What is “not assignable to parameter of type never” error in typescript?

Question

Code is:

const foo = (foo: string) => {
  const result = []
  result.push(foo)
}

I get the following TS error:

[t

Answer 1

You need to type result to an array of string const result: string[] = [];.

Answer 2

All you have to do is define your result as a string array, like the following:

const result : string[] = [];

Without defining the array type, it by default will be never. So when you tried to add a string to it, it was a type mismatch, and so it threw the error you saw.

Answer 3

Remove "strictNullChecks": true from "compilerOptions" or set it to false in the tsconfig.json file of your Ng app. These errors will go away like anything and your app would compile successfully.

Disclaimer: This is just a workaround. This error appears only when the null checks are not handled properly which in any case is not a good way to get things done.

Answer 4

I got the same error in ReactJS function component, using ReactJS useState hook. The solution was to declare the type of useState at initialisation:

const [items , setItems] = useState<IItem[]>([]); // replace IItem[] with your own typing: string, boolean...

Answer 5

The solution i found was

const [files, setFiles] = useState([] as any);

Answer 6

I was able to get past this by using the Array keyword instead of empty brackets:

const enhancers: Array<any> = [];

Use:

if (typeof devToolsExtension === 'function') {
  enhancers.push(devToolsExtension())
}