How to check if an object conforms to the schema of an interface in typescript?

Question

I have an interface which has certain properties defined. For example:

interface Student {
name: string;
dob:string;
age:number;
city:string;
}

I read a JSON file which contains a record in this format and assign it to a variable.

let s1:Student = require('./student.json');

Now, I want to verify if s1 contains all the properties mentioned in interface Student. At runtime this is not validated. Is there any way I can do this? There is an option of type guards, but that won't serve the purpose here. I do not know which fields would come from the JSON file. I also cannot add a discriminator(No data manipulation);

Answer 1

Without explicitly writing code to do so, this isn't possible in TypeScript. Why? Because once it's gone through the compiler, your code looks like this:

let s1 = require('./student.json');

Everything relating to types gets erased once compilation completes, leaving you with just the pure JavaScript. TypeScript will never emit code to verify that the type checks will actually hold true at runtime - this is explicitly outside of the language's design goals.

So, unfortunately, if you want this functionality you're going to have to write if (s1.name), if (s1.dob), etc.

(That said, it is worth noting that there are third-party projects which aim to add runtime type checking to TypeScript, but they're still experimental and it's doubtful they'll every become part of the actual TypeScript language.)