With example data like this:

var data = {
      "animals": [
          {
              "name": "cat",
              "legs": 4
          },
          {
              "name": "bird",
              "legs":2
          }
      ]
  };

Object.keys(data.animals).forEach(function(index) {
        if(data.animals[index].legs === 2){
          console.log(data.animals[index].name);
        }
      });

which console logs “bird”