From 37c1cf0a6ccbc573aa6d8a4e5324ec752f289061 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Mon, 5 Sep 2022 18:58:18 +0200
Subject: [PATCH 01/10] fix

---
 .changeset/fresh-suns-listen.md               |  5 +++++
 .../mobx/src/types/legacyobservablearray.ts   | 21 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 .changeset/fresh-suns-listen.md

diff --git a/.changeset/fresh-suns-listen.md b/.changeset/fresh-suns-listen.md
new file mode 100644
index 000000000..320dc290a
--- /dev/null
+++ b/.changeset/fresh-suns-listen.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+fix regression #3514: LegacyObservableArray compat with Safari 9.\*
diff --git a/packages/mobx/src/types/legacyobservablearray.ts b/packages/mobx/src/types/legacyobservablearray.ts
index 253f0c177..dab40117d 100644
--- a/packages/mobx/src/types/legacyobservablearray.ts
+++ b/packages/mobx/src/types/legacyobservablearray.ts
@@ -14,6 +14,21 @@ import {
     defineProperty
 } from "../internal"
 
+// Bug in safari 9.* (or iOS 9 safari mobile). See #364
+const ENTRY_0 = createArrayEntryDescriptor(0)
+
+const safariPrototypeSetterInheritanceBug = (() => {
+    let v = false
+    const p = {}
+    Object.defineProperty(p, "0", {
+        set: () => {
+            v = true
+        }
+    })
+    Object.create(p)["0"] = 1
+    return v === false
+})()
+
 /**
  * This array buffer contains two lists of properties, so that all arrays
  * can recycle their property definitions, which significantly improves performance of creating
@@ -57,6 +72,12 @@ class LegacyObservableArray<T> extends StubArray {
             this.spliceWithArray(0, 0, initialValues)
             allowStateChangesEnd(prev)
         }
+
+        if (safariPrototypeSetterInheritanceBug) {
+            // Seems that Safari won't use numeric prototype setter untill any * numeric property is
+            // defined on the instance. After that it works fine, even if this property is deleted.
+            Object.defineProperty(this, "0", ENTRY_0)
+        }
     }
 
     concat(...arrays: T[][]): T[] {

From 6cca25748b1a63f1cac10f0387d4221d83e3a93f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Mon, 31 Oct 2022 23:53:39 +0100
Subject: [PATCH 02/10] fix-3537

---
 .changeset/grumpy-dots-wink.md             |  5 ++
 packages/mobx/__tests__/v5/base/array.js   | 66 ++++++++++++++++------
 packages/mobx/src/types/observablearray.ts | 37 +++++++-----
 3 files changed, 77 insertions(+), 31 deletions(-)
 create mode 100644 .changeset/grumpy-dots-wink.md

diff --git a/.changeset/grumpy-dots-wink.md b/.changeset/grumpy-dots-wink.md
new file mode 100644
index 000000000..a9ae22d5d
--- /dev/null
+++ b/.changeset/grumpy-dots-wink.md
@@ -0,0 +1,5 @@
+---
+"mobx": minor
+---
+
+Proxied observable arrays can now safely read/write out of bound indices. See https://github.com/mobxjs/mobx/discussions/3537
diff --git a/packages/mobx/__tests__/v5/base/array.js b/packages/mobx/__tests__/v5/base/array.js
index fa23d0c83..5ef2de096 100644
--- a/packages/mobx/__tests__/v5/base/array.js
+++ b/packages/mobx/__tests__/v5/base/array.js
@@ -4,9 +4,9 @@ const mobx = require("../../../src/mobx.ts")
 const { observable, when, _getAdministration, reaction, computed, makeObservable, autorun } = mobx
 const iterall = require("iterall")
 
