I'm writing a test where I'm creating an object literal. For example:
const myObject = {
id: 1,
name: 'egg',
error: null
}
The initialised value for myObject.error is null. However, when I try set it to a new value later on. For example:
myObject.error = ['exception', 'mapping']
I assign it a new value of string[] but I get an error:
Type 'string[]' is not assignable to type 'null'.ts(2322)
myObject is not an interface or a type so why does it expect it to always be a null value?
Thanks!
That happens because you didn't specifically define any other type for the myObject const, which will lead to TypeScript assigning the following type definition by default:
const myObject: {
id: number;
name: string;
error: null;
}
Yeah, null is a TypeScript type, and obviously, string[] is not compatibale with that. You can however, define the type yourself, and make it compatible like so:
type ObjectType = {
id: number;
name: string;
error: null | string[];
}
const myObject: ObjectType = {
id: 1,
name: 'egg',
error: null
}
myObject.error = ['exception', 'mapping']
This way, you are letting TypeScript know, that myObject.error can either assume the null or string[] type values.
TypeScript infers (guesses) the type of myObject, in your case it's:
{
id: number;
name: string;
error: null;
}
When you try to assign to error it fails as it's expecting null. You could create an interface:
interface Produce {
id: number;
name: string;
error: string[] | null;
}
const myObject: Produce = {
id: 1,
name: 'egg',
error: null
};
myObject.error = ['a']; // <== This works
Regarding the usage of null, you could use an optional T? instead, which is equivalent to T | undefined:
interface Produce {
id: number;
name: string;
error?: string[];
}
const myObject: Produce = {
id: 1,
name: 'egg'
};