CODEWITHSUNDEEP
interfacetypescript
Eugene
Eugene

Reputation: 11

TypeScript interface type checking from the beginning tutorial

I've just started learning TypeScript and I'm having a hard time wrapping my head around an interface behavior from the beginning tutorial on http://www.typescriptlang.org/Playground/#tut=ex5

As I understand it, the interface should enforce the type of the parameters, however this particular line throws me off

var user = new Student("Jane", "M.", "User");

It compiles correctly and all is well, but when I modify it to 

var user = new Student(1, 2, 3);

it also compiles just fine.

Could anyone elaborate why it's working ?

I understand that this is a beginners question but I could not find any info on this searching online and I don't have any TypeScript experts around me.

Thanks in advance, Eugene

Upvotes: 1

Views: 49

Answers (1)

Ryan Cavanaugh
Ryan Cavanaugh

Reputation: 220884

The type of the Student constructor parameters is any because there is no type annotation:

class Student {
    fullname : string;
    constructor(public firstname, public middleinitial, public lastname) {
        this.fullname = firstname + " " + middleinitial + " " + lastname;
    }
}

If we change it to have some type annotations, we'll get an error:

class Student {
    fullname : string;
    constructor(public firstname: string, // <-- add ': string' here
                public middleinitial: string, // and here
                public lastname: string) { // and here
        this.fullname = firstname + " " + middleinitial + " " + lastname;
    }
}

var x = new Student(1, 2, 3); // Error

Upvotes: 1

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