How to return array with one field defined by a string argument

Question

I have a function that returns an array, how do I define the name of one of the columns in the return array with an argument?

Let's say instead of field 'qty' I want to have field whose name is passed in myFieldName argument.

var results = _.map(
    _.where(data, {
        UNDERLYING: product
    }),
    function (r) {
        if (r["QUANTITY"] != 0 && !isInArray(Date.parse(r["REPORT_DATE"]), errorDates)) {
            return {
                dt: Date.parse(r["REPORT_DATE"]),
                qty: (r["QUANTITY"] * multiplier)
            };
        }
    }
);

Closely related: [Dynamically access object property using variable](http://stackoverflow.com/questions/4244896/dynamically-access-object-property-using-variable) — apsillers, May 06 '15 at 13:56
Most closely a duplicate of [Using a variable for a key in a JavaScript object literal](http://stackoverflow.com/questions/2274242/using-a-variable-for-a-key-in-a-javascript-object-literal) — apsillers, May 06 '15 at 13:59
As you can see I am already using that notation, the issue is I can't refer to r, as the new array is not r, I can't write r["qty"] instead of qty: — LucasSeveryn, May 06 '15 at 14:07

thefourtheye · Answer 1 · 2015-05-06T14:01:30.623

You cannot use the variable name to define a property in an Object literal. So, you may have to create an object and add properties to it, like this

var obj = {};
obj["dt"] = Date.parse(r["REPORT_DATE"]);
obj[myFieldName] = r["QUANTITY"] * multiplier;
return obj;

If you know few of the properties which are going to be there already, then you can define them in the object literal itself, like this

var obj = {
    dt: Date.parse(r["REPORT_DATE"])
};
obj[myFieldName] = r["QUANTITY"] * multiplier;
return obj;

If you are in an environment which supports ECMAScript-6's Computed Property names, then you can simply do

return {
    dt: Date.parse(r["REPORT_DATE"]),
    [myFieldName] : r["QUANTITY"] * multiplier
}

ssube · Accepted Answer · 2015-05-06T14:47:10.327

You can assign properties, new or existing, after creating an object like so:

let foo = {bar: 1};
foo.baz = 2;

To do that with a dynamic name, you need to use the brackets accessor, like:

function assign(obj, field, value) {
    obj[field] = value;
}

In your case, you would do something like:

function getQuantity(data, field) {
  var results = _.map(
    _.where(data, {
      UNDERLYING: product
    }),
    function(r) {
      if (r["QUANTITY"] != 0 && !isInArray(Date.parse(r["REPORT_DATE"]), errorDates)) {
        var returnValue = {
          dt: Date.parse(r["REPORT_DATE"])
        };
        returnValue[field] = r["QUANTITY"] * multiplier;
        return returnValue;
      }
    }
  );
  return results;
}

Thank you, this is missing one bit, after the `returnValue[Field]` we need to add `return returnValue;` — LucasSeveryn, May 06 '15 at 14:46

How to return array with one field defined by a string argument

2 Answers2