-let consoleWarnMock
+let consoleWarnSpy
 afterEach(() => {
-    consoleWarnMock?.mockRestore()
+    consoleWarnSpy?.mockRestore()
 })
 
 test("test1", function () {
@@ -402,8 +402,8 @@ test("array exposes correct keys", () => {
     expect(keys).toEqual(["0", "1"])
 })
 
-test("accessing out of bound values throws", () => {
-    const a = mobx.observable([])
+test("legacy array: accessing out of bound values throws", () => {
+    const a = mobx.observable([], { proxy: false })
 
     let warns = 0
     const baseWarn = console.warn
@@ -666,25 +666,41 @@ test("very long arrays can be safely passed to nativeArray.concat #2379", () =>
     const nativeArray = ["a", "b"]
     const longNativeArray = [...Array(10000).keys()] // MAX_SPLICE_SIZE seems to be the threshold
     const longObservableArray = observable(longNativeArray)
+    const longLegacyArray = observable(longNativeArray, { proxy: false })
     expect(longObservableArray.length).toBe(10000)
+    expect(longLegacyArray.length).toBe(10000)
+
     expect(longObservableArray).toEqual(longNativeArray)
-    expect(longObservableArray[9000]).toBe(longNativeArray[9000])
-    expect(longObservableArray[9999]).toBe(longNativeArray[9999])
-    consoleWarnMock = jest.spyOn(console, "warn").mockImplementation(() => {})
-    expect(longObservableArray[10000]).toBe(longNativeArray[10000])
-    expect(consoleWarnMock).toMatchSnapshot()
+    expect(longLegacyArray).toEqual(longNativeArray)
 
-    const expectedArray = nativeArray.concat(longNativeArray)
-    const actualArray = nativeArray.concat(longObservableArray)
+    expect(longObservableArray[9000]).toBe(longNativeArray[9000])
+    expect(longLegacyArray[9000]).toBe(longNativeArray[9000])
 
-    expect(actualArray).toEqual(expectedArray)
+    expect(longObservableArray[9999]).toBe(longNativeArray[9999])
+    expect(longLegacyArray[9999]).toBe(longNativeArray[9999])
 
-    const anotherArray = [0, 1, 2, 3, 4, 5]
-    const observableArray = observable(anotherArray)
-    const r1 = anotherArray.splice(2, 2, ...longNativeArray)
-    const r2 = observableArray.splice(2, 2, ...longNativeArray)
+    expect(longObservableArray[10000]).toBe(longNativeArray[10000])
+    consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {}) // Out of bound warn
+    expect(longLegacyArray[10000]).toBe(longNativeArray[10000])
+    expect(consoleWarnSpy).toMatchSnapshot()
+
+    const expectedNativeArray = nativeArray.concat(longNativeArray)
+    const actualObservableArray = nativeArray.concat(longObservableArray)
+    const actualLegacyArray = nativeArray.concat(longLegacyArray.slice()) // .slice because legacy isn't concat spreadable
+
+    expect(actualObservableArray).toEqual(expectedNativeArray)
+    expect(actualLegacyArray).toEqual(expectedNativeArray)
+
+    const anotherNativeArray = [0, 1, 2, 3, 4, 5]
+    const anotherObservableArray = observable(anotherNativeArray)
+    const anotherLegacyArray = observable(anotherNativeArray)
+    const r1 = anotherNativeArray.splice(2, 2, ...longNativeArray)
+    const r2 = anotherObservableArray.splice(2, 2, ...longNativeArray)
+    const r3 = anotherLegacyArray.splice(2, 2, ...longNativeArray)
     expect(r2).toEqual(r1)
-    expect(observableArray).toEqual(anotherArray)
+    expect(r3).toEqual(r1)
+    expect(anotherObservableArray).toEqual(anotherNativeArray)
+    expect(anotherLegacyArray).toEqual(anotherNativeArray)
 })
 
