Add objects dinamically to an array of objects

Question

I am wondering if there is a way to dynamically add objects to an array of objects based on a value? For example, I have an array of objects:

[
    {category:A, num:5}, 
    {category:B, num:2}
]

I want to create another array of objects where objects would be the same, but repeated based on the value of num (so 5 times for category A and 2 times for category B) :

[
    {category:A, num:5, repeated:1},
    {category:A, num:5, repeated:2},
    {category:A, num:5, repeated:3},
    {category:A, num:5, repeated:4},
    {category:A, num:5, repeated:5},
    {category:B, num:2, repeated:1}, 
    {category:B, num:2, repeated:2}
]

I have tried map, forEach, for loop, but nothing worked. I am quite new to javascript, how some one could help!

This Wikipedia page (en.wikipedia.org/wiki/Java_(programming_language) states: Not to be confused with JavaScript. — Abra
– Abra, Commented Sep 29, 2020 at 14:13

Jamiec · Accepted Answer · 2020-09-29 14:25:34Z

1

You can do this using a combination of flatMap and map:

var input = [
    {category:"A", num:5}, 
    {category:"B", num:2}
] ;

var result = input.flatMap(e => [...new Array(e.num)].map( (x,i) => ({
    category:e.category,
    num: e.num,
    repeated: i+1
})));
console.log(result);

answered Sep 29, 2020 at 14:25

Jamiec

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Mulan · Accepted Answer · 2020-09-29 14:25:05Z

1

You could do it using flatMap -

const repeat = ({ num = 0, ...t }) =>
  num === 0
    ? []
    : [ ...repeat({ ...t, num: num - 1 }), { ...t, num, repeated: num } ]

const input = 
  [ { category: "A", num: 5}, { category: "B", num: 2 } ]
  
const output =
  input.flatMap(repeat)
  
console.log(output)

Output -

[
  { category: "A", num: 1, repeated: 1 },
  { category: "A", num: 2, repeated: 2 },
  { category: "A", num: 3, repeated: 3 },
  { category: "A", num: 4, repeated: 4 },
  { category: "A", num: 5, repeated: 5 },
  { category: "B", num: 1, repeated: 1 },
  { category: "B", num: 2, repeated: 2 }
]

answered Sep 29, 2020 at 14:25

Mulan

136k35 gold badges240 silver badges276 bronze badges

1 Comment

Maria Over a year ago

Thank you so much! Didnt know about flatMap, very useful.

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