typeof & Nan & ==

readed 'JavaScript:the good part',and Add some note here :

 

1)Several Types in JS:numbers,strings,booleans,null,undefined,objects.

numbers&strings have methods,but the diff is objects are keyed collections . arrays,functions,regular expressions are objects.we can dig into it by typeof operator.

typeof 1.00 ; // return numbertypeof null ; // return object,because null is object toomy_value === null ;// test my_value for null// so test the value for object but not nullif(my_value && typeof my_value === 'object'){ // my_value is an object or an array!}typeof /a/ ; // return object // test my_value for array:if(my_value && typeof my_value ==='object' && my_value.constructor === Array){}// the better one if(my_value && typeof my_value ==='object' &&typeof my_value.length === 'number' &&!(my_value.propertyIsEnumerable('length')){ // my_value is truly an array!}

2)'Nan' and ' =='

 

JS has two sets of equality '==='and !===,and their evil twins == and !=.

 

IF the two operands are of the same type and have the same value,then === produces true and !== produces false;

the EVIL twins do right thing when the operands are of the same type,BUT IF THEY ARE DIFFERENT TYPE ,THEY ATTEMPT TO COERCE THE VALUE,unforunately,the rules are complicated,SO DO NOT USE IT IF POSSIBLE

 

 

'' == '0'0 == '' // true;0 == '0' // true;false == 'false' // falsefalse == '0' // truefalse = undefined // falsefalse == null // falsenull == undefined // true'/t/r/n' == 0 // true

By the way ,the operator NaN is the evil part too, so if you want to check the number,use this:

function isNumber(){return(typeof my_value === 'number' && isFinite(my_value));}