 describe("dehances", () => {
@@ -867,3 +883,19 @@ test("reduce without initial value #2432", () => {
     expect(observableArraySum).toEqual(arraySum)
     expect(arrayReducerArgs).toEqual(observableArrayReducerArgs)
 })
+
+test("proxied arrays can access out-bound indices", () => {
+    consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {
+        throw new Error(`Unexpected console.warn call`)
+    })
+
+    const array = observable([])
+
+    array[1]
+    array[2]
+    array[1001] = "foo"
+    expect(array.length).toBe(1002)
+    expect(array[1001]).toBe("foo")
+
+    consoleWarnSpy.mockRestore()
+})
diff --git a/packages/mobx/src/types/observablearray.ts b/packages/mobx/src/types/observablearray.ts
index 6c720f2dd..4e2aa572a 100644
--- a/packages/mobx/src/types/observablearray.ts
+++ b/packages/mobx/src/types/observablearray.ts
@@ -345,19 +345,24 @@ export class ObservableArrayAdministration
     }
 
     get_(index: number): any | undefined {
-        if (index < this.values_.length) {
-            this.atom_.reportObserved()
-            return this.dehanceValue_(this.values_[index])
-        }
-        console.warn(
-            __DEV__
-                ? `[mobx] Out of bounds read: ${index}`
-                : `[mobx.array] Attempt to read an array index (${index}) that is out of bounds (${this.values_.length}). Please check length first. Out of bound indices will not be tracked by MobX`
-        )
+        if (this.legacyMode_ && index >= this.values_.length) {
+            console.warn(
+                __DEV__
+                    ? `[mobx] Out of bounds read: ${index}`
+                    : `[mobx.array] Attempt to read an array index (${index}) that is out of bounds (${this.values_.length}). Please check length first. Out of bound indices will not be tracked by MobX`
+            )
+            return undefined
+        }
+        this.atom_.reportObserved()
+        return this.dehanceValue_(this.values_[index])
     }
 
     set_(index: number, newValue: any) {
         const values = this.values_
+        if (this.legacyMode_ && index > values.length) {
+            // out of bounds
+            die(17, index, values.length)
+        }
         if (index < values.length) {
             // update at index in range
             checkIfStateModificationsAreAllowed(this.atom_)
@@ -380,12 +385,16 @@ export class ObservableArrayAdministration
                 values[index] = newValue
                 this.notifyArrayChildUpdate_(index, newValue, oldValue)
             }
-        } else if (index === values.length) {
-            // add a new item
-            this.spliceWithArray_(index, 0, [newValue])
         } else {
-            // out of bounds
-            die(17, index, values.length)
+            // For out of bound index, we don't create an actual sparse array,
+            // but rather fill the holes with undefined (same as setArrayLength_).
+            // This could be considered a bug.
+            const newItems = new Array(index + 1 - values.length)
+            for (let i = 0; i < newItems.length - 1; i++) {
+                newItems[i] = undefined
+            } // No Array.fill everywhere...
+            newItems[newItems.length - 1] = newValue
+            this.spliceWithArray_(values.length, 0, newItems)
         }
     }
 }

From 9802ef4e5eaa303fb9cde354470081aa83d8c97f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 00:04:22 +0100
Subject: [PATCH 03/10] dunno how it got in here

---
 .changeset/fresh-suns-listen.md | 5 -----
 1 file changed, 5 deletions(-)
 delete mode 100644 .changeset/fresh-suns-listen.md

diff --git a/.changeset/fresh-suns-listen.md b/.changeset/fresh-suns-listen.md
deleted file mode 100644
index 320dc290a..000000000
--- a/.changeset/fresh-suns-listen.md
+++ /dev/null
@@ -1,5 +0,0 @@
----
-"mobx": patch
----
-
-fix regression #3514: LegacyObservableArray compat with Safari 9.\*

From 1bee07ba87547c8bc26ed5fa508ad0905202ba20 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 00:07:43 +0100
Subject: [PATCH 04/10] suprefluous mockRestore

