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

Question

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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);
}

Answer 1

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