Using TypeScript to define types for an instance on the fly can be challenging when strictness is a requirement

My TypeScript code is compiling correctly with a focus on strict typing.

When defining the user1 instance, I am successfully forced to include all members for IUser. If any properties are missing, such as the id property, the TypeScript linter throws an error:

TSError: ⨯ Unable to compile TypeScript:
error TS2322: Type '{ username: string; password: string; email: string; age: number; }' is not assignable to type 'IUser'.
  Property 'id' is missing in type '{ username: string; password: string; email: string; age: number; }'.

However, for the user associated with "Steve Jobs," I didn't use the property id, yet no error was thrown by the linter.

I'm seeking a way to define a user dynamically like the second user (without declaring another variable) while maintaining strict typing. I have attempted using angle brackets without success.

interface IUser {
    id: number;
    username: string;
    password: string;
    email: string;
    age: number;
}

let user1: IUser = {
    id: 1,
    username: 'Bill Gates',
    password: 'windows',
    email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="c7a5aeababa0a6b3a2b487aaaea4b5a8b4a8a1b3e9a4a8aa">[email protected]</a>',
    age: 61
};

var outputObject = function(obj: any) {
    console.log(JSON.stringify(obj));
}

outputObject(user1);

outputObject(<IUser> {
    username: 'Steve Jobs',
    password: 'mac',
    email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="324146574457585d5041725342425e571c515d5f">[email protected]</a>',
    age: 62
});

Any suggestions on how to achieve this?

Thank you!

Answer №1

It seems like there is some confusion in your inquiry, but it looks like you are searching for Partial<T>. This particular code snippet utilizes Partial<T> to make all properties of type T optional. You can pass Partial<IUser> as the argument to the function outputObject, or alternatively, we can modify outputObject itself by adding a generic parameter that can be explicitly specified as Partial<IUser> for accurate checking of the input parameter:

var outputObject = function<T>(obj: T) {
    console.log(JSON.stringify(obj));
}

outputObject<Partial<IUser>>({
    username: 'Steve Jobs',
    password: 'mac',
    email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="cebdbaabb8aba4a1acbd8eafbebea2abe0ada1a3">[email protected]</a>',
    age: 62
});

outputObject<Partial<IUser>>({
    age: '62' // Error incompatible types
});

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