---
 packages/mobx/__tests__/v5/base/array.js | 2 --
 1 file changed, 2 deletions(-)

diff --git a/packages/mobx/__tests__/v5/base/array.js b/packages/mobx/__tests__/v5/base/array.js
index 5ef2de096..518425abd 100644
--- a/packages/mobx/__tests__/v5/base/array.js
+++ b/packages/mobx/__tests__/v5/base/array.js
@@ -896,6 +896,4 @@ test("proxied arrays can access out-bound indices", () => {
     array[1001] = "foo"
     expect(array.length).toBe(1002)
     expect(array[1001]).toBe("foo")
-
-    consoleWarnSpy.mockRestore()
 })

From 149580bb58d8c274cddceb4c7497410b343ad8f4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 13:01:26 +0100
Subject: [PATCH 05/10] fix swapped dev/prod warnings

---
 packages/mobx/src/types/observablearray.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/mobx/src/types/observablearray.ts b/packages/mobx/src/types/observablearray.ts
index 4e2aa572a..d5e46a03a 100644
--- a/packages/mobx/src/types/observablearray.ts
+++ b/packages/mobx/src/types/observablearray.ts
@@ -348,8 +348,8 @@ export class ObservableArrayAdministration
         if (this.legacyMode_ && index >= this.values_.length) {
             console.warn(
                 __DEV__
-                    ? `[mobx] Out of bounds read: ${index}`
-                    : `[mobx.array] Attempt to read an array index (${index}) that is out of bounds (${this.values_.length}). Please check length first. Out of bound indices will not be tracked by MobX`
+                    ? `[mobx.array] Attempt to read an array index (${index}) that is out of bounds (${this.values_.length}). Please check length first. Out of bound indices will not be tracked by MobX`
+                    : `[mobx] Out of bounds read: ${index}`
             )
             return undefined
         }

From efae44a7108b533ff1df1de58b5c52783f520a00 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 13:03:36 +0100
Subject: [PATCH 06/10] fix test description

---
 packages/mobx/__tests__/v5/base/array.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/mobx/__tests__/v5/base/array.js b/packages/mobx/__tests__/v5/base/array.js
index 518425abd..552582495 100644
--- a/packages/mobx/__tests__/v5/base/array.js
+++ b/packages/mobx/__tests__/v5/base/array.js
@@ -884,7 +884,7 @@ test("reduce without initial value #2432", () => {
     expect(arrayReducerArgs).toEqual(observableArrayReducerArgs)
 })
 
-test("proxied arrays can access out-bound indices", () => {
+test("proxied arrays can access out of bound indices", () => {
     consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {
         throw new Error(`Unexpected console.warn call`)
     })

From 1b1b9d78b754edd7a5532b2e635977431fb51a45 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 13:31:21 +0100
Subject: [PATCH 07/10] update snapshot

---
 packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap b/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap
index 8363de5cf..1a4890e74 100644
--- a/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap
+++ b/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap
@@ -4,7 +4,7 @@ exports[`very long arrays can be safely passed to nativeArray.concat #2379 1`] =
 [MockFunction] {
   "calls": Array [
     Array [
-      "[mobx] Out of bounds read: 10000",
+      "[mobx.array] Attempt to read an array index (10000) that is out of bounds (10000). Please check length first. Out of bound indices will not be tracked by MobX",
     ],
   ],
   "results": Array [

From 88c41f27827026cc6a2328168c7ff0218b4d2af6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 13:43:06 +0100
Subject: [PATCH 08/10] update v4 snapshot

