Javascript Promises

Very details article about promises: https://web.dev/promises/

  1. Support by Chrome 32, Opera 19, Firefox 29, Safari 8 & Microsoft Edge, and up
  2. A Promise can be chained with .then(CALL_BACK_FUNCTION(VARIABLE){…}) and ,catch(CALL_BACK_FUNCTION(VARIABLE){…})
  3. .then(…), and .catch(…) return type of Promise
  4. A Promise can either be fulfilled or rejected, so the .then(…) can actually take up to 2 call-back functions, the first one for fulfill, and the second one for rejection call-back. Like:
          .then(res => res.json())
            (result) => {
                isLoaded: true,
                items: result.items
            (error) => {
                isLoaded: true,
                error: error
  5. .catch(…) is the syntactic sugar for then(undefined, CALL_BACK_FUNCTION(VARIABLE){…})
  6. The rejection call-back (second functional parameter of a Promise), and the .catch(…) are different! If we do .then(function(result){…}, function(error){…}), either one of them will be called. For .catch(…) if anything in the chain before it it rejected without a rejection call-back, any .then(…) in between will be skip, and the .catch(…) will be called.
  • js/javascript_promises.txt
  • Last modified: 2020/09/08 15:23
  • by chongtin