Type widening in Typescript

1. Declare a variable in a way that allows it to be mutated later(e.g., with let, var), its type is widened from its literal value to the base type that literal belongs to:

let a = 'x'; // string
a = 'y';
let b = 3; // number
b = 4;
var c = true; // boolean
c = false;
const d = { x: 3}; // { x: number }
d.x = 4;

enum E{X, Y, Z};
let e = E.Y; // E
e = E.Z;

1. Variables initialized to null or undefined are widened to any:

let a = null;
a = 3;
a = 'b';

But if this variable leaves out of the scope(e.g., return from a function) it was declared in, Typescript assigns it a definite type.