---
 packages/mobx/__tests__/v4/base/__snapshots__/array.js.snap | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/mobx/__tests__/v4/base/__snapshots__/array.js.snap b/packages/mobx/__tests__/v4/base/__snapshots__/array.js.snap
index 8363de5cf..1a4890e74 100644
--- a/packages/mobx/__tests__/v4/base/__snapshots__/array.js.snap
+++ b/packages/mobx/__tests__/v4/base/__snapshots__/array.js.snap
@@ -4,7 +4,7 @@ exports[`very long arrays can be safely passed to nativeArray.concat #2379 1`] =
 [MockFunction] {
   "calls": Array [
     Array [
-      "[mobx] Out of bounds read: 10000",
+      "[mobx.array] Attempt to read an array index (10000) that is out of bounds (10000). Please check length first. Out of bound indices will not be tracked by MobX",
     ],
   ],
   "results": Array [

From 7c54b738297784a46b5d0f9d333e047ca30f3cb9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 13:51:35 +0100
Subject: [PATCH 09/10] refactor tests

---
 .../v5/base/__snapshots__/array.js.snap       | 17 -----
 packages/mobx/__tests__/v5/base/array.js      | 64 ++++---------------
 2 files changed, 13 insertions(+), 68 deletions(-)
 delete mode 100644 packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap

diff --git a/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap b/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap
deleted file mode 100644
index 1a4890e74..000000000
--- a/packages/mobx/__tests__/v5/base/__snapshots__/array.js.snap
+++ /dev/null
@@ -1,17 +0,0 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
-
-exports[`very long arrays can be safely passed to nativeArray.concat #2379 1`] = `
-[MockFunction] {
-  "calls": Array [
-    Array [
-      "[mobx.array] Attempt to read an array index (10000) that is out of bounds (10000). Please check length first. Out of bound indices will not be tracked by MobX",
-    ],
-  ],
-  "results": Array [
-    Object {
-      "type": "return",
-      "value": undefined,
-    },
-  ],
-}
-`;
diff --git a/packages/mobx/__tests__/v5/base/array.js b/packages/mobx/__tests__/v5/base/array.js
index 552582495..025c56b7e 100644
--- a/packages/mobx/__tests__/v5/base/array.js
+++ b/packages/mobx/__tests__/v5/base/array.js
@@ -402,26 +402,6 @@ test("array exposes correct keys", () => {
     expect(keys).toEqual(["0", "1"])
 })
 
-test("legacy array: accessing out of bound values throws", () => {
-    const a = mobx.observable([], { proxy: false })
-
-    let warns = 0
-    const baseWarn = console.warn
-    console.warn = () => {
-        warns++
-    }
-
-    a[0] // out of bounds
-    a[1] // out of bounds
-
-    expect(warns).toBe(2)
-
-    expect(() => (a[0] = 3)).not.toThrow()
-    expect(() => (a[2] = 4)).toThrow(/Index out of bounds, 2 is larger than 1/)
-
-    console.warn = baseWarn
-})
-
 test("replace can handle large arrays", () => {
     const a = mobx.observable([])
     const b = []
@@ -664,43 +644,25 @@ test("correct array should be passed to callbacks #2326", () => {
 
