Convert nested array to an object

Question

I am trying to use reduce to convert a nested array to an object.

I want to convert var bookprice = [["book1", "$5"], ["book2", "$2"], ["book3", "$7"]];

to

var bookpriceObj = {
    "book1": "$5", 
    "book2": "$2",
    "book3": "$7"
};

here is what I tried

var bookprice = [["book1", "$5"], ["book2", "$2"], ["book3", "$7"]];
bookpriceObj = {};
bookprice.reduce(function(a, cv, ci, arr){
    for (var i = 0; i < arr.length; ++i)
        bookpriceObj [i] = arr[i];

    return bookpriceObj ;
})

but the below result is not the desired result

{
    ["book1", "$5"]
    ["book2", "$2"]
    ["book3", "$7"]
}

Seems to me like you're trying to use a reducer like it's a for loop, but it's a different concept. Do not reference variables that are outside it, all the things are already in the arguments of the reducer. The same accumulator is passed through the whole thing element by element, and you "collect" things to it (or sum or what have you). But in this case a simple for loop would probably be way faster. — user1425689
– user1425689, Commented Aug 24, 2018 at 15:46

Emeeus · Accepted Answer · 2018-08-24 15:32:35Z

12

Using forEach is shorter

var bookprice = [["book1", "$5"], ["book2", "$2"], ["book3", "$7"]];

var bookpriceObj = {};


bookprice.forEach(e=>bookpriceObj[e[0]] = e[1]);

console.log(bookpriceObj)

answered Aug 24, 2018 at 15:32

Emeeus

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2 Comments

Orelsanpls Over a year ago

You have an entry value, a loop over an array and an end value, it's definitely a fit for Array.reduce

Emeeus Over a year ago

@GrégoryNEUT I know, but I'm not a big fan of reduce for this situations, the end value is an object, not a single value (which is the purpose). I think is more clear using loops. In my opinion, reduce() is better to reduce, to find one single value as end value.

mpm · Accepted Answer · 2018-08-24 15:37:52Z

4

You need to return an object from the "reducer" AKA the first argument of reduce, and use an empty object as second argument.

var bookprice = [
  ["book1", "$5"],
  ["book2", "$2"],
  ["book3", "$7"]
];

var result = bookprice.reduce(function(object, el) {
  object[el[0]] = el[1]
  return object;
}, {})

console.log(result)

You don't need a for loop since reduce already iterates over the array.

edited Aug 24, 2018 at 15:37

answered Aug 24, 2018 at 15:31

mpm

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Comments

zfrisch · Accepted Answer · 2018-08-24 15:45:05Z

You can use reduce with Array Destructoring.

    bookpriceObj = bookprice.reduce((a, [b, c]) => (a[b] = c, a), {});

Here's what's happening:

a is the accumulator which has an initial value of the final parameter passed to reduce. The last value we pass is {} so it's an object
Array Destructoring can immediately assign values in an array to variables. a.e. [a, b] = ["one", "two"] assigns a with a value of "one" and b with a value of "two"
with each iteration over each item we assign the value of b(a.e. "book1") as a property on the object(a) and give it a value equal to c(a.e. "$2")
Each iteration of reduce you must return the accumulator(a)
When the whole thing is finally done, it'll store in bookpriceObj the result!

var bookprice = [ ["book1", "$5"], ["book2", "$2"], ["book3", "$7"]],
bookpriceObj = bookprice.reduce((a, [b, c]) => (a[b] = c, a), {});

console.log(bookpriceObj)

Alex Dovzhanyn · Accepted Answer · 2018-08-24 15:35:53Z

1

bookprice.reduce((acc, price) => {
  let tmp = {}
  tmp[price[0]] = price[1]
  return Object.assign(acc, tmp)
}, {})

answered Aug 24, 2018 at 15:35

Alex Dovzhanyn

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1 Comment

dwjohnston Over a year ago

Please add some explanation to your answer.

Orelsanpls · Accepted Answer · 2018-08-24 15:36:42Z

1

Using Array.reduce(), Array destructuration, Dynamical keys and Spread operator.

const bookprice = [
  ['book1', '$5'],
  ['book2', '$2'],
  ['book3', '$7'],
];

const ret = bookprice.reduce((tmp, [
  name,
  price,
]) => ({
  ...tmp,

  [name]: price,
}), {});

console.log(ret);

answered Aug 24, 2018 at 15:36

Orelsanpls

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Comments

Joliver · Accepted Answer · 2018-08-24 16:23:48Z

1

You are not returning the accumulated object in your reduce function and the for loop is not necessary.

Try:

const bookprice = [["book1", "$5"], ["book2", "$2"], ["book3", "$7"]];
const nameIndex = 0;
const priceIndex = 1;

const bookPricesObj = bookprice.reduce((prices, bookInfo) => {
    const name = bookInfo[nameIndex];
    const price = bookInfo[priceIndex];

    prices[name] = price;
    return prices;
}, {});

answered Aug 24, 2018 at 16:23

Joliver

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Comments

A l w a y s S u n n y · Accepted Answer · 2018-08-24 15:41:23Z

0

using Array.prototype.reduce to convert a nested array to an object

var bookprice = [
  ["book1", "$5"],
  ["book2", "$2"],
  ["book3", "$7"]
];
bookpriceObj = bookprice.reduce(function(acc, cur, i) {
  acc[cur[0]] = cur[1];
  return acc;
}, {})

console.log(bookpriceObj);

edited Aug 24, 2018 at 15:41

answered Aug 24, 2018 at 15:36

A l w a y s S u n n y

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