Lead everyone to learn the basic concepts of javascript basics (1)_javascript skills

Study Catalog

1. Data type

Data type

2. Operators

Increment and decrement operators:

bit operator:
Boolean operator:
Additive operator:
Equality operator

3. Statements
for-in statement
label statement

1. Data type

Data type

Basic data types (five types)

Undefined
Null
Boolean
Number
String

Complex data type (one type)

Object
Undefined: There is only one value undefined (defined but not assigned).

Example:

var str;alert(str);//undefined

Note: If it is not declared with var, an exception will be reported if used directly (... is not defined).

Suggestion: Assign an initial value to each var declaration, which can reduce many unexpected situations.

For example:

var str;var str2="Zhang San";str = str str2;alert(str);//It is not the "Zhang San" we expected

Null: slightly

Boolean: There are only two literal values: true and false.

Any data type can be converted to Boolean type through the Boolean() function.

String: non-empty character=>true Empty character=>false
Number: any non-zero =>true 0 and NaN=>false
Object: any object=>true null=>false
Undefined          undefined=>false

For example:

var str = Boolean(NaN);var str2 = Boolean("Zhang San");var str3 = Boolean(undefined);alert(str " " str2 " " str3);

In this case, we will automatically perform the corresponding Boolean conversion when using the if statement.

if (str) {
  //str只要是非空字符、任何非零数字、任何非null对象 都是true
  //str只要是空字符、0、NaN、null、undefined 都是false
  //这样就省去了 我们一个个的判断了
  alert("true");
}

String: slightly

2. Operators

Increment and decrement operators:

【 num, --num, num, num--】

var age = 3, age2 = 3;age ; age2;alert(age " " age2);

Both are 4, I can’t seem to see any difference.

var age = 3, age2 = 3,num1 ,num2;num1 = age 1;num2 = age2 1;alert(num1 " " num2);

Okay, here’s the difference. One is still a 4, and the other becomes a 5. Why is this happening? The priorities are just different. Age first executes the operator and then adds 1 to itself. Age first adds 1 to itself and then adds 1 through the operator.

Bitwise operators:

[Bitwise NOT (~), bitwise AND (&), bitwise OR (|), bitwise XOR (^), left shift (<<), signed right Shift (>>), unsigned right shift (>>>)】
In fact, we rarely use it in daily life, so let’s learn a little bit about it here.

Example: bitwise not

var num1 = 25; // Binary 00000000000000000000000000011001
var num2 = ~num1; // Binary 11111111111111111111111111100110
alert(num2); // -26

Example: Bitwise AND

var result = 25 & 3;
alert(result); //1
//*********************
25 = 0000 0000 0000 0000 0000 0000 0001 1001
3 = 0000 0000 0000 0000 0000 0000 0000 0011
---------------------------------------------
AND = 0000 0000 0000 0000 0000 0000 0000 0001 

Example: Bitwise XOR

var result = 25 ^ 3;
alert(result); //26
//*************
25 = 0000 0000 0000 0000 0000 0000 0001 1001
3 = 0000 0000 0000 0000 0000 0000 0000 0011
---------------------------------------------
XOR = 0000 0000 0000 0000 0000 0000 0001 1010 //十进制值 26

Example: Move left

var oldValue = 2; // 等于二进制的 10
var newValue = oldValue << 5; // 等于二进制的 1000000，十进制的 64

Example: Signed right shift

var oldValue = 64; // 等于二进制的 1000000
var newValue = oldValue >> 5; // 等于二进制的 10 ，即十进制的 2 

Example: unsigned right shift (this difference is a bit big, because the value of 1 representing a negative number in the 32nd position is also moved over)

var oldValue = -64; // 等于二进制的 11111111111111111111111111000000
var newValue = oldValue >>> 5; // 00000111111111111111111111111110 等于十进制的 134217726

Boolean operators:

【Logical NOT (!), logical AND (&&), logical OR (||)】

What needs to be said here is that && and || are short-circuit operations. ||If the previous one satisfies the condition, the subsequent condition will not be tested. && On the contrary, if the previous one does not satisfy the condition, the subsequent condition will not be detected.

Example:

var age = 3;
if(age||aaa){//这里的age转Boolean为ture，所以后面的aaa就不检测了。
alert(age);}
if(aaa||age){//这里将会报错
alert(age);}

测试：

var age = 3;if(age||aaa){//这里的age转Boolean为ture，所以后面的aaa就不检测了。alert(age);}if(aaa||age){//这里将会报错alert(age);}

这里可以告诉大家一个小小的技巧，很多时候我们并不知道某个变量是否有值，但是我们又需要正常使用，那么我们可以给个备胎。

如：浏览器的兼容问题，在旧式浏览器中是用window.event来取事件的相关属性值，新式浏览器中是直接通过形参的形式传过来的。我们就可以。

function myonclick(ev) {
  var myevent = ev || window.event;//这样就可以保证新旧浏览器的兼容了
}

实验:　　

var obj = null;var obj2 = obj||"备胎";alert(obj2);

加性操作符：

【加法（+）、减法（-）】

这个再熟悉不过了，不过还是有一点需要注意。

var strnum1 = "5";
var num2 = strnum1 + 3;
var num3 = strnum1 - 3;
alert("num2="+num2+" num3="+num3);//这里会是什么结果呢？

Let’s test it out.

var strnum1 = "5";var num2 = strnum1 3;var num3 = strnum1 - 3;alert("num2=" num2 " num3=" num3);//What will be the result here?

