The role of two exclamation marks in JavaScript

var a;

var b=!!a;

a defaults to undefined. !a is true, !!a is false, so the value of b is false, not undefined or other values, mainly to facilitate subsequent judgments.

!! Generally used to force the following expression to Boolean type data (boolean), which can only be true or false;

Because JavaScript is a weakly typed language (variables do not have a fixed data type) Sometimes it is necessary to cast to the corresponding type, similar to:

a=parseInt(“1234″)

a=”1234″-0 //Convert to number

b=1234+”” //Convert to character String

c=someObject.toString() //Convert the object to a string

The 1st and 4th are explicit conversions, 2 and 3 are implicit conversions

Boolean conversion, javascript convention rules For

false, undefined, null, 0, "" for false

true, 1, "somestring", [Object] for true

For null, undefined and other values ​​that are implicitly converted, when using the ! operator will produce true results, so the purpose of using two exclamation marks is to convert these values ​​into "equivalent" Boolean values;

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Let’s first explain with a simple example:
var o={flag:true};
var test=!!o.flag;//Equivalent to var test=o.flag||false;
alert(test);
Because the ! operator is used for null and undefined will produce a true result, so the purpose of using two exclamation marks is that if the value of flag in o is explicitly set (not null/undefined/0""/etc.), naturally test will be the same as o.flag The value; if not set, test will default to false, not null or undefined.
The classic example in jQuery is as follows: (jQuery 1.7.0.js: Line 748)
grep: function(elems, callback, inv) {
var ret = [], retVal;
inv = !!inv;

// Go through the array, only saving the items
// that pass the validator function
for ( var i = 0, length = elems.length; i < length; i++ ) {
retVal = !!callback( elems [ i ], i );
if ( inv !== retVal ) {
ret.push( elems[ i ] );
}
}

return ret;
}
When using the grep function, if given If the third parameter is not null/undefined/0""/, then inv is true, otherwise it is false. The purpose of this is to ensure that the values ​​​​of inv and retVal can only be taken from true/false, not other values, to facilitate subsequent judgments.