How do I assert an ojbect to only have properties from a string union with TypeScript?

Here's an example:

type supportedChains = 'eth_mainnet' | 'eth_testnet'

// I want to make sure fixtures only have eth_mainnet and eth_testnet props
const fixtures = {
    eth_testnet: {
        // ...
    },
} as ??

What I've tried for ??:

• initially { [chain: supportedChain]: any } – getting An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.ts(1337)
• next, { [chain in supportedChain]: any } – this doesn't give an error, but also doesn't guard against unwanted props

Not sure how to seach for this, please suggest how to implement this and ideally some further reading.

You should use satisfies operator for this and additionally adding const assertion would be great:

const fixtures = {
   eth_mainnet: 'some value',
   eth_testnet: 'some value 2',
} as const satisfies Record<supportedChains, string>

The benefit of this approach is that it prevents the compiler from widening the type of the fixtures. With regular type definition the type of fixtures will be: Record<supportedChains, string, however with the satisfies and const assertion it will take the exact values from the object:

{
   eth_mainnet: 'some value',
   eth_testnet: 'some value 2',
}

Set the type explicitly and your object will be guarded against unwanted props. When using the as keyword you only tell the compiler to treat the object as something different than the type the compiler infers the object to be. This is called Type Assertion.

Now what you want to use is a Mapped Type. As the name implies this maps your union values as object property keys. Additionally you could add a ? to make the properties optional. (=> object is still valid if not all union values are there as properties)

type SupportedChains = "eth_mainnet" | "eth_testnet";

const fixtures: {[Chain in SupportedChains]?: any} = {
  eth_testnet: {},
  eth_mainnet: {},
  extra_prop: {}
//~~~~~~~~~~ -> Type '{ eth_testnet: {}; eth_mainnet: {}; extra_prop: {}; }' is not assignable to...
};