Default value for each type in typescript

You have to remember that Typescript transpiles to javascript. In Javascript, the default value of an unassigned variable is undefined, as defined here.

For example, the following typescript code:

let a: string; console.log(a);

will transpile to the following javascript and logs undefined.

var a; console.log(a);

This also applies when you pass parameters to a function or a constructor of a class:

// Typescript
function printStr(str: string) {
    console.log(str);
}

class StrPrinter {
    str: string;
    constructor(str: string) {
        this.str = str;
        console.log(this.str);
    }
}

printStr();
let strPrinter = StrPrinter();

In the code example above, typescript will complain that the function and the class constructor is missing a parameter. Nevertheless, it will still transpile to transpile to:

function printStr(str) {
    console.log(str);
}
var StrPrinter = (function () {
    function StrPrinter(str) {
        this.str = str;
        console.log(this.str);
    }
    return StrPrinter;
}());
printStr();
var strPrinter = StrPrinter();

You might also want to see how typescript transpiles to javascript here.

The default value of every type is undefined

From: MDN - 'undefined'

A variable that has not been assigned a value is of type undefined.

For example, invoking the following will alert the value 'undefined', even though greeting is of type String

let greeting: string;
alert(greeting);