javascript - Why is .forEach returning undefined?

Question

I know there already are multiple questions to this topic https://stackoverflow.com/search?q=%5Bjavascript%5D+return+forEach+undefined but none of this seemed to help me out.

So I have the following data:

 const testsMap = {
            0: ["just", "test"],
            1: ["bla", "asdf"]
        }

 const testArray = [{
    id: "1",
    segments: null,
    tests: [{
            id: "1",
            segments: "1"
        },
        {
            id: "2",
            segments: "0"
        }
    ]
},
{
    id: "2",
    segments: "1",
    tutorials: [{
            id: "1",
            segments: "1"
        },
        {
            id: "2",
            segments: "0"
        }
    ]
}];

What I want to achieve with the output without using .map() or .reduce since I do not want a new array, I just want to overwrite the existing one, is the following:

[{
    display: true,
    id: "1",
    segments: null,
    tests: [{
            display: true,
            id: "1",
            segments: "1",
            newSegments: ["bla", "asdf"]
        },
        {
            display: true,
            id: "2",
            segments: "0",
            newSegments: ["just", "test"]
        }
    ]
},
{
    display: false,
    id: "2",
    segments: "1",
    tutorials: [{
            id: "1",
            segments: "1"
        },
        {
            id: "2",
            segments: "2"
        }
    ]
}];

The function I have looks something like this - please note that it has some helper fns which you can ignore - it's just about that the fn returns undefined:

function SOtest () {
  const returnedValue = testArray.forEach(test => {
    test.newSegments = test.segments ? testsMap[test.segments] : [];
    test.display = helperFn(); // will add true/false to the test prop

    if (test.display) {
      test.tests.map(t => {
        t.newSegments = t.segments ? testsMap[t.segments] : [];
        t.display = helperFn(); // will add true/false to the test prop
      })
    }
    return test;
  })
  return returnedValue;
}

Now the forEach itself works just fine when executed by itself in the console - but as soon as I want to return it, it equals undefined.

What am I missing?

Answer 1

forEach doesn't return anything. It just loops through elements and while looping you can change element data

And so you can change your function SOtest to

function SOtest () {
  testArray.forEach(test => {
    test.newSegments = test.segments ? testsMap[test.segments] : [];
    test.display = helperFn(); // will add true/false to the test prop

    if (test.display) {
      test.tests.map(t => {
        t.newSegments = t.segments ? testsMap[t.segments] : [];
        t.display = helperFn(); // will add true/false to the test prop
      })
    }
  })
  return testArray;
}