it on Baidu The difference goes to

typeof

六尊Data type

Number
string
boolean
undefined
object
function

typeof("里面放數(shù)據(jù)")var num = 123;var str = "123";var a = true;var b = null; 
var c = undefined;console.log(typeof(num));//打印-->numberconsole.log(typeof(str));//打印-->stringconsole.log(typeof(a));//打印-->booleanconsole.log(typeof(b));//打印-->objectconsole.log(typeof(c));//打印-->undefined//第二種方法console.log(typeof c);//打印-->undefined 空格也可以

Type conversion

Display type conversion

Number(mix)
parseInt(string,radix)
parseFloat(string)
toString(radix)
String(mix)
Boolean()
//Number轉(zhuǎn)換成數(shù)var num = Number("123");        = true;        = false;        = null;        = undefined;        = "a";        = "123abc";console.log(num)//打印123                //打印1                //打印0                //打印0                //打印NaN                //打印NaN                //打印NaN//parseInt轉(zhuǎn)換成整數(shù)//parseInt(String,radix)//radix 是調(diào)整進制取值范圍是2-36//parseInt 是用數(shù)字為一直往后面看,看到截止,一直看到非數(shù)字位截止,把之前的數(shù)字返回var num = parseInt("123.9");        = true;        = false;        = null;        = undefined;        = "a";        = "123abc";console.log(num)//打印123不是四舍五入                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印123                //parseFloat//把數(shù)字轉(zhuǎn)換為浮點數(shù)var num = parseFloat("123.9");        = true;        = false;        = null;        = undefined;        = "a";        = "123.2abc";console.log(num)//打印123.9                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印NaN                //打印123.2                //String把內(nèi)容換成字符串var num = String(123.9);        = undefined;console.log(num)//打印"123.9"                //打印"undefined"//Boolean轉(zhuǎn)換成布爾值//除了undefined、null、NaN、""、0、false 打印出來的是false以外, 其他的全是truevar num = Boolean(123.9);        = undefined;console.log(num)//打印true                //打印false                                                //toString(radix) 轉(zhuǎn)換成為字符串//兩個不能用一個undefined一個null會報錯undefined和unll沒有這個toString屬性//radix 是以10進制為基底轉(zhuǎn)換為別的進制//用法:要轉(zhuǎn)的數(shù)據(jù).toStringvar demo = 123;var str = demo.toString();console.log(str)//打印出"123"

Quote content conversion

isNaN()
++/-- +/-(一元正負)
-，*，/，%
&&，||，!
<，>，<=，>=
== !=

//isNaN()當你把數(shù)放在括號里面的時候他能判斷這個是是不是NaN,然后給你返回回來console.log(isNaN(NaN))//打印trueconsole.log(isNaN(123))//打印falseconsole.log(isNaN("123"))//打印falseconsole.log(isNaN("adc"))//打印trueconsole.log(isNaN(null)//打印falseconsole.log(isNaN(undefined)//打印true//isNaN在內(nèi)部 執(zhí)行了一個Numbar方法//比如isNaN("abc")//首先執(zhí)行了Numbar("abc")看是不是NaN 如果是就返回NaN//所以這個numbar它沒有顯示的去調(diào)用,是隱式的去調(diào)用
//++,先調(diào)用numbarvar a = "123";
a++;//打印出a-->124;var a = "abc";
a++;//打印出a-->NaN;//+/- 先調(diào)用numbarvar a = + "abc";console.log(typeof(a))//打印出numbar//+ 隱式類型轉(zhuǎn)換調(diào)用的是stringvar a = "1" + 1;console.log(typeof(a))//打印出string
//-，*，/，%隱式類型轉(zhuǎn)換調(diào)用的是numbar
// <，>，<=，>=如果有數(shù)字就調(diào)用numbarvar a = "1" < 2;console.log(typeof(a))//打印出boolean
// == ，!=var a = "1" == 1;console.log(typeof(a))//打印出boolean true
//特殊的undefined>0//打印falseundefined<0//打印falseundefined==0//打印falsenull>0//打印falsenull<0//打印falsenull==0//打印falseundefined == null//打印trueNaN == NaN//不等于任何東西

No type conversion occurs

=== !==(絕對的等于 絕對不等于)
1 !== "1" //true1 !== 1 //falseNaN === NaN //false

There is also a special

//a在沒有定義的情況下typeof(a);//用console.log打印出undefined//typeof返回的值類型 都是string類型typeof(typeof(a))//用console.log打印出string

I believe you have mastered the method after reading the case in this article. Please pay attention for more exciting things. Other related articles on php Chinese website!

Related reading:

Basic JavaScript knowledge summary (4) Conditional statements, loop statements

Basics Summary of JavaScript knowledge (3) Comparison operators, logical operators

Summary of basic JavaScript knowledge (2) Introduction, variables, value types, operators

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