Javascript unable to return variable

Question

I have an array of arrays with objects that looks like this:

var data = [
[{'name':'John', 'value':2},
{'name':'Randy', 'value':4}
],

[{'name':'Sarah', 'value':10},
{'name':'Julie', 'value':10}
]
];

I want to iterate through each array and add up values and store the totals in an array. What I came up with looks like this:

function add(accumulator, a) {
    return accumulator + a;
}

var totals = [];

var yExtent = data.forEach(function(item) {
  item.forEach(function(jtem) {
    var subTotals = [];
    subTotals.push(jtem.value);
    return subTotals;
  })
  var localTotal = subTotals.reduce(add,0);
  totals.push(localTotal);
  return totals;
});

console.log(totals)

However, this results in the error: "subTotals is not defined"

Question

Is this happening because subTotals is not in the same scope as the other part of the function? If so, how should I proceed? I cannot put var localTotal = subTotals.reduce(add,0); in scope because the forEach() would calculate it too frequently and give me the wrong values. How should I modify this code to return my inner forEach() calculations to the outer?

Except for a useless undefined, forEach doesn't return anything. Besides that, you have the scope issue of that variable. — Gerardo Furtado
– Gerardo Furtado, Commented May 17, 2019 at 1:21
move var subTotals = []; one line up out of the inner forEach — Lux
– Lux, Commented May 17, 2019 at 1:24
^--- this, and also remove the return total (which is doing nothing anyway). — Gerardo Furtado
– Gerardo Furtado, Commented May 17, 2019 at 1:25

Sean Vieira · Accepted Answer · 2019-05-17 01:29:46Z

2

forEach is for side effects. You really want to write your transforms using transforming functions instead of side effects. First, it will be shorter. Second, it will be clearer - you don't want a side effect, you want a pure transformation, and your code will reflect that. Using the same code, but with map instead results in this:

var totals = data.map(item => {
  var localTotal = item.map(datum => datum.value).reduce(add, 0);
  return localTotal;
});

answered May 17, 2019 at 1:29

Sean Vieira

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Comments

Adam B · Accepted Answer · 2019-05-17 01:28:41Z

1

You might want to do something like

function selectValue(item) {
  return item.value;
}

const totals = data.map(
  subList => subList.map(selectValue)
                    .reduce(add, 0)
);

answered May 17, 2019 at 1:28

Adam B

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Comments

Sloan Tash · Accepted Answer · 2019-05-17 01:34:14Z

1

You need to move the declaration of subTotals outside of the forEach loop like this-

Also you shouldn't reuse item.

function add(accumulator, a) {
    return accumulator + a;
}

var totals = [];

var yExtent = data.forEach(function(item) {
    var subTotals = [];
    item.forEach(function(subItem) {
        var subs = [];
        subTotals.push(subItem.value);
        return subs;
    })
    var localTotal = subTotals.reduce(add,0);
    totals.push(localTotal);
    return totals;
});

console.log(totals)

This will still give you a multi-dimensional array so you may want to change the console log to console.log(totals.flat(2)).

Or you may want to add the numbers..

answered May 17, 2019 at 1:34

Sloan Tash

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Comments

Maiya · Accepted Answer · 2019-05-17 04:42:02Z

0

The array.forEach function is made so that it is impossible for it to return any value. There is not a way to over-ride this behavior. It's something they did under the hood. So, all you can do with .forEach is go through an array and do something with the items that are stored to the array, like console.log() them, etc.

answered May 17, 2019 at 4:42

Maiya

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