• Phen@lemmy.eco.br
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    1 year ago

    Except you can add the key with an undefined value and it may have different behaviors than if the key was really not there.

      • activ8r@sh.itjust.works
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        1 year ago

        In JS a Boolean has 4 states.
        true
        false
        undefined (where the key is set to undefined)
        undefined (where the key doesn’t exist on the object)

        if (obj.value === true) {  
            console.log(1);
        } else if (obj.value === false) {
            console.log(2);
        } else if ("value" in obj) { // key "value" is in the obj, but it is set to undefined 
            console.log(3);
        } else { // key "value" is not in object
            console.log(4);
        }
        

        You’re welcome.