Typescript interface accepts wrong type

Question

I'm new to Typescript. I'm stuck at the interfaces part. Using Nodejs I send an object from the client to the server called startData. This object contains an ID and Username. I'm using an interface to define the types in this object. I define testId as a number and Username as a string.

However, when I send for example testId as a string and Username as an object, no error gets thrown. Why does the interface accept the wrong types? I would expect it to throw some kind of unexpected type error.

interface startDataInterface{
    testId: number;
    username: string;
}

socket.on('startData', function(startData: startDataInterface) {
   ...

I'm sending this on the client:

socket.emit('startData', {testId: "string", username: {}});

Steven Spungin · Accepted Answer · 2018-11-25 22:59:24Z

1

The socket method on only accepts generic functions, so typescript will not report an error on the receiving side. Extend or delegate socket and write a typesafe wrapper if you need that degree of type checking.

edited Nov 25, 2018 at 22:59

answered Nov 25, 2018 at 20:23

Steven Spungin

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Evert · Accepted Answer · 2018-11-25 20:19:56Z

0

You are probably extending EventEmitter, which, by default has any for it's emit arguments.

Anytime any is used, typechecking effectively is disabled. By default there's also no relationship between what was emitted and what was caught with on.

It's possible to create very specific on and emit method, via overloading. For instance, you can define your emit method as such:

emit: function(eventName: 'startData', startData: StartDataInterface);
emit: function(eventName: string...args: any) {
}

With a signature like this, typescript will know how to enforce these types for the startData event, but you will also have to add the same type of overloaded signature on the on method.

answered Nov 25, 2018 at 20:19

Evert

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2 Comments

P.Yntema Over a year ago

I'm sorry, I don't understand. Why are you using two emit functions here to catch one message?

Evert Over a year ago

@P.Yntema they are not 2 functions. It's one function with multiple overloaded definitions. Take a look at the official documentation (search for overloads on this page) typescriptlang.org/docs/handbook/functions.html

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