how to access array inside object without keys

Question

I have a JSON data which look like this:

 {
  "chart":[
     [
        1627776011000,
        28
     ],
     [
        1627776371000,
        38
     ],
     [
        1627776731000,
        48
     ],
     ...
    ]
}

I want to use this data in Chart.js where 1627776011000 ... are x axis values and 28 ... are y axis values.

I know I can do it with an array push

const myArr = JSON.parse(dataJSON);

const data = []; 
const labels = [];

myArr.chart.forEach((item) => {
    labels.push(item[0]);
    data.push(item[1]);
});

but could there be a better way, for example with a map?

You could use reduce, but the way you do it is just fine. Try not to look for the "perfect" solution, it is much more practical to settle with a working solution. — Akxe
– Akxe, Commented Dec 13, 2021 at 18:14
What do you mean by "better"? Opinion-based questions are off-topic on Stack Overflow. — TylerH
– TylerH, Commented Dec 14, 2021 at 15:52

Barmar · Accepted Answer · 2021-12-13 18:15:19Z

1

You can do it as two calls to map():

const labels = myArr.chart.map(el => el[0]);
const data = myArr.chart.map(el => el[1]);

answered Dec 13, 2021 at 18:15

Barmar

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2 Comments

koubin Over a year ago

thank you. perfect! Would you know roughly how much faster is this than array push from question?

Barmar Over a year ago

maybe slower since it has to do two loops. But maybe faster because it can allocate the result arrays at once instead of growing incrementally. You'll need to profile it.

Nicholas Hayashi · Accepted Answer · 2021-12-13 18:19:39Z

1

This seems to me like an invented problem.

If your data points are just x and y values, there's nothing wrong with saying the first array element is x and the second is y.

There are some people who would argue and say you should make it an Object with properties like xValue and yValue.

However, any upsides or downsides in either case would be marginal. You could imagine someone saying the Object version is more readable. The Object version is harder to optimize by the javascript runtime (V8 at least): https://v8.dev/blog/elements-kinds

Unless you have measured it, and it matters, it doesn't matter.

answered Dec 13, 2021 at 18:19

Nicholas Hayashi

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Comments

PeterKA · Accepted Answer · 2021-12-13 19:06:32Z

1

You can use .reduce() in as follows:

const myArr = {
  "chart":[
     [
        1627776011000,
        28
     ],
     [
        1627776371000,
        38
     ],
     [
        1627776731000,
        48
     ]
    ]
};

const [labels, data] = myArr.chart.reduce((acc,cur) => [
    [...acc[0], cur[0]],
    [...acc[1],cur[1]]],
[[],[]]);
console.log( labels, data );

answered Dec 13, 2021 at 19:06

PeterKA

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