TypeScript: Check if variable has been initialised

Question

I have some function as follows:

const myFunction = (columns: any, data: any) => {
  let label: string;
  let value: number;

  //some section where it assigns label and value
  for (let index = 0; index < columns.length; index++) {
        const column: string = columns[index].fieldName;
        const datapoint: string = data[index].formattedValue;

        if (column.includes("KeyPointLabel")) {
            label = datapoint;
        }
        if (column.includes("KeyPointValue")) {
            value = Number(datapoint);
        }
    }

  //if the above section worked as expected, they should have a value.
  //so if it doesn't, throw an error.
  if (typeof label === undefined) throw new Error() <--- ERROR HERE!!

  return {label, value}
}

I want to null check for label and value. I have looked at a couple of other threads where it suggested to do if (typeof label === undefined) or if (label == null), etc. but any statement that references label or value gives me 'used before assigned' error.

The section where it assigns label and value is just a for-loop which looks through the columns to see if it has a column name that contains KeyPointLabel, assigns label = data[same index as column], and similarly for value. The form of columns and data is not something I can control.

Technically, I could do

  let label: string = "";
  let value: number = -1;

  ...

  if(label === "" || value < 0) throw new Error();

  return { label, value }

because my value is never expected to have a negative number and label never to be an empty string, but I was wondering if there is a bit more elegant solution.

What is //some section where it assigns label and value? Sounds like it'd make the most sense by far to throw in there when the conditional assignment fails, instead of afterwards — CertainPerformance
– CertainPerformance, Commented May 10, 2022 at 0:15

Azarro · Accepted Answer · 2022-05-10 00:32:52Z

3

The reason you get the used before assigned error is because Typescript doesn't know that undefined is a possible value/type for label or even the other value.

Your first codeblock actually works if you just simply specify the type as:

  let label: string | undefined;
  let value: number | undefined;

TS Playground

answered May 10, 2022 at 0:32

Azarro

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brunnerh · Accepted Answer · 2022-05-10 00:33:05Z

2

I would just declare it as possibly being undefined, because it is:

  let label: string | undefined;
  let value: number | undefined;

answered May 10, 2022 at 0:33

brunnerh

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CER · Accepted Answer · 2022-05-10 00:45:38Z

0

You can also use the '!' (Definite Assignment Assertion).

Example:

let label!: string;
let value!: number;

Example: TS Playground

answered May 10, 2022 at 0:45

CER

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