Names, values and shared objects

First, build a mental model

A variable is a named binding to a value. An object reference is itself a value that can be copied: two bindings can then reach the same mutable object. Rebinding a name and mutating an object are different operations.

1. Values represent information: numbers, text, decisions, collections and structured records.
2. An assignment changes the value held by one binding. It does not search for every other binding that previously held an equal value.
3. Copying a reference can share an object; changing one of that object's fields is observable through other references.
4. Types describe the operations a value supports. A static checker and a runtime value check act at different times.
5. The JavaScript example makes the idea concrete. Use the comparison lab to notice where C# structs or another language's rules differ.

let first = 3;
let second = first;
second = 5;

const basket = { count: 3 };
const shared = basket;
shared.count = 5;

console.log(first);
console.log(basket.count);

1. Copy a number

   second receives 3. Reassigning second to 5 leaves first at 3.

2. Copy a reference

   shared and basket reach the same object.

3. Mutate the object

   That object's count becomes 5, so basket.count is also 5.