Double exclamation mark in JavaScript (!!) Role

Often see such an example:
var A;
var b =!! A

A default is undefined.!a is true,!! A is false, so the value of B is false, not undefined, nor other values, mainly to facilitate subsequent judgments.

!! Generally used to cast the subsequent expression to a Boolean type of data (Boolean), which can only be true or false;
Because JavaScript is a weakly typed language (variables do not have a fixed data type), it is sometimes necessary to cast to the appropriate type, similar to the following:

A=parseint ("1234″")
A= "1234″-0//Convert to Digital
b=1234+ ""//Convert to String
C=someobject.tostring ()//Convert an object to a string
The 1th and 4th are explicit conversions, 2 and 3 are implicit conversions

Boolean conversions, JavaScript Convention rules are

False, undefinded, NULL, 0, "" is false
True, 1, "somestring", "[Object] is true"
The null and undefined other implicitly converted values, using the! Operator will produce true results, so the use of two exclamation points is to convert these values to "equivalent" Boolean value;
Take a look again:

var foo;
alert (!foo);//undifined case, an exclamation point returns true;
alert (!goo);//null case, an exclamation point returns true;
var o={flag:true};
var test=!! o.flag;//equivalent to Var test=o.flag| | False
alert (test);

This example demonstrates that when undifined and Null are returned with an exclamation point, the return is true, with two exclamation points returning the false, so the two exclamation point is that if the value of the variable is explicitly set (not null/undifined/0/"" equivalent), The result is returned based on the actual value of the variable, and if not set, the result returns false.

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Double exclamation mark in JavaScript (!!) Role