The + has been overloaded to do different things

let subtractionTest = "5" - 3;
console.log("subtractionTest is ");
console.log(subtractionTest);

let additionTest = "5" + 3;
console.log("additionTest is ");
console.log(additionTest);

/*
subtractionTest is
2
additionTest is
53
*/

In javascript, a true hash is an object with function

someVar = new Map();
someVar.name = "Bob";
someVar.age = 24;

console.log(JSON.stringify(someVar));

//OUTPUT IS...
/*
{"name":"Bob","age":24}
*/

console.log(someVar.get("name"));

//OUTPUT IS...
/*
undefined
*/

someVar.set("name", "Maxwell");
someVar.set("age", 64);
console.log(JSON.stringify(someVar));

//OUTPUT IS...
/*
{"name":"Bob","age":24}
*/

for (const [key, value] of Object.entries(someVar)) {
  console.log(key, value);
}

//OUTPUT IS...
/*
name Bob
age 24
*/

for (const [key, value] of someVar) {

    console.log(key, value);
}

//OUTPUT IS..
/*
name Maxwell
age 64
*/

const nextVar = new Map();
nextVar.set("name", "Jane");
nextVar.set("age", 21);
console.log(JSON.stringify(nextVar))

//OUTPUT IS...
/*
{}
*/

console.log(JSON.stringify(Object.fromEntries(nextVar)));

//OUTPUT IS
/*
{"name":"Jane","age":21}
*/

pass-by-value for primitives, pass-by-value of the reference for objects

let a = 10;
function change(x) {
  x = 20;
}
change(a);
console.log(a); // 10 is unchanged, because x is a copy of the value

let obj = { name: "Alice" };
function modify(o) {
  o.name = "Bob";  // modifies the original object
}
modify(obj);
console.log(obj.name); // "Bob" did we pass a reference?

function reassign(o) {
  o = { name: "Charlie" }; // creates a new object locally
}
reassign(obj);
console.log(obj.name); // still "Bob" o was a copy of the reference!

This or this or?

const person = {
  name: "Alice",
  greet: function() {
    console.log(`Hello, my name is ${this.name}`);
  }
};

person.greet(); // Hello, my name is Alice" (this = person)

const greetFunction = person.greet;
greetFunction(); // Hello, my name is undefined" (this = global or undefined in strict mode)