how can I dynamically get the variables in javascript?

Question

I am working on React app, but I think this is most likely JavaScript problem. I have many variables with a pattern, VARIABLE_NAME_{number}, example, FOO_1, FOO_2 ... so on. In my function, it takes index as input and return mapped output.

import power from 'customClass';
const getOneOfFoo = (theIndex) => {
  power()
   .then(data => {
      let result = data?.first?.second?.FOO_{theIndex}?.name ; // how can I declare this one with passing input?
     // example, if theIndex=59, I want to have
     // let result = data?.first?.second?.FOO_59?.name;
      resolve({
          result: result
      });
     })
   .catch(err => console.log(err))
  });

The data object structure is like this,

data
 |-first
     |-second
         |-FOO_1
            |-name
         |-FOO_2
            |-name
         |-FOO_3
            |-name
         |-FOO_4
            |-name
         ...

In line 5, I want to assign result dynamically. How can I achieve this?

If I understand your question right, you can access properties like []. For example SomeObject["someProperty"] or in your case SomeObject[INDEX] — terpinmd
– terpinmd, Commented Dec 1, 2021 at 2:06

Capt 171 · Accepted Answer · 2021-12-01 02:09:10Z

You can treat it like a dictionary. Here's an example:

const data = {
  'FOO_1': { name: 'Kenobi' },
  'FOO_2': { name: 'Skywalker' },
  'FOO_3': { name: 'Yoda' },
  'FOO_4': { name: 'Kylo' },
  'FOO_5': { name: 'Sidious' }
}

function getVar(index) {
  let result = data?.[`FOO_${index}`]?.name;
  return result;
}

console.log(getVar(1)); // expected output: Kenobi
console.log(getVar(2)); // expected output: Skywalker
console.log(getVar(3)); // expected output: Yoda
console.log(getVar(4)); // expected output: Kylo
console.log(getVar(5)); // expected output: Sidious
console.log(getVar(6)); // expected output: undefined

So for your case, it would probably be something like this:

import power from 'customClass';
const getOneOfFoo = (theIndex) => {
  power()
   .then(data => {
      let result = data?.first?.second?.[`FOO_${theIndex}`]?.name;
      resolve({
          result: result
      });
     })
   .catch(err => console.log(err))
  });

And btw, resolve doesn't seem to be defined.

Nhut Pham · Accepted Answer · 2021-12-01 02:38:02Z

-1

let result = data?.first?.second[`FOO_${theIndex}`]?.name ;

u can use [] with inside is string which is name of property

edited Dec 1, 2021 at 2:38

answered Dec 1, 2021 at 2:10

Nhut Pham

1859 bronze badges

3 Comments

Capt 171 Over a year ago

Wrong syntax. Supposed to be let result = data?.first?.second?.[`FOO_${theIndex}`]?.name;

Nhut Pham Over a year ago

i just try one and it's okay,

Capt 171 Over a year ago

Backticks for template literals DO NOT mix with normal strings. Also see the TC39 proposal for optional chaining. It goes something like this: obj?.[index]

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