Question

19

Type 'string | ArrayBuffer' is not assignable to type 'string'

rated 0 times [ 20] [ 1] / answers: 1 / hits: 43605 / 6 Years ago, tue, october 23, 2018, 12:00:00

TypeScript error for reading string from FileReader

Simple code to read file contents:

const reader: FileReader = new FileReader();

       reader.readAsText(file);

       reader.onload = (e) => {

          const csv: string = reader.result; -> getting TS error on this line

}

TypeScript error I get:

Type 'string | ArrayBuffer' is not assignable to type 'string'.

  Type 'ArrayBuffer' is not assignable to type 'string'.

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kaitlynnb

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answered 6 Years ago tyriquehenryq · Accepted Answer

The error message says it all.

You declare a string type of csv variable.
You then assign string | ArrayBuffer type (of reader.result) to the string type, you just assigned. You cannot. You only can assign string to string.

So, if you 100% sure that reader.result contains string you could assert this:

const csv: string = reader.result as string;

However if you are not sure, do this:

const csv: string | ArrayBuffer = reader.result;

// or simply:

const csv = reader.result; // `string | ArrayBuffer` type is inferred for you

Then you typically should have some check like:

if (typeof csv === 'string') {/*use csv*/}

else {/* use csv.toString() */}