What is pass by value and pass by reference?

Pass-by-value creates a new space in memory and makes a copy of a value. Primitives such as string, number, boolean etc will actually create a new copy. Hence, updating one value doesn't impact the other value. i.e, The values are independent of each other.

let a = 5;
let b = a;

b++;
console.log(a, b); //5, 6

In the above code snippet, the value of a is assigned to b and the variable b has been incremented. Since there is a new space created for variable b, any update on this variable doesn't impact the variable a.

Pass by reference doesn't create a new space in memory but the new variable adopts a memory address of an initial variable. Non-primitives such as objects, arrays and functions gets the reference of the initiable variable. i.e, updating one value will impact the other variable.

let user1 = {
name: "John",
age: 27,
};
let user2 = user1;
user2.age = 30;

console.log(user1.age, user2.age); // 30, 30

In the above code snippet, updating the age property of one object will impact the other property due to the same reference.