How can I add a new key:value pair to an existing Javascript object dynamically within a loop

Question

I would like to calculate a new value based on two objects and add the result as new object to the existing array.

The input looks like:

[{
    "trades": [
        {
        "fields": {
                "orderOpenPrice": "1.40000",
                "orderTakeProfit": "1.50000",
                [...]

        },
        },

        {
          "fields": {
                "orderOpenPrice": "1.30000",
                "orderTargetPrice": "1.50000",
                [...]
        }
        },
        {
        "fields": {
                "orderOpenPrice": "1.50000",
                "orderTargetPrice": "1.55000",
                [...]
        }
        },

        [...]

}]

This is the desired output:

[{
    "trades": [
        {
        "fields": {
                "orderOpenPrice": "1.40000",
                "orderTakeProfit": "1.50000",
                "pipsTargetedKey": "10000",
                [...]

        },
        },

        {
          "fields": {
                "orderOpenPrice": "1.30000",
                "orderTakeProfit": "1.50000",
                "pipsTargetedKey": "20000",
                [...]
        }
        },
        {
        "fields": {
                "orderOpenPrice": "1.50000",
                "orderTakeProfit": "1.55000",
                "pipsTargetedKey": "5000",
                [...]
        }
        },

        [...]

}]

I tried two different approaches using this thread: How can I add a key/value pair to a JavaScript object?:

Using assign:

[...]
for (var i = 0; i < tradesTotal; i++) {
    pipsTargeted = Math.abs(trades[i].fields.orderOpenPrice - trades[i].fields.orderTakeProfit);
    trades[i].fields.assign(trades[i].fields, {pipsTargetedKey: pipsTargeted});
}
[...]

Using dot notation:

[...]
for (var i = 0; i < tradesTotal; i++) {
    pipsTargeted = Math.abs(trades[i].fields.orderOpenPrice - trades[i].fields.orderTakeProfit);
    trades[i].fields.pipsTargetedKey = pipsTargeted
}
[...]

However, both attempts do not add another key:value pair.

Edit on request:

tradesTotal = Object.keys(trades).length;

// manipulate trades object
for (var i = 0; i < tradesTotal; i++) {

    // format dateTime
    trades[i].fields.orderOpenTime = (trades[i].fields.orderOpenTime).replace('T', ' ');
    if (trades[i].fields.orderCloseTime !== null)
    trades[i].fields.orderCloseTime = (trades[i].fields.orderCloseTime).replace('T', ' ');

    // format orderType
    if (trades[i].fields.orderType === 0) {
        trades[i].fields.orderType = 'Buy'
    } else if (trades[i].fields.orderType === 1) {
        trades[i].fields.orderType = 'Sell'
    } else if (trades[i].fields.orderType === 2) {
        trades[i].fields.orderType = 'Buy Limit'
    } else if (trades[i].fields.orderType === 3) {
        trades[i].fields.orderType = 'Sell Limit'
    } else if (trades[i].fields.orderType === 4) {
        trades[i].fields.orderType = 'Buy Stop'
    } else if (trades[i].fields.orderType === 5) {
        trades[i].fields.orderType = 'Sell Stop'
    } else if (trades[i].fields.orderType === 6) {
        trades[i].fields.orderType = 'Bank Transaction'
    }

    // calculate R:R and TP + SL in pips and add result to object
    if (stopLoss && takeProfit > 0) {
        pipsRisked = Math.abs(trades[i].fields.orderOpenPrice - trades[i].fields.orderStopLoss);
        pipsTargeted = Math.abs(trades[i].fields.orderOpenPrice - trades[i].fields.orderTakeProfit);
        rrRatio = (pipsTargeted / pipsRisked);
        trades[i].fields.pipsRiskedKey = pipsRisked;
        trades[i].fields.pipsTargetedKey = pipsTargeted;
        trades[i].fields.pipsRRKey = rrRatio;
    }
}

if that's your inner loop you'll need to add `.length` to `tradesTotal` — Nick Parsons, Dec 21 '19 at 12:19
@NickParsons tradesTotal is a variable assigned with the object.length already, this is just fine. — JSRB, Dec 21 '19 at 12:24
Is the `trades` variable the property trades inside your array of objects? Or is it your entire array input? — Nick Parsons, Dec 21 '19 at 12:33

score 1 · Accepted Answer · answered Dec 21 '19 at 12:38

As you mentioned trades is an array. When you do Object.keys() on an array, you get the indexes of that array. This can be simplified to just trades.length as they are the same thing.

Currently, you're looping over trades, which allow you to access each object in your array. Each object has a trades property with an array, which you also need to loop over. This means you need a nested loop. One to loop over all your objects in your larger, and another to loop over all your objects in your trades property array. This can be done like so:

const tradesTotal = [{
  "trades": [{
      "fields": {
        "orderOpenPrice": "1.40000",
        "orderTakeProfit": "1.50000",


      },
    },

    {
      "fields": {
        "orderOpenPrice": "1.30000",
        "orderTargetPrice": "1.50000",

      }
    },
    {
      "fields": {
        "orderOpenPrice": "1.50000",
        "orderTargetPrice": "1.55000",

      }
    },
  ]
}];


for (var i = 0; i < tradesTotal.length; i++) {
  var trades = tradesTotal[i].trades;
  for (var j = 0; j < trades.length; j++) {
    var pipsTargeted = Math.abs(trades[j].fields.orderOpenPrice - trades[j].fields.orderTakeProfit);
    trades[j].fields.pipsTargetedKey = pipsTargeted
  }

}
console.log(tradesTotal);

@Phanti no worries. Currently the equation you're using to calculate `pipsTargetedKey` is incorrect so it won't give you you're desired result exactly. (some of your objects use `orderTakeProfit` and some use `orderTargetPrice`) — Nick Parsons, Dec 21 '19 at 12:41

score 0 · Answer 2 · answered Dec 21 '19 at 12:25

Just use Array.prototype.map function and spead operator:

const trades = [
  {
    "fields": {
      "orderOpenPrice": "1.40000",
      "orderTakeProfit": "1.50000",
    },
  },
  {
    "fields": {
      "orderOpenPrice": "1.30000",
      "orderTakeProfit": "1.50000",
    }
  },
  {
    "fields": {
      "orderOpenPrice": "1.50000",
      "orderTakeProfit": "1.55000",
    }
  }
]

const mappedTrades = trades.map(trade => {
  const {orderOpenPrice, orderTakeProfit} = trade.fields
  return {
     ...trade,
     fields: {
        ...trade.fields,
        pipsTargetedKey: Math.abs(Number(orderOpenPrice) - Number(orderTakeProfit))
     }
  }
})

Another way is to use something like Proxy or Object.observe.

How can I add a new key:value pair to an existing Javascript object dynamically within a loop

2 Answers2