Evolving Any

When you create a new empty array, TypeScript infers it as any[].

let inventory = [];
// inventory: any[]

If you then push a type into it, TypeScript will infer the array as that type.

inventory.push(42);
// inventory: number[]

Where it gets weird is that you're actually still allowed to push other types into the array, it just keeps updating the underlying type:

inventory.push("robe");
// inventory: (number | string)[]

This is so fascinating because if we had explicitly typed the array as number[], we would have gotten an error when trying to push a string into it.

let inventory: number[] = [];
inventory.push("robe");
// Error: Argument of type 'string' is not assignable to parameter of type 'number'

The "evolving any" is a special type inference feature. It's not very useful if you're trying to restrict what can be pushed into an array within the initial scope, but it is useful outside of that scope. Let me show you what I mean. Let's say I make a function like this:

function getConfig() {
  let config = [];
  // config: any[]
  config.push("api-key");
  // config: string[]
  config.push(8080);
  // config: (string | number)[]
  return config;
}

Within getConfig, the array feels like any... I can just keep adding stuff. However, when I use getConfig:

let config = getConfig();
// config: (string | number)[]

config.push(false);
// Error: Argument of type 'boolean' is not assignable to parameter of type 'string | number'

Now I get an error! The evolving any stops evolving when it's passed around.