javascript error-prone in the knowledge

• undefined : Refers to the not yet set;
• null : That had given over the value, but there is no value;
• NaN: Invalid value, the value fails, refers to perform mathematical operations did not succeed, as a result of failure to return. NaN != NaN； 值为true, It is determined whether the new ES6 NaN need Number.isNaN( .. )not use window.isNaN (...), because window.isNaN ( 'foo') is true.

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• typeof null === 'object'; //true
• typeof undefined === 'undefined'; //true
• typeof there is a security mechanism:

   var a;
   typeof a;  //undefined
   typeof b ;  //undefined

   a //undefined
   b //ReferenceError:b is not defined
   
  解释： undefined : 变量在未持有值的时候为 undefined;
  这里 b 的报错信息改成b is not declared会更准确.（declared：声明，defined:下定义） 

使用 typeof b === 'undefined'  可以在程序中用来判断变量是否存在。

如 
   if( b ){   //报错
   	console.log(b)
   }

   if( typeof b !== 'undefined'){  //安全
   	console.log(b)
   }
判断全局是否存在变量b也可以用 window.b 来进行,这样写也不会报错；

• Similar strings and arrays, which arrays are based, has lengthproperties as well indexOfand concatmethods. But does not change the character string by a numerical method, the array may be.
• 42.toFixed(2)Invalid (42).toFixed(2); 0.42.toFixed(2); 42..toFixed(2); effective
• 0.1 + 0.2 === 0.3 //值为 falseBinary floating-point numbers 0.1, 0.2 is not very accurate, but infinitely close, the result is 0.1 + 0.2 .3000000000000004.