Typescript promise losing values after .then

Question

I have a function within an Angular2 service that is returning Mock data via Promise.resolve which when unwrapped with .then gives me an empty promise object. I can see that the calling function is receiving the Promise with the payload in the __zone_symbole__value property before it is passed into .then however inside of.thenI seem to be left with just an empty promise.

  getTemperatureData(): Promise<any> {

    let data = this.convertJSONToGoogleChartTable(temperatureData_JSON);
    let p = Promise.resolve(data);
    return p;
  }

Using Chrome I see that p above looks like

ZoneAwarePromise {__zone_symbol__state: true, __zone_symbol__value: "[["Date","Temperature","LowTemperature"],["05/11/2…",69.02,null],["06/11/2016 23:54:34",69.99,null]]"}

The calling code which is broken into two lines to debug is below.

getTemperatureData() {
    var d = this.dataService.getTemperatureData();
    d.then(data => this.line_ChartData = data);
}

When I look at d I see the same as p above

ZoneAwarePromise {__zone_symbol__state: true, __zone_symbol__value: "[["Date","Temperature","LowTemperature"],["05/11/2…",69.02,null],["06/11/2016 23:54:34",69.99,null]]"}

The problem occurs with the .then where the value of "d" is just an empty promise. The below was taken from the Chrome dev tools console to show what I am seeing.

d.then(data => console.log(data))
ZoneAwarePromise {__zone_symbol__state: null, __zone_symbol__value: Array[0]}

No matter what I do and how many combinations I have tried I cannot get to my data inside of d. (Note that p and d are only temporary to break the code down for now.)

My package.json is below:

{
  "name": "angular2",
  "version": "0.0.0",
  "license": "MIT",

  "angular-cli": {},
  "scripts": {
    "start": "ng serve",
    "lint": "tslint \"src/**/*.ts\"",
    "test": "ng test",
    "pree2e": "webdriver-manager update",
    "e2e": "protractor"
  },
  "private": true,
  "dependencies": {
    "@angular/common": "~2.1.0",
    "@angular/compiler": "~2.1.0",
    "@angular/core": "~2.1.0",
    "@angular/forms": "~2.1.0",
    "@angular/http": "~2.1.0",
    "@angular/material": "^2.0.0-alpha.9-3",
    "@angular/platform-browser": "~2.1.0",
    "@angular/platform-browser-dynamic": "~2.1.0",
    "@angular/router": "~3.1.0",
    "core-js": "^2.4.1",
    "ng2-bootstrap": "^1.1.16",
    "node-mysql": "^0.4.2",
    "rxjs": "5.0.0-beta.12",
    "ts-helpers": "^1.1.1",
    "zone.js": "^0.6.23"
  },
  "devDependencies": {
    "@types/jasmine": "^2.2.30",
    "@types/node": "^6.0.42",
    "angular-cli": "1.0.0-beta.19-3",
    "codelyzer": "1.0.0-beta.1",
    "jasmine-core": "2.4.1",
    "jasmine-spec-reporter": "2.5.0",
    "karma": "1.2.0",
    "karma-chrome-launcher": "^2.0.0",
    "karma-cli": "^1.0.1",
    "karma-jasmine": "^1.0.2",
    "karma-remap-istanbul": "^0.2.1",
    "protractor": "4.0.9",
    "ts-node": "1.2.1",
    "tslint": "3.13.0",
    "typescript": "~2.0.3",
    "webdriver-manager": "10.2.5"
  }
}

Answer 1

the value of data is just an empty promise

That tells me that convertJSONToGoogleChartTable is returning a promise, and you're not chaining your promise to it.

Note that if you were using a stronger type than any, the TypeScript compiler would probably have caught this for you.

Since you're not doing anything after getting that data, you could just do this:

  getTemperatureData(): Promise<any> {
    return this.convertJSONToGoogleChartTable(temperatureData_JSON);
  }

But if you want to do something with that data before returning it, you can do that in a then, chained off of the original promise:

  getTemperatureData(): Promise<any> {
    return this.convertJSONToGoogleChartTable(temperatureData_JSON)
        .then(data => {
            console.log(data);
            return data;
        });
  }