 test("very long arrays can be safely passed to nativeArray.concat #2379", () => {
     const nativeArray = ["a", "b"]
-    const longNativeArray = [...Array(10000).keys()] // MAX_SPLICE_SIZE seems to be the threshold
+    const longNativeArray = [...Array(10000).keys()]
     const longObservableArray = observable(longNativeArray)
-    const longLegacyArray = observable(longNativeArray, { proxy: false })
     expect(longObservableArray.length).toBe(10000)
-    expect(longLegacyArray.length).toBe(10000)
-
     expect(longObservableArray).toEqual(longNativeArray)
-    expect(longLegacyArray).toEqual(longNativeArray)
-
     expect(longObservableArray[9000]).toBe(longNativeArray[9000])
-    expect(longLegacyArray[9000]).toBe(longNativeArray[9000])
-
     expect(longObservableArray[9999]).toBe(longNativeArray[9999])
-    expect(longLegacyArray[9999]).toBe(longNativeArray[9999])
-
     expect(longObservableArray[10000]).toBe(longNativeArray[10000])
-    consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {}) // Out of bound warn
-    expect(longLegacyArray[10000]).toBe(longNativeArray[10000])
-    expect(consoleWarnSpy).toMatchSnapshot()
-
-    const expectedNativeArray = nativeArray.concat(longNativeArray)
-    const actualObservableArray = nativeArray.concat(longObservableArray)
-    const actualLegacyArray = nativeArray.concat(longLegacyArray.slice()) // .slice because legacy isn't concat spreadable
-
-    expect(actualObservableArray).toEqual(expectedNativeArray)
-    expect(actualLegacyArray).toEqual(expectedNativeArray)
-
-    const anotherNativeArray = [0, 1, 2, 3, 4, 5]
-    const anotherObservableArray = observable(anotherNativeArray)
-    const anotherLegacyArray = observable(anotherNativeArray)
-    const r1 = anotherNativeArray.splice(2, 2, ...longNativeArray)
-    const r2 = anotherObservableArray.splice(2, 2, ...longNativeArray)
-    const r3 = anotherLegacyArray.splice(2, 2, ...longNativeArray)
+
+    const expectedArray = nativeArray.concat(longNativeArray)
+    const actualArray = nativeArray.concat(longObservableArray)
+
+    expect(actualArray).toEqual(expectedArray)
+
+    const anotherArray = [0, 1, 2, 3, 4, 5]
+    const observableArray = observable(anotherArray)
+    const r1 = anotherArray.splice(2, 2, ...longNativeArray)
+    const r2 = observableArray.splice(2, 2, ...longNativeArray)
     expect(r2).toEqual(r1)
-    expect(r3).toEqual(r1)
-    expect(anotherObservableArray).toEqual(anotherNativeArray)
-    expect(anotherLegacyArray).toEqual(anotherNativeArray)
+    expect(observableArray).toEqual(anotherArray)
 })
 
 describe("dehances", () => {
@@ -884,7 +846,7 @@ test("reduce without initial value #2432", () => {
     expect(arrayReducerArgs).toEqual(observableArrayReducerArgs)
 })
 
-test("proxied arrays can access out of bound indices", () => {
+test("accessing out of bound values does NOT throw", () => {
     consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {
         throw new Error(`Unexpected console.warn call`)
     })

From 487f1cbaa637c1ffdc13c2aaba3ad4b2290e3bff Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Pla=C4=8Dek?=
 <11457665+urugator@users.noreply.github.com>
Date: Tue, 1 Nov 2022 14:07:15 +0100
Subject: [PATCH 10/10] finishing touches

---
 packages/mobx/__tests__/v5/base/array.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/mobx/__tests__/v5/base/array.js b/packages/mobx/__tests__/v5/base/array.js
index 025c56b7e..8b9cc576c 100644
--- a/packages/mobx/__tests__/v5/base/array.js
+++ b/packages/mobx/__tests__/v5/base/array.js
@@ -644,7 +644,7 @@ test("correct array should be passed to callbacks #2326", () => {
 
 test("very long arrays can be safely passed to nativeArray.concat #2379", () => {
     const nativeArray = ["a", "b"]
-    const longNativeArray = [...Array(10000).keys()]
+    const longNativeArray = [...Array(10000).keys()] // MAX_SPLICE_SIZE seems to be the threshold
     const longObservableArray = observable(longNativeArray)
     expect(longObservableArray.length).toBe(10000)
     expect(longObservableArray).toEqual(longNativeArray)
@@ -846,7 +846,7 @@ test("reduce without initial value #2432", () => {
     expect(arrayReducerArgs).toEqual(observableArrayReducerArgs)
 })
 
-test("accessing out of bound values does NOT throw", () => {
+test("accessing out of bound indices is supported", () => {
     consoleWarnSpy = jest.spyOn(console, "warn").mockImplementation(() => {
         throw new Error(`Unexpected console.warn call`)
     })