var, let and const

var

• Scope: Global or function.
• Variables declared by var can be redifined or updated.

var bar = "foo1";
var bar = "foo2";
bar = "foo3";

• Problems: As in the above example, you don’t know if bar has been declared somewhere else and it can be a total surprise!
• Hoisting: top of the code and initialized to undefined

let

• Scope: block
• Variables declared by let can be updated but not re-declared within its scoped.

let bar = "foo";
bar = "new foo";

This won’t work

let bar = "foo";
let bar = "new foo"; // error: Identifier 'bar' has already been declared

Re-declaration outside of scope will be allowed (WTF?!):

let bar = "foo";
if (someCondition) {
    let bar = "some foo";
}
console.log(bar); // "some foo"

• Hoisting: does not initialize variables to undefined.

Const

• Scope: block
• Can’t be re-declared or updated.
• Must be initialized at the time of declaration.
• Objects property can be updated, although the object itself cannot.

const contact1 = {
    emailAddress: "dummy@example.com",
    firstName: "Dummy"
}
contact1.firstName = "John"; // This is ok.

• Hoisting: same as let. Hoisted to the top but does not initialize declarations.