A daily javascript learning summary (basic knowledge)_javascript skills

1. Character conversion

 var s1 = "01";
  var s2 = "1.1";
  var s3 = "z";//字母'z'无法转换为数字，所以或返回NaN
  var b = false;
  var f = 1.1;
  var o = { 
   valueOf: function() {
    return -1;
   }
  };
  
  s1 = -s1; //value becomes numeric -1
  s2 = -s2; //value becomes numeric -1.1
  s3 = -s3; //value becomes NaN
  b = -b;  //value becomes numeric 0
  f = -f;  //change to -1.1
  o = -o;  // 1 先执行对象的valueOf()方法返回-1,--1为1
　　　　o = +o;//-1 o的valueOf()值为-1，+-1还是-1

2. Special character operations

 var result1 = 5 - true; //4 because true is converted to 1
  var result2 = NaN - 1;  //NaN NaN不是一个数字，和任何数字做任何运算都是NaN
  var result3 = 5 - 3;  //2
  var result4 = 5 - "";  //5 because "" is converted to 0
  var result5 = 5 - "2";  //3 because "2" is converted to 2
  var result6 = 5 - null; //5 because null is converted to 0

3. Variable to string operation

 var value1 = 10;
  var value2 = true;
  var value3 = null;
  var value4;//value4 没有赋值就是underfined 转换为字符串就是'underfined'
  
  alert(String(value1));  //"10"
  alert(String(value2));  //"true"
  alert(String(value3));  //"null"
  alert(String(value4));  //"undefined"

4. Number conversion

var num = 10;
  alert(num.toString());  //"10"默认十进制
  alert(num.toString(2));  //"1010"二进制
  alert(num.toString(8));  //"12"八进制
  alert(num.toString(10));  //"10"十进制
  alert(num.toString(16));  //"a"十六进制

5. String comparison operation

var result1 = 5 > 3; //true
  var result2 = 5 < 3; //false
  var result3 = "Brick" < "alphabet"; //true 字符串比较按照字母表顺序进行比较 小写字母在大写字母后面
  var result4 = "Brick".toLowerCase() < "alphabet".toLowerCase(); //false字母表顺序比较
  var result5 = "23" < "3"; //true 字符2小于字符3
  var result6 = "23" < 3; //false 此时'23'转换成23了
  var result7 = "a" < 3; //false because "a" becomes NaN 字符'a'无法转换成数字
  var result8 = NaN < 3; //false NaN 和任何数字比较，都无法转换成数字，所以一直是false
  var result9 = NaN >= 3; //false

6. Character conversion

  var num1 = parseInt("AF", 16);  //175 按照16进制输出十进制数据 10*16+15*1
  var num2 = parseInt("AF");   //NaN 没有指定进制，默认按照10进制转换，由于AF不在十进制范围，返回NaN

  alert(num1);
  alert(num2);

7. Use of parseInt

  var num1 = parseInt("1234blue"); //1234
  var num2 = parseInt("");   //NaN 字符''无法转换成数字
   var num3 = parseInt("0xA");   //10 - hexadecimal 16进制的A
  var num4 = parseInt(22.5);   //22
  var num5 = parseInt("70");   //70 - decimal
  var num6 = parseInt("0xf");   //15 16进制为15

8. Use of Number objects

  var num1 = Number("Hello world!"); //NaN
  var num2 = Number("");    //0 空字符串可以转换成0 这个parseInt()不一样
   var num3 = Number("000011");  //11
  var num4 = Number(true);   //1

9. NaN usage 

  alert(NaN == NaN);  //false
  alert(isNaN(NaN));  //true
  alert(isNaN(10));  //false ?10 is a number
  alert(isNaN("10"));  //false ?can be converted to number 10
  alert(isNaN("blue")); //true ?cannot be converted to a number
  alert(isNaN(true));  //false ?can be converted to number 1

10. System maximum number

var result = Number.MAX_VALUE + 1;
alert(isFinite(result)); // false

11. Infinity

  alert(5 * 6);   //30
  alert(5 * NaN);   //NaN
  alert(Infinity * 0); //NaN
  alert(Infinity * 2); //Infinity
  alert("5" * 5);   //25
  alert(true * 10);  //10
  alert(false * 10);  //0
　　　　　　alert(26 % 5); //1
　　　　　　alert(Infinity % 3); //NaN
　　　　　　alert(3 % 0); //NaN
　　　　　　alert(5 % Infinity); //5
　　　　　　alert(0 % 10); //0
　　　　　　alert(true % 25); //1
　　　　　　alert(3 % false); //NaN

12. for in loop

for (var propName in window) {
    document.write(propName);
    document.write("<br />");
  }

13. Comparison of special characters

 alert(null == undefined); //true
  alert(null === undefined); //false
  
  alert("NaN" == NaN);  //false
  alert("NaN" === NaN);  //false
  alert(NaN == NaN);   //false
  alert(NaN === NaN);   //false
  alert(NaN != NaN);   //true
  alert(NaN !== NaN);   //true
  
  alert(false == 0);   //true
  alert(false === 0);   //false
  alert(true == 1);   //true
  alert(true === 1);   //false
  
  alert(null == 0);   //false
  alert(undefined == 0);  //false
  
  alert(5 == "5");   //true
  alert(5 === "5");   //false  

14. Boolean object

 var message = "Hello world!";
  var messageAsBoolean = Boolean(message);
  
  alert(messageAsBoolean); //true

15. Use for loop with tags
Both the break statement and the continue statement can be used in conjunction with labeled statements to return to a specific location in the code.
Usually this is done when there is a loop inside a loop, for example:

 var num = 0;
  //返回到特定的位置
  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

In the above example, the label outermost represents the first for statement. Normally, each for statement executes the code block 10 times, which means that num will be executed 100 times under normal circumstances, and when the execution is completed, num should equal 100. The break statement here has one parameter, which is the label of the statement to jump to after stopping the loop. In this way, the break statement can not only jump out of the inner for statement (that is, the statement that uses variable j), but also jump out of the outer for statement (that is, the statement that uses variable i). Therefore, the final value of num is 55, because the loop will terminate when the values ​​of i and j are both equal to 5.
The continue statement can be used in the same way:

var iNum = 0;

outermost:
for (var i=0; i<10; i++) {
 for (var j=0; j<10; j++) {
 if (i == 5 && j == 5) {
 continue outermost;
 }
 iNum++;
 }
}

alert(iNum); //输出 "95"

In the above example, the continue statement will force the loop to continue, not only the inner loop, but also the outer loop. This occurs when j equals 5, meaning that the inner loop will be reduced by 5 iterations, resulting in iNum having a value of 95.

Tip: As you can see, labeled statements used in conjunction with break and continue are very powerful, but overusing them can cause trouble when debugging your code. Make sure the labels you use are descriptive and don't nest too many levels of loops.

The above is today’s summary of JavaScript learning, and it will continue to be updated every day. I hope you will continue to pay attention.