I am coding a function that return a promise with a given type of data. My entity name is Groups and i want to return a array of Groups Groups[] This is my function

async filterGroupsByAvailableSpots (groups: Groups[] ) :Promise<Groups[]> {
    let groupsAvailableToMatch : Groups[] = [] ;
    
    let searchAvailableGroupsLoop = new Promise((resolve, reject) => {
      groups.forEach((group, index, array) => {
        this.getStudentsByActiveGroup (group.id)
          .then((students) => {
            //If the group have spots to match
            if (students.length < 6) {
              groupsAvailableToMatch.push(group)
            }
            // Stop FLag    
            if (index === array.length -1) {
              resolve(groupsAvailableToMatch);
            } 
          })
      });
    });

    return searchAvailableGroupsLoop;
  }

But Im facing an error in the return promise

let searchAvailableGroupsLoop: Promise<unknown>
Type '{}' is missing the following properties from type 'Groups[]': length, pop, push, concat, and 26 more.ts(2740)

I am coding a function that return a promise with a given type of data. My entity name is Groups and i want to return a array of Groups Groups[] This is my function

async filterGroupsByAvailableSpots (groups: Groups[] ) :Promise<Groups[]> {
    let groupsAvailableToMatch : Groups[] = [] ;
    
    let searchAvailableGroupsLoop = new Promise((resolve, reject) => {
      groups.forEach((group, index, array) => {
        this.getStudentsByActiveGroup (group.id)
          .then((students) => {
            //If the group have spots to match
            if (students.length < 6) {
              groupsAvailableToMatch.push(group)
            }
            // Stop FLag    
            if (index === array.length -1) {
              resolve(groupsAvailableToMatch);
            } 
          })
      });
    });

    return searchAvailableGroupsLoop;
  }

But Im facing an error in the return promise

let searchAvailableGroupsLoop: Promise<unknown>
Type '{}' is missing the following properties from type 'Groups[]': length, pop, push, concat, and 26 more.ts(2740)

• javascript
• node.js
• typescript
• nestjs

Share Follow asked Oct 18, 2020 at 1:27 Anderson LaverdeAnderson Laverde 39522 gold badges55 silver badges1616 bronze badges 3

• (∩⌣̀_⌣́) that async looks suspicious, you're not using await inside your function, try not to mix them blog.logrocket./async-await-in-typescript – diedu Commented Oct 18, 2020 at 1:56
• stackoverflow./questions/54385676/… – diedu Commented Oct 18, 2020 at 2:03
• did you solve your problem? – diedu Commented Oct 23, 2020 at 1:10

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2 Answers 2

Sorted by: Reset to default 2

I think TS is not able to infer the type for the returned variable, try specifying the type when creating the promise

  let searchAvailableGroupsLoop = new Promise<Groups[]>((resolve, reject) => {

You need to resolve your Promise for every case. If in your groups array there are none "Stop flags" your promise resolves to {}. So resolve with an empty array or reject with an error if that suits your requirements better.

async filterGroupsByAvailableSpots (groups: Groups[] ) :Promise<Groups[]> {
let groupsAvailableToMatch : Groups[] = [] ;

let searchAvailableGroupsLoop = new Promise((resolve, reject) => {
  groups.forEach((group, index, array) => {
    this.getStudentsByActiveGroup (group.id)
      .then((students) => {
        //If the group have spots to match
        if (students.length < 6) {
          groupsAvailableToMatch.push(group)
        }
        // Stop FLag    
        if (index === array.length -1) {
          resolve(groupsAvailableToMatch);
        } 
      })
  });
  resolve([]); // add this line or reject("Error");
});

return searchAvailableGroupsLoop;

}