Why is the response object from JavaScript fetch API a promise?

by

If your question is “why does response.json() return a promise?” then @Bergi provides the clue in comments: “it waits for the body to load”.

If your question is “why isn’t response.json an attribute?”, then that would have required fetch to delay returning its response until the body had loaded, which might be OK for some, but not everyone.

This polyfill should get you what you want:

var fetchOk = api => fetch(api)
  .then(res => res.ok ? res : res.json().then(err => Promise.reject(err)));

then you can do:

fetchOk(API)
  .then(response => response.json())
  .catch(err => console.log(err));

The reverse cannot be polyfilled.

More Related Contents:

  1. How to return many Promises and wait for them all before doing other stuff
  2. How can I fetch an array of URLs with Promise.all?
  3. ES6 promise settled callback?
  4. When is the body of a Promise executed?
  5. fetch: Reject promise with JSON error object
  6. ES6 Promise.all() error handle – Is .settle() needed? [duplicate]
  7. Fulfill (don’t resolve) promise with another promise
  8. ES6 Promises – something like async.each?
  9. Filtering an array with a function that returns a promise
  10. using a fetch inside another fetch in javascript
  11. Why does the Promise constructor require a function that calls ‘resolve’ when complete, but ‘then’ does not – it returns a value instead?
  12. What is the best general practice to timeout a function in promise [closed]
  13. When would someone need to create a deferred?
  14. ES6 promise execution order for returned values
  15. Promises, pass additional parameters to then chain
  16. How to extract data out of a Promise
  17. What happens if you don’t resolve or reject a promise?
  18. React cannot read property map of undefined
  19. Are JavaScript forever-pending promises bad?
  20. Why does javascript ES6 Promises continue execution after a resolve?
  21. set withCredentials to the new ES6 built-in HTTP request API : Fetch
  22. using await on global scope without async keyword
  23. ES6 Promise replacement of async.eachLimit / async.mapLimit
  24. Why does JavaScript Promise then handler run after other code?
  25. When is the body of a Promise constructor callback executed?
  26. Which browsers support import and export syntax for ECMAScript 6?
  27. What do double brackets mean in javascript and how to access them
  28. ECMAScript 6 class destructor
  29. String interpolation on variable
  30. Node.js: When to use Promises vs Callbacks

Leave a Comment