Cannot assign an array of Typescript type Never[] to an array of objects

Creating an object array can sometimes lead to errors, let's take a look at an example:

objectExample[a].push({ id: john, detail: true });
objectExample[a].push({ id: james, detail: false});

const objectExample = {
   a = [ { id: john, detail: true}, 
         { id: james, detail: false}];
   }

In TypeScript, attempting to define the object as follows may result in an error:

const objectExmaple: { [key: string]: { [key: string]: string | boolean}[]} = [];

The error message might look like this:

Type 'never[]' is not assignable to type '{ [key: string]: { [key: string]: string | boolean; }[]; }'.
  Index signature is missing in type 'never[]'.ts(2322)

To resolve this error and successfully create your object array, you need to...

typescript

Answer №1

exampleObject is actually an object, not an array, so the proper way to initialize it would be using {}. Furthermore, if you wish for key a to contain an array, you must specify this either during initialization or before utilizing the push method:

const exampleObject: { [key: string]: { [key: string]: string | boolean }[] } = {
  a: []
};

exampleObject['a'].push({ id: 'john', detail: true });
exampleObject['a'].push({ id: 'james', detail: false});

Another suggestion could be to declare the type in a more readable manner such as 

Record<string, Array<Record<string, string | boolean>>>

Explore More

Answer №2

There are a couple of issues:

  • If the variable objectExample is an object, you cannot initialize it to []
  • The type definition seems overly complicated for no reason
type Item = { [key: string]: string | boolean}
// Same as type Item = { [key: string]: string | boolean}

const objectExample: Record<string, Item[]> = {
   a: [ { id: 'john', detail: true}, 
         { id: 'james', detail: false}]
}

objectExample.a.push({ id: 'john', detail: true });
objectExample.a.push({ id: 'james', detail: false});

Here is a useful link to try this code out in a playground

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