Some JS basic knowledge review notes

1. var a=123 b=++a//b=124,a=124

var a=123 b=a++//b=123,a=124

+ + priority problem, the execution order is different;

2, uninitialized variable value is Undefined,var A;alert (a)//undefined;

JavaScript will automatically give you an uninitialized variable set to undefined;

3, five basic types: typeof (digital type)//number;

typeof (String)//string;

typeof (bool type)//boolean;

typeof (undefined)//undefined;

typeof (null)//object;

4, str= "020";     parseint (str); 16;0 will be treated as octal;

If you do not want to be treated as octal to handle =>parseint (str, 10); 20;10 on behalf of decimal;

5. Exponential notation:

1E1 = = 1e1 = = 1e+1 = 1E+1//10;e equivalent to 10,e+1 equivalent to e1;e+3 equivalent to E3;

Tip: The maximum value JS can handle is 1.7976931348623157E + 308

The minimum value is 5E-324

Values outside these two are displayed as infinity (no infinity); Infinity and any number operations are infinity;

Minimum use-infinity

Maximum Use Infinity

6, Nan is a numeric value; typeof (Nan)//number;nan can be understood as a set, contagious, with a Nan in the operation, and all the equations as Nan;

7. Some operations of implicit type conversion

var a = "2", B = "3", C = 1;

A*b//6; (number);

A+b//"at"; (string);

A*c//2; (number);

Some numbers are converted to number, which can be used in the form of * *, but it is recommended to use parseint ();

Some JS basic knowledge review notes