Don't understand why a variable is undefined

Question

This little function uses tt (an array of test objects) and is expected to return a coherent array of objects like [{ testNum: [e, e, e, e] }, ...] where "e" is an empty array element.

  buildValues = tt => {
    tt.map( aTest => {
      const tn = aTest.testNum;
      return { tn: new Array( aTest.items.length )};
    })
  };

The linter gives a warning in the const line:
TestsPane.js|93 col 13 warning| 'tn' is assigned a value but never used. (no-unused-vars)
And effectively the return line does not use tn as I expected.
What am I doing wrong?

Below, a simplified element of the tt array:

  let tt = [
    { testNum: 1,
      items: [ { value: 0 },  { value: 1 },  { value: 0 }, { value: 0 } ]
    },
  ...

I've tried this in nodejs and it effectively fails as the linter suggests.

THE SOLUTION, built on top of the great answers I got:

  buildValues = tt => tt.map( aTest => {
      return { [aTest.testNum]: new Array( aTest.items.length )};
  });

you even never return the result of `map`. please add what you want as result. — Nina Scholz, Mar 22 '21 at 13:18
`const tn` and `{ tn: ... }` are two separate things and they're not related to each other. the first one is a variable name and the second one is an object's key. So it's normal that the linter is giving you that warning. — pouria, Mar 22 '21 at 13:18
You may want to see this post - [How to use map](https://stackoverflow.com/a/66519768/2873538) — Ajeet Shah, Mar 22 '21 at 13:19
https://stackoverflow.com/questions/2241875/how-to-create-an-object-property-from-a-variable-value-in-javascript — CBroe, Mar 22 '21 at 13:19
`tn` in the return object is not the same as the const declared outside it. The error basically says that you are declaring `tn` with `aTest.testNum`, but you are not using it for anything. — olawrdhalpme, Mar 22 '21 at 13:19
may be you need the `tn` to be the returned object `key` so that would be something like this: `return {[tn] : new Array( aTest.items.length )}` — kunal panchal, Mar 22 '21 at 13:20
Thanks to all who answered or commented. I learned how to code an object's key dinamicaliy: the key expression has to be enclosed in brackets for JS to consider it a reference, not a literal. — Juan Lanus, Mar 22 '21 at 15:43
Also, my code example shed additional confusion because I omitted the `return` when I edited my original one-liner that leveraged the implicit arrow-functions return for simple cases. — Juan Lanus, Mar 22 '21 at 15:46

score 2 · Answer 1 · answered Mar 22 '21 at 13:20

2

Indeed you are not using the variable tn. You return an object with the key "tn" and not the value of the variable tn.

const tn = aTest.testNum;
const myValue = {};
myValue[tn] = new Array( aTest.items.length );
return myValue

answered Mar 22 '21 at 13:20

alaric

169
6

score 1 · Accepted Answer · answered Mar 22 '21 at 13:22

1

I think you wanted it to be like so:

buildValues = tt => tt.map( aTest => {
    const tn = aTest.testNum;
    return { [tn]: new Array( aTest.items.length )};
});

answered Mar 22 '21 at 13:22

n--

3,563
4
9

Initially I wanted to be like this: `tt.map( aTest => ({ aTest.testNum: new Array( aTest.items.length )})` a single liner, but the linter compleined of the dot I'll try it with your brackets. – Juan Lanus Mar 22 '21 at 13:25
it would work like this: tt.map( aTest => ({ [aTest.testNum]: new Array( aTest.items.length )}) – n-- Mar 22 '21 at 13:28

score 0 · Answer 3 · answered Mar 22 '21 at 13:25

0

items: [ { value: 0 }.  { value: 1 }.  { value: 0 }.  { value: 0 } ]

You should be separating them with a comma.

return { tn: new Array( aTest.items.length )};

If you have a dynamic key you should put them in brackets like this

return { [tn]: new Array( aTest.items.length )};

answered Mar 22 '21 at 13:25

Marko

66
6

1

Yes @Marko, the dots instead of commas are a typo, I already fixed it to avoid confusion. And yes, the midding brackets seem to be my issue. – Juan Lanus Mar 22 '21 at 14:38

Don't understand why a variable is undefined

3 Answers3