Discussion on: JavaScript-30-Day-14

[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.