Compiler option to disallow assignment of any to a more narrow type

[tcrognon]

This currently compiles with no errors:

function deserialize(untrustedData: any): number {
    return untrustedData;
}
const myNum = deserialize('bad data');

If any can only be assigned to any, the compiler can assist in making sure untrusted data is properly type checked. So the example above would have to be changed to

function deserialize(untrustedData: any): number {
    if (typeof untrustedData !== 'number') {
        throw new Error('Not a number.');
    }
    // control flow analysis narrows type to number
    return untrustedData;
}
const myNum = deserialize('bad data');

or

function deserialize(untrustedData: any): number {
    // not recommended, but prevents this new option from being too draconion
    return <number>untrustedData;
}
const myNum = deserialize('bad data');

This is a simplified example, but reflects the frequent task of deserializing untrusted data to typed classes/interfaces where certain types are expected but should be checked.

More generally, it gives the developer confidence that a type really is a certain type in their code in a similar way that the non-nullable option does.

[yortus]

The motivation here looks very similar to #10715.

[caseyhoward]

This also works but shouldn't:

const x: any = 'not a number';
const y: number = x;

[mhegazy]

Is this just no-any ?

[caseyhoward]

No, no-any disallows any's altogether. I want to be able to use any but I don't want it to be assignable to a variable with a concrete type.

[mhegazy]

humm.. so what do you do with that any? it has to go somewhere? you never return it from a function, nor send it to another... so only console.log would be allowed? would not it be better to use {} or object in this case?

[caseyhoward]

You have to validate it. If it's any there is a good chance it's some sort of input from the outside.

function myPublicAddingFunction(x: any, y: any): number {
  if (typeof x === 'number' and typeof y === 'number') {
    // typescript now knows that x and y are numbers
    return x + y;
  } else {
    throw 'numbers only';
  }
}

[mhegazy]

u can replace that with number | string | boolean | undefined | null | symbol | object

[mhegazy]

or just {} | null | undefined

[caseyhoward]

Replace what? And how does it fix my issue?

[mhegazy]

replace any with {} | null | undefined, then use no-any tslint rule. this will guarantee that there are no unsafe assignments to any.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.