How to change values of an array and store in new array?

Question

For several hours I've been searching on SO and trying to solve this: I have an array with objects. Each object has a key/value pair. I want to change the values of a certain key and store this in a new array.

Please have a look at this stackblitz

data:any[] = [
  { color: "brown", nr: 1},
  { color: "red", nr: 2},
  { color: "green", nr: 3}
]

newData: any[];

text() {
  const array = [];

  for (let i = 0; i < this.data.length; i++) {
    this.data['nr'] = i * 2;
    this.data['color'] = "yellow";
    array.push(this.data[i]);
    console.log(this.data[i]);
  }

  this.newData = array;
  console.log(array);
}

I expect the newData array would have the new values, but no success. Obviously I am doing something wrong. Has anyone a clue?

Hint: this.data is an array...it has no properties nr or color...it's elements do — charlietfl
– charlietfl, Commented Aug 9, 2019 at 17:41

cullanrocks · Accepted Answer · 2019-08-09 17:53:21Z

2

Try using the .map function:

let newArray = data.map((x, index) => ({
  color: 'yellow',
  nr: x.nr * index
}));

edited Aug 9, 2019 at 17:53

answered Aug 9, 2019 at 17:43

cullanrocks

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1 Comment

Jojoseph Over a year ago

This is for me the best solution: Nice and short. The other solutions (@MrRobboto, @randomSoul, @Islam) worked as well, with or without changing the original Array. Great! I Updated the stackblitz with the different solutions.

random · Accepted Answer · 2019-08-09 17:47:58Z

0

this.data is an array. In order to change the object key values, use this.data[i][keyName] instead of this.data[keyName].

newData:any[] = []; // <-- Initiate to empty array

text (){
    const array = [];
    for (let i = 0; i < this.data.length; i++) {
        this.data[i]['nr'] = i*2;
        this.data[i]['color'] = "yellow";

        array.push(this.data[i]);
    }
    this.newData.push(...array);
    console.log(JSON.stringify(this.newData));
  }

answered Aug 9, 2019 at 17:47

random

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Comments

MrRobboto · Accepted Answer · 2019-08-09 18:23:08Z

0

A key question I think would be if you plan to mutate the original array or not - other answers I believe are changing the values of the original array and there's no point in newData in that case. Also as randomSoul pointed out, you need to drill into each element of the array and access properties from there.

So assuming you are not trying to mutate the original - this is what I would do following your code style:

data:any[] = [
  { color: "brown", nr: 1},
  { color: "red", nr: 2},
  { color: "green", nr: 3}
];

newData: any[];

text() {
  // There are better ways to deep copy but out of scope...
  // Need to create new reference if we do not want to mutate original
  const array = JSON.parse(JSON.stringify(this.data));

  for (let i = 0; i < array.length; i++) {
    array[i]['nr'] = i * 2;
    array[i]['color'] = "yellow";
  }

  this.newData = array;
}

answered Aug 9, 2019 at 18:23

MrRobboto

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1 Comment

MrRobboto Over a year ago

Side note - TypeScript can really help you here if you type your data object, i.e.,: data: {color:string, nr: number}[] Then to access in your loop you could actually just do data[i].nr, data[i].color and if you change the types you'll get compile-time errors.

Islam · Accepted Answer · 2019-08-10 00:49:07Z

0

text (){
    const array = [...this.data]; // copy an array
    array.map((item, i) => {
      item["color"] = "yellow";
      item["nr"] = i * 2;
      console.log(item)
    })
    this.newData = [...array]; // copy an array
    console.log(this.newData);
  }

answered Aug 10, 2019 at 0:49

Islam

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