Dynamically assign value to undefined variable

Question

I'm sure there is a simple way to do this, but am stumped for now.

I have many variables defined at the top of my script (here is an example of two):

var firstVar,secondVar;

Then I have an object which contains those variables:

var myObj = { a: {name:firstVar, number:1}, b: {name:secondVar, number:2}

I want to assign values to those variables:

keys = Object.keys(myObj);

function getAll(e){
   var myArray = [];
   for (var prop in myObj){
      myArray.push(myObj.prop[e]);
   }
return myArray; 
}

The behaviour I want is:

var nameVars = getAll(name);
// [firstVar,secondVar]

But instead it returns:

// [undefined,undefined]

How else can I get the variables before defining them?

T.J. Crowder · Accepted Answer · 2016-08-07 10:08:45Z

2

Then I have an object which contains those variables:

No, it doesn't. It contains a copy of the value those variables contained as of when you created the object (which is undefined, since you've never assigned a value to them). Once created, there is no ongoing link between the object property you've copied the value to and the variable.

Since the object has no enduring link to the variables, there's no way for getAll to return the information you've said you want.

You've said in a comment that you're building d3 graphs and have the same structure with some variables, and want to avoid repeating yourself. It sounds to me like you want a builder function:

function buildObject(firstVar, secondVar) {
    return { a: {name:firstVar, number:1}, b: {name:secondVar, number:2} };
}

...which you would then use like this:

var obj1 = buildObject("value1", "value2");
// do a graph
var obj2 = buildObject("valueA", "valueB");
// do a graph

...or possibly even something that just takes the variables and produces the graph:

function makeGraph(firstVar, secondVar) {
    buildTheGraph({ a: {name:firstVar, number:1}, b: {name:secondVar, number:2} });
}

I don't think it is, but if it's the names you want, just put them in quotes (and also myArray.push(myObj.prop[e]); should be myArray.push(myObj[prop][e]); and getAll(name) should be getAll("name")), but again there's no link to the variables at all:

// Since they're not used, we don't even need these: var firstVar, secondVar;

var myObj = { a: { name: "firstVar", number: 1 }, b: { name: "secondVar", number: 2 } };

function getAll(e) {
  var myArray = [];
  for (var prop in myObj) {
    myArray.push(myObj[prop][e]);
  }
  return myArray;
}

var nameVars = getAll("name");
console.log(nameVars);

...but note that having the names doesn't help you get the variable values later (unless you use eval, which you should seek to avoid).

edited Aug 7, 2016 at 10:08

answered Aug 7, 2016 at 9:50

T.J. Crowder

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10 Comments

Jonathan_W Over a year ago

So does this mean that if I want this behaviour, I should define all the variables with some dummy value up front?

T.J. Crowder Over a year ago

@Jonathan_W: The value of the variables is irrelevant. The key thing here is that there's no link between the object property and the variable.

T.J. Crowder Over a year ago

@Jonathan_W: I think this is an X/Y problem. What is it you expect to do with the value of myObj.a.name?

Akintayo Jabar Over a year ago

@Jonathan_W try using prop[e] only instead of myObj.prop[e] in your getAll function and your e should be a string i.e "name"

T.J. Crowder Over a year ago

@Jonathan_W: I suggest posting a new question asking how to do that, with an minimal reproducible example showing where you're repeating yourself. Then we can help you solve the real problem (and possibly demonstrate a technique you can reuse for similar problems in the future).

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