Discussion on: JavaScript-30-Day-14

[KUMAR HARSH]

"This is where it leaks through that JavaScript doesn't have "variables" in the sense of

the name that refers to one and only one memory location which contains some value that can change
but instead implements

the name that refers to one and only one memory location which contains a reference to the value which the name is bound to at this point in time"

I would like to understand this in more detail, about how variables work, can you point me to a source where I can learn about this in greater depth.

[peerreynders]

Look, there are lots of sources that will say that I'm wrong (and perhaps I am) - for example:

When assigning a primitive value to a variable or passing it as an argument to a function, its content is copied.

While this superficially works as a mental model (especially if you are trying to avoid talking about references) it makes no sense if all primitive values are immutable - duplicating immutable values is simply wasteful.

From What Does it Mean that Primitive Values are Immutable in JavaScript?:

Variables just point to values.

The "content is copied" mindset fosters the view that "variables point to the same memory address for their entire lifetime" - for example:

let x = 123;

// Content for variable `x` will always 
// live at address `#a` so:
// #a: 123

let y = x;
// Content for variable `y` will always 
// live at address `#b` 
// and content is copied, so:
// #a: 123
// #b: 123

y += 1;
// We only change the content for `y`, so:
// #a: 123
// #b: 124
//
// Note that the value at `#b` changed. 
// This violates the assertion 
// that primitive values are immutable.

Compare that to the "variables just point to values" mindset:

let x = 123;

// Create a new immutable value at memory location `#a` 
// and have `x` point to it: 
// #a: 123
// x: #a

let y = x;
// `y` needs to point to the same value as `x` 
// so just _copy the reference_:
// #a: 123
// x: #a
// y: #a

y += 1;
// Primitive values are immutable 
// so we create the new value `124` 
// and store it at memory location `#b` 
// and have `y` refer to it: 
// #a: 123
// #b: 124
// x: #a
// y: #b

The thing is the "variables just point to values" view simplifies everything because now variables work exactly the same for primitive and non-primitive values and it also explains the behaviour of const - const makes the reference used by the variable immutable, so you're stuck with the same primitive or non-primitive value for the lifetime of that variable; however that doesn't affect the internal mutability of non-primitive values.

[KUMAR HARSH]

That was some great explanation, thanks for taking out the time to explain it so clearly.