Valor de clave de matriz de JavaScript HTML SELECCIONAR

var massiv = [ { "key": 0, "duration": 4, "price": 1000000 },{ "key": 1, "duration": 6, "price": 1250000 },{ "key": 2, "duration": 12, "price": 1500000 }, ];

var elementString = "<select>";
for (var i in massiv) {
  elementString += '<option value="'+massiv[i].duration+'">'+massiv[i].price+'</option>';
}    
elementString += "</select>";

console.log(elementString)
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