Call variable using another variable whos string content is the other variables' name

Question

So I have a bunch of variables such as:

var HelloBob = '123';
var HelloSally = 'abc';
var HelloTim = 'pie';

I need to reference these variables by constructing their names and so having their names in another variable.

So in which this would output '123' instead of 'Hellobob'

var name = 'bob';
console.log('Hello'+bob)

I've done this previously using Window[] (I think), but I attempt using it and it doesn't work. Such as...

var HelloBob = '123';
var name = 'bob';
if(typeof window['Hello'+name] !== undefined){
   console.log('Variable Exists')
}

Which should only be true if a variable with the name 'Hellobob' exists, but it is always true when I run it.

I'll need to be able to reference the variables fully, so able to do .toLowerCase()

why not collect all values as properties of a custom object, this is easily accessable, without the need for beeing global. — Nina Scholz, Dec 19 '18 at 18:07
`typeof something` returns a string, which is `!== undefined`, but is `== "undefined"` — Reinstate Monica Cellio, Dec 19 '18 at 18:07
Code that is organized around variable names like this has a strong code smell. You should be using real data structures like objects for this. Everything will be easier to code, read, maintain and will be less fragile. — Mark, Dec 19 '18 at 18:10

MTCoster · Accepted Answer · 2018-12-19T18:12:06.500

4

There are a couple issues with your snippet:

typeof returns a string - you’re comparing the result to undefined (a property of the global scope) instead of 'undefined' (the type string)
You’re forming the variable name Hellobob which doesn’t exist. Setting name to Bob (instead of bob) should fix this.

var HelloBob = '123';
var name = 'Bob';

if (typeof window['Hello' + name] !== 'undefined') {
  console.log('Variable exists:', window['Hello' + name]);
}

However...

As mentioned in the comments, this is usually not a good pattern to follow. Instead, consider creating an object to contain these variables:

var hello = {
  Bob: '123',
  Sally: 'abc',
  Tim: 'pie',
};

var name = 'Bob';

if (hello.hasOwnProperty(name)) {
  console.log('Variable exists:', hello[name]);
}

edited Dec 19 '18 at 18:12

answered Dec 19 '18 at 18:08

MTCoster

5,868
3
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uhh `undefined` is *not* an object, but you're right generally. The `null` constant is termed an "object" by `typeof`, but that's just a long-time weirdness. – Pointy Dec 19 '18 at 18:10
@Pointy Good point, is my update more accurate? – MTCoster Dec 19 '18 at 18:12
In reference to your first code, this never passes the if check for me, I've found the project where I previously use window[] and the code there doesn't work in my current project. I do "console.log(window['Hello' + name]);" with your corrected code and it out puts 'undefined' – Vereonix Dec 20 '18 at 09:12
@Vereonix Can you provide a reproducible example? The snippet in my answer appears to work fine – MTCoster Dec 20 '18 at 14:50

Scott E · Answer 2 · 2018-12-19T18:16:46.520

-3

The best way I know is to use eval:

var HelloBob = '123';
var HelloSally = 'abc';
var HelloTim = 'pie';

function getGreeting(name) {
    var greetingVar = 'Hello'+name;
    var greeting = null;
    try {
        greeting = eval(greetingVar);
    } catch (err) {
        console.error("Tried to get greeting for a person that doesn't exist.");
    }
    return greeting;
}

console.log(getGreeting('Bob'));
console.log(getGreeting('Scott'));

See also: Use dynamic variable names in JavaScript

Edit: I agree with other comments though, this seems like a terrible idea that is probably best implemented another way.

edited Dec 19 '18 at 18:16

answered Dec 19 '18 at 18:12

Scott E

13
1
4

This could not be less relevant and could not be worse advice. The problem that the OP has is merely a couple of missing quotation marks. I don't know who upvoted this but they should really, **really** pay attention to what they're voting on. – Reinstate Monica Cellio Dec 19 '18 at 18:15
There's no reason to use eval here. – Taplar Dec 19 '18 at 18:17
Doesn't the window[] reference depend on the variables being globals run in a browser environment? Is that really a better solution than eval? – Scott E Dec 19 '18 at 18:18
Considering that `eval()` actually executes whatever you pass to it, yes, `window[varname]` definitely is better, even if it fails due to scope. – Reinstate Monica Cellio Dec 19 '18 at 18:19
Yes, that could certainly be a security concern in the real world. Fair point. – Scott E Dec 19 '18 at 18:25

Call variable using another variable whos string content is the other variables' name

2 Answers2

However...