• 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.