What is the best approach to retrieve a value from a function that includes a Map.prototype.forEach loop with asynchronous calls?

Question

What is the best approach to retrieve a value from a function that includes a Map.prototype.forEach loop with asynchronous calls?

I have a block of code that looks like this:

async retrieveExportData() {
   const exportStatistics: Object[] = [];
   this.mongoRepositories.forEach( async (repository, key) => {
       await repository.connect();
       let queryResults = await repository.getData();
       exportStatistics.push(...queryResults);
       repository.close();
    });        
  return exportStatistics;
}

this.mongoRepositories is a Map<string, MongoRepo>.

How can I properly return the complete exportStatistics array? The current implementation leaves it empty at the end of the block.

EDIT: This question is distinct from the potential duplicate because of the differences in iterating over a Map compared to iterating over an Array. Updating the question title to reflect this distinction.

typescript async-await

Answer 1

Answer №1

After attempting to iterate without using Map.prototype.forEach and encountering compilation errors, I realized that I needed to switch my target environment to es6 from es5 and enable downlevelIteration in my tsconfig.json file. Once I made these changes, the code compiled successfully.

Subsequently, I restructured the code block in the following manner:

async getExportStats() {
  return await this.queryMongo();
}

private async queryMongo() {
   const exportStats: Object[] = [];
   for (const [key, repo] of this.mongoRepos.entries()) {
     await repo.connect();
     const queryResult = await repo.getExportStats();
     exportStats.push(...queryResult);
 }
   return exportStats;
}

Implementing this restructuring resolved the issue that I was facing.

Answer 2

After attempting to iterate without using Map.prototype.forEach and encountering compilation errors, I realized that I needed to switch my target environment to es6 from es5 and enable downlevelIteration in my tsconfig.json file. Once I made these changes, the code compiled successfully.

Subsequently, I restructured the code block in the following manner:

async getExportStats() {
  return await this.queryMongo();
}

private async queryMongo() {
   const exportStats: Object[] = [];
   for (const [key, repo] of this.mongoRepos.entries()) {
     await repo.connect();
     const queryResult = await repo.getExportStats();
     exportStats.push(...queryResult);
 }
   return exportStats;
}

Implementing this restructuring resolved the issue that I was facing.

Answer 3

Answer №2

Avoid using await within a for loop. To enhance throughput, utilize Array.prototype.map() along with Promise.all() to simultaneously connect to each repository and fetch data. Then, employ Array.prototype.flat() to merge all the results into a single-dimensional array of objects that is returned from the collection of query results.

async getExportData() {
  const queryResults: Object[][] = await Promise.all(
    Array.from(this.mongoRepositories).map(async ([key, repo]) => {
      await repo.connect();
      const queryResult: Object[] = await repo.getData();
      repo.close();
      return queryResult;
    })
  );
  return queryResults.flat();
}

Answer 4

Avoid using await within a for loop. To enhance throughput, utilize Array.prototype.map() along with Promise.all() to simultaneously connect to each repository and fetch data. Then, employ Array.prototype.flat() to merge all the results into a single-dimensional array of objects that is returned from the collection of query results.

async getExportData() {
  const queryResults: Object[][] = await Promise.all(
    Array.from(this.mongoRepositories).map(async ([key, repo]) => {
      await repo.connect();
      const queryResult: Object[] = await repo.getData();
      repo.close();
      return queryResult;
    })
  );
  return queryResults.flat();
}

What is the best approach to retrieve a value from a function that includes a Map.prototype.forEach loop with asynchronous calls?

Answer №1

Answer №2

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