javascript: What is a NOT NOT? (!! operator )

2018-06-03 18:41:38

Possible Duplicate:
What is the !! operator in JavaScript?

What is a not not in javascript I have seen this a few times :)

function foo(){
    return !!(window.history);
}

i believe it is used for enforcing boolean types...

for example

if("true" == true){
    alert("1");
}else{
    if(!!"true" == true){
        alert("2");
    }
}

alerts 2 not 1

我认为它曾经[不那么]隐式地将值转换为布尔类型

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