Why is this happening? Add strings and numbers, and the numbers will be converted to strings. Strings and numbers are subtracted, and strings are converted to numbers.

Equality operator

var num1 = "5";var num2 = 5;var isEqual = num1==num2;alert("Is num1 equal to num2:" isEqual);

It’s strange, how come strings are equal to numbers. Yes, automatic transformation is used here. But what if we don’t want to see such a result?

var num1 = "5";var num2 = 5;var isEqual = num1===num2;alert("Is num1 equal to num2:" isEqual);

Yes, we can use === all equals. Not only compare values, but also types.

3. Statements

for-in statement

The for loop statement is used a lot, but for-in may be used less often. (For-in loops should be used to traverse non-array objects. Using for-in to loop is also called "enumeration".)

Example:

var obj = {id:2,name:"Zhang San"};var str = "";for(o in obj){ str = "Attribute: " o " Value: " obj[o] " ";}alert(str);

label statement

It feels very similar to goto in C#, but different.

Have we ever wanted to jump out of the second level of loops within multiple nested loops? Generally we can only use break or continue to jump out of the innermost loop, or return to jump out of the entire function. What I never expected is that there is also a label statement that can jump out of any loop.
Example:

var num = 0;outermost:for (var i=0; i < 10; i ) { for (var j=0; j < 10; j ) { if (i == 5 && j == 5) { break outermost; } num ; }}alert(num); //55

Okay, the first article is almost finished here. Finally, I will give you some interesting questions.

1. Why 1＝0.9999999999……
2. Why alert(0.1 0.2)//0.30000000000000004
3. Why alert(0123 4)//87

Four.

var obj1 = {}; obj1.name2 = "Zhang San";
var str1 = "test"; str1.name2 = "李思";
alert(obj1.name2 " " str1.name2);
//What is the pop-up value

5.

var obj1 = { name2: "111" };
var obj2 = obj1;
obj1.name2 = "222";
alert(obj2.name2);//What pops up here?
obj1 = { name2: "333" };
alert(obj2.name2);//What pops up here?

Six,

var fun = function (num1) {
If (num1 <= 1) {
          return 1;
}
       else {
            return num1 * fun(num1 - 1);
}
}
var fun2 = fun;
fun = function () {
Return 1;
}
alert(fun2(5));//What pops up here?

I still have some time to introduce the basic data types of JavaScript to everyone

JavaScript’s basic data types include 6 types: number/string/boolean/object/function/undefined.

　2.1 number type

The number type is used to store numerical values. It describes a 64-bit floating point value. But Javascript cannot represent all values ​​between 0-2e64, because it also needs to represent non-integers, including complex numbers, fractions, etc. For 64-bit, 11 bits are needed to store the decimal part of the number, and 1 bit is used to represent the sign, so JavaScript can actually represent values ​​between -2e52 and 2e52.

　2.2 string type

The string type is used to represent text. You can use single quotes or double quotes to include text. Any symbol placed within quotes will be considered a string, but special symbols may need to be escaped.

　2.3 boolean type

The boolean type only includes two values: true and false. We can use various boolean expressions in the program to get true or false to implement different business branch processing.

We can include multiple conditions in the expression, and the conditions can be AND or NOT. When calculating, the priority is as follows: || has the lowest priority, followed by &&, and then the comparison operator. Finally there are other operators (eg!).

　　和其他许多语言一样，对于&&来说，当前面的条件为false时，后面的条件不再计算，对于||来说，当前面的条件为true时，后面的条件不再计算。

　　来看下面的例子：

function conditionTest(){
   var a = ;
   var b = ;
   var c = {"key":"old"};
   print(c["key"]);
   if (a==) print("a = ");
   if (a== && b==) print("a == && b == ");
   if (a== || changeValue(c)) print(c["key"]);
   if (a== && changeValue(c)) print(c["key"]);
 }
 function changeValue(obj){
   obj["key"] = "changed";
   return true;
 }

　　它的输出结果如下：

old
a = 1
a == 1 && b == 1
old
changed

　　可以看出，在使用||时，没有调用changeValue方法。

　　2.4 undefined类型

　　当我们声明了一个变量，但是没有对其赋值时，它就是undefined的，就像下面这样

var b;
print(b);

　　在Javascript中，还有一个和undefined类似的值：null。undefined表示“变量已声明但是没有复制”，null表示“变量已赋值但为空”，需要注意的是undefined==null的值为true。

　　2.5 类型转换

　　我们在上面提到了undefined == null的值是true，但我们使用typeof操作时可以发现，null是object类型，这说明在比较的过程中，发生了类型转换。

　　类型转换是指将一种类型的值转换成另外一种类型的值。我们使用==进行比较时，会有类型转换，我们可以使用===来禁止类型转换。

　　来看下面的例子：

function convertTypeTest(){
   var a = ;
   var b = "";
   print ("a:" + a);
   print ("b:" + b);
   print ("type of a:" + typeof a);
   print ("type of b:" + typeof b);
   print ("a==b:" + (a == b));
   print ("a===b:" + (a === b));
   print ("a===Number(b):" + (a === Number(b)));
   print ("String(a)===b:" + (String(a) === b));
   print ("type of undefined:" + typeof undefined);
   print ("type of null:" + typeof null);
   print ("undefined==null:" + (undefined == null));
   print ("undefined===null:" + (undefined === null));
 }

　　输出结果如下：

a:1
b:1
type of a:number
type of b:string
a==b:true
a===b:false
a===Number(b):true
String(a)===b:true
type of undefined:undefined
type of null:object
undefined==null:true
undefined===null:false

可以很明显看到==和===的区别。