perf: fast-path primitives in deepEnhancer (mobxjs#4683)

Author: gesposito

.changeset/deep-enhancer-primitive-fast-path.md

@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+perf: fast-path primitives in `deepEnhancer`. Writing a primitive into a deep observable no longer runs the observable/array/plain-object/Map/Set/function type checks; primitives can never be made observable, so they are returned immediately. Creating an observable array of primitives is ~4x faster, and observable Set/Map writes are ~20-25% faster in the perf suite.

packages/mobx/__tests__/base/makereactive.js

@@ -715,3 +715,18 @@ test("#1650, toString is not treated correctly", () => {
     const oo = mobx.observable(o)
     expect(oo.toString).toBe("toString")
 })
+
+test("deep enhancer passes primitives through unchanged", () => {
+    const x = mobx.observable({ v: undefined })
+    const primitives = [1, "a", true, null, undefined, Symbol("test"), BigInt(42)]
+    primitives.forEach(value => {
+        x.v = value
+        expect(x.v).toBe(value)
+    })
+})
+
+test("deep enhancer still wraps functions assigned after creation into autoActions", () => {
+    const x = mobx.observable({ fn: undefined })
+    x.fn = function () {}
+    expect(mobx.isAction(x.fn)).toBe(true)
+})

packages/mobx/src/types/modifiers.ts

@@ -22,6 +22,11 @@ export interface IEnhancer<T> {
 }
 
 export function deepEnhancer(v, _, name) {
+    // primitives can never be made observable; skip the type checks below
+    if (v === null || (typeof v !== "object" && typeof v !== "function")) {
+        return v
+    }
+
     // it is an observable already, done
     if (isObservable(v)) {
         return v