Make a promise wait for a specific object property to be set

make promise wait for property

// Semi reusable factory for promises waiting on an object property to be defined
function waitForIt (objectToObserve, propertyToObserve) {
  return new Promise(function (resolve, reject) {
    var checkIt = function (objectToObserve, propertyToObserve){
      if (typeof objectToObserve[propertyToObserve] == 'undefined'){
        setTimeout(checkIt, 500, objectToObserve, propertyToObserve)
      } else {
        resolve(objectToObserve[propertyToObserve])
      }
    }
    checkIt(objectToObserve, propertyToObserve)
  })
}

// Sampe object
var foo = {
      "envTime":12345678,
      "aDependency":"{{envTime}}",
      "bDependency":"{{extJSON.response.answer}}", // <-- waiting on this.extJSON.response.answer to be defined
      "extJSON":{
        "GET":"http://example.com/some.json",
        "response":{}
      }
    }

// Create a promise using our factory
var waitingForTheAnswer = waitForIt(foo.extJSON.response, 'answer')
    .then(function (val) {
      console.log('It is finished, and the answer is', val)
    })

// Wait a little, then add the 'answer' property to our object
setTimeout(function (objectToSet, propToSet) {
  objectToSet[propToSet] = 42
}, 50, foo.extJSON.response, 'answer')

// Wait a little longer and verify that our promise was fulfilled
setTimeout(function (promiseToCheck) {
  console.log('promiseToCheck.isFulfilled()', promiseToCheck.isFulfilled())
}, 600, waitingForTheAnswer)