Get open, close from json in Binance Data

Question

How to get open, baseAssetVolume of a json. I'm working with Binance NodeJs package, and I'll like to query open and other values.

This is the code from the site docs to get the full data

const api = require('binance');
const binanceWS = new api.BinanceWS(true);
const streams = binanceWS.streams;

binanceWS.onCombinedStream(
    [
     streams.ticker('BNBBTC')
    ],
    streamEvent => {
        switch (streamEvent.stream) {
            case streams.ticker('BNBBTC'):
            console.log(
               'Ticker event, update market stats\n',
                    streamEvent.data
            );
            break;
        }
    }
);

This is the data returned.

   {
  eventType: '24hrTicker',
  eventTime: 1619369591449,
  symbol: 'BNBBTC',
  priceChange: '0.00014790',
  priceChangePercent: '1.474',
  weightedAveragePrice: '0.01006976',
  previousClose: '0.01003790',
  currentClose: '0.01018500',
  closeQuantity: '40.36000000',
  bestBid: '0.01018460',
  bestBidQuantity: '7.55000000',
  bestAskPrice: '0.01018500',
  bestAskQuantity: '58.91000000',
  open: '0.01003710',
  high: '0.01026090',
  low: '0.00977950',
  baseAssetVolume: '401687.43000000',
  quoteAssetVolume: '4044.89583374',
  openTime: 1619283191026,
  closeTime: 1619369591026,
  firstTradeId: 136462988,
  lastTradeId: 136653082,
  trades: 190095
}

Answer 1

In node.js, you can declare variables using let and declare constants using const. You can access properties of an object with a dot, as well as like an array key.

switch (streamEvent.stream) {
    case streams.ticker('BNBBTC'):
        // declaring constant, accessing the `open` property with a dot
        const open = streamEvent.data.open;

        // declaring variable, accessing the property as it was an array key
        let baseAssetVolume = streamEvent.data['baseAssetVolume'];

        break;
}

---

You might also run into scoping issues. In Javascript, variables are visible only in the block that created them (in your case streamEvent => {) and nested blocks.

If you want to access the values in a higher scope, you need to define the variables in the higher scope.

const api = require('binance');
const binanceWS = new api.BinanceWS(true);
const streams = binanceWS.streams;

let open; // declaring the variable here

binanceWS.onCombinedStream(
    [
     streams.ticker('BNBBTC')
    ],
    streamEvent => {
        switch (streamEvent.stream) {
            case streams.ticker('BNBBTC'):
            open = streamEvent.data.open; // assigning the value here
            break;
        }
    }
);

---

Mind that the onCombinedStream event handler is probably going to be executed multiple times, and open = streamEvent.data.open is going to assign a new value each time it's executed.

To prevent this from happening, you can check whether the value has been already assigned (default value is null).

/*
 * Three equal signs for strict comparison.
 * Not necessary in this case, but it's a good practice.
 */
if (open === null) {
    open = streamEvent.data.open; // assigning the value here
}

instead of the previous

open = streamEvent.data.open; // assigning the value here