Use reduce with map and array

Question

I am creating small app so that i learn angular, typescript and so on. I find out about reduce function which i can use to sum fields in array. I am trying to implement it in my app but with no luck so far. I have something like this:

    const sumTest: any = data.grades?.map(async (studentNumber) => {
    const getGrade= await dataSources.getStudenGrade(studentNumber);
    return getGrade.grade
    }).reduce((a: any, b: any) =>{ a + b});

That is clearly wrong because i am getting [ Promise { <'pending'> }, Promise { <'pending'> } ] as result. Can somebody help me with that?

Stephen Cleary · Accepted Answer · 2020-11-23 18:44:15Z

1

Your mapping result is an array of promises. That's normal; by mapping each item to a promise, you end up with multiple promises.

If you want to asynchronously wait for all the promises to complete, you would use Promise.all:

const sumTest: any = data.grades?.map(async (studentNumber) => {
  const getGrade= await dataSources.getStudenGrade(studentNumber);
  return getGrade.grade;
});
const values = await Promise.all(sumTest);
values.reduce((a: any, b: any) =>{ a + b});

answered Nov 23, 2020 at 18:44

Stephen Cleary

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4 Comments

Eddy Over a year ago

Hi Stephen. I works but still i am not getting sum. Result is [ 4, 8 ] and another thing typescript is complaining about : Expected an assignment or function call and instead saw an expression.eslint in values.reduce((a: any, b: any) =>{ a + b});. I am calling console.log(values) to get above result.

Stephen Cleary Over a year ago

@Sasa: You should remove the curly braces: values.reduce((a: any, b: any) => a + b);

Eddy Over a year ago

values.reduce((a: any, b: any) => a + b) console.log(values); I did but result is still: [ 4, 8] it is not summing it.

Eddy Over a year ago

Ok it works now :):) I have put entire velues.reduce in console log :). Thanks a lot @Stephen

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