Weak Type Detection

TypeScript 2.4 introduces the concept of “weak types”.Any type that contains nothing but a set of all-optional properties is considered to be weak.For example, this Options type is a weak type:

interface Options {
    data?: string;
    timeout?: number;
    maxRetries?: number;
}

In TypeScript 2.4, it’s now an error to assign anything to a weak type when there’s no overlap in properties.For example:

function sendMessage(options: Options) {
    // ...
}
const opts = {
    payload: "hello world!",
    retryOnFail: true,
}
// Error!
sendMessage(opts);
// No overlap between the type of 'opts' and 'Options' itself.
// Maybe we meant to use 'data'/'maxRetries' instead of 'payload'/'retryOnFail'.

You can think of this as TypeScript “toughening up” the weak guarantees of these types to catch what would otherwise be silent bugs.

Since this is a breaking change, you may need to know about the workarounds which are the same as those for strict object literal checks:

• Declare the properties if they really do exist.
• Add an index signature to the weak type (i.e. [propName: string]: {}).
• Use a type assertion (i.e. opts as Options).