Javascript: How can I create an object by specifying the fields in strings?

Question

I'm trying to update a value in a MongoDB document. The document has many fields but I only want to specify a few of them depending on the fields I've changed in the UI.

var monthField='calendar.m'+month+'.result';
var triField='calendar.t'+trimester+'.result';
var yearField="calendar.year.result";

Objective.update({_id:{$in:objective.parents}},{
    $inc:
    {
        yearField:transaction.value,
        monthField:transaction.value,
        triField:transaction.value
    }},
    {multi:true, upsert:true}
  )

Unfortunately the above code does not "eval" the yearField, monthField and triField to their string values and instead is trying to update as if those fields exist in the document.

I know I can just find the documents, alter the values, and save them all one by one but is there a way to do what I'm trying to do? It's just so much better doing it in one update line.

You are not dealing with JSON here. JSON always is a string, and nothing else (the contents of that string looks a like Javascript, but it isn't). What you are dealing with here are simply native objects, i.e. object literals. — Tomalak
– Tomalak, Commented Oct 7, 2016 at 11:23

James Thorpe · Accepted Answer · 2016-10-07 11:20:36Z

5

Since you're using node.js, you can make use of computed property names, just by wrapping the names in brackets:

var monthField='calendar.m'+month+'.result';
var triField='calendar.t'+trimester+'.result';
var yearField="calendar.year.result";

Objective.update({_id:{$in:objective.parents}},{
    $inc:
    {
        [yearField]:transaction.value,
        [monthField]:transaction.value,
        [triField]:transaction.value
    }},
    {multi:true, upsert:true}
)

This syntax was added to the spec in ES2015, and while not ubiquitous yet, it is supported in node.js.

answered Oct 7, 2016 at 11:20

James Thorpe

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3 Comments

Ricardo Mota Over a year ago

Perfect. Thank you. So that is the equivalent of using {{ }} in angular?

James Thorpe Over a year ago

@RicardoMota No idea - I'm not an angular guy :)

Ricardo Mota Over a year ago

Fair enough :) I'll accept your answer as soon as the system allows.

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