computed() is a cache, not a derivation

Ask a Vue developer how to derive a value and the answer arrives before the question ends: computed(). It's in every tutorial, every codebase, every muscle. It is also — structurally — the wrong default.

Vue's actual derivation primitive is the tracked read. Any function that reads reactive state inside an effect subscribes to that state without creating a separate derivation node. That's the whole engine. computed() is a cache bolted on top: ~300–400 bytes per instance, allocated at creation, paid whether the value is ever read or not, in exchange for memoization you almost never need on firstName + ' ' + lastName.

In ivue, a derivation is a plain getter:

class $Box {
  get width() {
    return ref(4);
  }
  get height() {
    return ref(3);
  }
  get area() {
    return this.width.value * this.height.value; // 0 bytes per instance
  }
}

area lives once, on the prototype. Whatever effect reads it — a render, a watcher — subscribes to width and height directly through it. It can never be stale, because it never stores anything. Watch the two behaviors side by side — the plain getter re-derives freely; the computed's body only runs when its dependency changes:

Plain getter vs computed(), side by sideLive · runs the shipped engine

celsius

21°

fahrenheit · plain getter

70°

body ran 0×

status · computed()

Comfortable

body ran 0×

Drag the slider: celsius is a dependency of BOTH, so both bodies run — memoization never skips real dependency changes. Now click re-render: the plain getter re-derives (that's its deal — zero bytes, re-run per render), while the computed body stays frozen. THAT skip is what its ~300 bytes buy.

The bill for the conventional default, measured: at 100,000 live instances of the same shape, the composable-with-eager-computeds pattern holds 5.3× the heap of the plain-getter class. Reserve computed() for the three cases that earn its bytes — genuinely expensive work, render suppression on equal values, a stable ref handle — and let the tracked read do what it always did.

The full argument, with the engine internals: The Engine. The heresy is measured, not rhetorical.