Running the following code will log undefined:

fn();
var foo = "hello";

function fn() {
    console.log(foo);
}

but after transforming it to let, the following will give "ReferenceError: foo is not defined":

fn();
let foo = "hello";

function fn() {
    console.log(foo);
}

The cause is that in first case the foo and fn variables get both hoisted, and when the function is called it can reference the variable.

But with let the variable foo is not hoisted, and when the function is called the variable doesn't exist yet, so we'll get ReferenceError.