JS_Boolean Logic

• Comparisons
  • >
  • <
  • >=
  • <=
  • ==
  • !=
  • ===
  • !== 
• == double equals
  • check for equality of value, but not equality of type
  • it coerces both values to the same type and then compares them
  • this can lead to some unexpected results.
    • 0 == ' ';                  //true
    • 0 == false;            //true
    • null == undefined; //true
• === triple equals.   checks for equality of value and type
  • 2 === '2';        //false
  • false === 0;   //false
  • 10 !== '10' ;   //ture
  • 10 != '10';     //false
• console.log().  prints arguments to the console
• Running code from a file
  • write code in a .js file and inculde the script in a .html file(abover the closing body tag)

  • "hi".toUpperCase(); //won't show up in the console
    
    console.log("hi".toUpperCase());   //will show up in the console.

• conditions
  • if
  • else if
  • else
  • nesting(多种条件语句嵌套)
• TRUTHY AND FALSY VALUES
  • all js values have an inherent truthyness or falseness about them
  • false values:
    • false
    • 0
    • ''''(empty string)
    • null
    • undefined
    • NaN
  • everything else is truthy!
• Logical Operations
  • &&.   AND
  • ||.    OR
  • !    NOT
    • ! null            //true
    • ! (0 === 0) //false
    • ! (3 <= 4).  //false
• How to check a string includes space or not?

if(string.indexOf(" ") === -1){

         console.log("no space")；

}

• 控制台基本操作

• alert("hello, welcome to my world!!!")   //在网页面出现弹框
  
  prompt("please introduce yourself~")   //在网页出现输入框，用户可以输入
  "Peiyu Li"
  
  let userInput = prompt("name")
  
  userInput
  "Peiyu"
  
  parseInt("78")     //字符转换成数字
  78
  
  parseInt("78dhkshdk")。 //字符转换成数字，自动滤去非数字部分
  78
  
  console.log("Good morning!!!")。  //输出，相当于print
  Good morning!!!