How do I wait for a promise to finish before returning the variable of a function

Question

I m still struggling with promises  but making some progress thanks to the community here   I have a simple JS function which queries a Parse database  It s supposed to return the array of results  but obviously due to the asynchronous nature of the query  hence the promises   the function returns before the results  leaving me with an undefined array   What do I need to do to make this function wait for the result of the promise   Here s my code   function resultsByName name           var Card   Parse Object extend  Card        var query   new Parse Query Card       query equalTo  name   name toString          var resultsArray            var promise   query find                  success  function results                      results is an array of Parse Object                               console log results                                  resultsArray   results                               return results                                    error  function error                      error is an instance of Parse Error                               console log  Error

User · Answer

What do I need to do to make this function wait for the result of the promise?

Use async/await (NOT Part of ECMA6, but available for Chrome, Edge, Firefox and Safari since end of 2017, see canIuse)
MDN

    async function waitForPromise() {
        // let result = await any Promise, like:
        let result = await Promise.resolve('this is a sample promise');
    }

Added due to comment: An async function always returns a Promise, and in TypeScript it would look like:

    async function waitForPromise(): Promise<string> {
        // let result = await any Promise, like:
        let result = await Promise.resolve('this is a sample promise');
    }

User · Answer

You re not actually using promises here   Parse lets you use callbacks or promises  your choice   To use promises  do the following   query find   then function         console log  success        function         console log  error          Now  to execute stuff after the promise is complete  you can just execute it inside the promise callback inside the then   call   So far this would be exactly the same as regular callbacks   To actually make good use of promises is when you chain them  like this   query find   then function         console log  success          return new Parse Query Obj  get  sOmE oBjEcT       function         console log  error       then function         console log  success on second callback        function         console log  error on second callback

User · Answer

You don t want to make the function wait  because JavaScript is intended to be non-blocking  Rather return the promise at the end of the function  then the calling function can use the promise to get the server response    var promise   query find     return promise      Or return query find

User · Answer

Instead of returning a resultsArray you return a promise for a results array and then then that on the call site - this has the added benefit of the caller knowing the function is performing asynchronous I O  Coding concurrency in JavaScript is based on that - you might want to read this question to get a broader idea   function resultsByName name           var Card   Parse Object extend  Card        var query   new Parse Query Card       query equalTo  name   name toString          var resultsArray            return query find                                        later resultsByName  Some Name   then function results          access results here by chaining to the returned promise       You can see more examples of using parse promises with queries in Parse s own blog post about it

[javascript] How do I wait for a promise to finish before returning the variable of a function?

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