What is "not assignable to parameter of type never" error in TypeScript?

Code is:

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

I get the following TS error:

[ts] Argument of type 'string' is not assignable to parameter of type 'never'.

What am I doing wrong? Is this a bug?

Solution 1:

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.

Solution 2:

Another way is :

const result = [] as  any;

Solution 3:

This seems to be some strange behavior in typescript that they are stuck with for legacy reasons. If you have the code:

const result = []

Usually it would be treated as if you wrote:

const result:any[] = []

however, if you have both noImplicitAny FALSE, AND strictNullChecks TRUE in your tsconfig, it is treated as:

const result:never[] = []

This behavior defies all logic, IMHO. Turning on null checks changes the entry types of an array?? And then turning on noImplicitAny actually restores the use of any without any warnings??

When you truly have an array of any, you shouldn't need to indicate it with extra code.