All fields in JavaScript (and in TypeScript) can have the value null or undefined.

You can make the field optional which is different from nullable.

interface Employee1 {
    name: string;
    salary: number;
}

var a: Employee1 = { name: 'Bob', salary: 40000 }; // OK
var b: Employee1 = { name: 'Bob' }; // Not OK, you must have 'salary'
var c: Employee1 = { name: 'Bob', salary: undefined }; // OK
var d: Employee1 = { name: null, salary: undefined }; // OK

// OK
class SomeEmployeeA implements Employee1 {
    public name = 'Bob';
    public salary = 40000;
}

// Not OK: Must have 'salary'
class SomeEmployeeB implements Employee1 {
    public name: string;
}

Compare with:

interface Employee2 {
    name: string;
    salary?: number;
}

var a: Employee2 = { name: 'Bob', salary: 40000 }; // OK
var b: Employee2 = { name: 'Bob' }; // OK
var c: Employee2 = { name: 'Bob', salary: undefined }; // OK
var d: Employee2 = { name: null, salary: 'bob' }; // Not OK, salary must be a number

// OK, but doesn't make too much sense
class SomeEmployeeA implements Employee2 {
    public name = 'Bob';
}

Typescript resolved that the newPerson.name has type of "null" since you put null there. The best way to get arround it is to be as specific as possible. If you want to allow null value and string, create an interface that says so:

interace Person {
    name: string | null;
    birthdate: Date | null;
}

let newPerson: Person = { ... }

This is most explicit. Probably better way is to use undefined instead of null. Then you can use just optional parameter:

interace Person {
    name?: string;
    birthdate?: Date;
}

It's worth noting that if you enable all of the strict checks in your tsconfig, you cannot pass null to a property defined as string. And that is a good thing, you don't want to see any "Cannot read property 'length' of null" erro message.