diff --git a/packages/array-compact/index.d.ts b/packages/array-compact/index.d.ts
index 31a698987..bec718074 100644
--- a/packages/array-compact/index.d.ts
+++ b/packages/array-compact/index.d.ts
@@ -9,4 +9,4 @@ type Falsy = false | null | undefined | '' | 0 | 0n;
  * compact([1, 2, [], 4, {}]);
  * // => [1, 2, [], 4, {}]
  */
-export default function compact<T>(arr: (Falsy | T)[]): T[];
+export default function compact<T>(arr: readonly (Falsy | T)[]): T[];
diff --git a/packages/array-compact/index.tests.ts b/packages/array-compact/index.tests.ts
index 20df27749..5ae9b9c77 100644
--- a/packages/array-compact/index.tests.ts
+++ b/packages/array-compact/index.tests.ts
@@ -9,6 +9,9 @@ const test4: bigint[] = compact([-0, null, '', BigInt(0), BigInt(1)]) // [BigInt
 // Inferred as (string | number)[] because of NaN
 const test5: (string | number)[] = compact(['a', 'b', 'c', NaN]) // ['a', 'b', 'c']
 
+const readOnlyArr: readonly (number | boolean)[] = [1, true, false]
+compact(readOnlyArr)
+
 // Not OK
 // @ts-expect-error
 compact()
diff --git a/packages/array-insert/index.d.ts b/packages/array-insert/index.d.ts
index 22510cbcf..397d78237 100644
--- a/packages/array-insert/index.d.ts
+++ b/packages/array-insert/index.d.ts
@@ -11,4 +11,8 @@
  * insert([1, 2, 5, 6], ['a', 'c', 'e'], 2);
  * // => [1, 2, 'a', 'c', 'e', 5, 6]
  */
-export default function insert<T, U>(arr: T[], val: U[] | U, index?: number): (T | U)[]
+export default function insert<T, U>(
+  arr: readonly T[],
+  val: readonly U[] | U,
+  index?: number
+): (T | U)[]
diff --git a/packages/array-insert/index.tests.ts b/packages/array-insert/index.tests.ts
index 6659d4c4d..4eee59fc8 100644
--- a/packages/array-insert/index.tests.ts
+++ b/packages/array-insert/index.tests.ts
@@ -14,6 +14,9 @@ test1 = insert(arr1, arr2, undefined); // throw
 const test2: (number | null)[] = insert(arr1, null, 1);
 const test3: unknown[] = insert([], []);
 
+const readOnlyArr: readonly unknown[] = []
+insert(readOnlyArr, readOnlyArr)
+
 // Not OK
 // @ts-expect-error
 insert(undefined, arr2, 4);
diff --git a/packages/array-mean/index.d.ts b/packages/array-mean/index.d.ts
index 8c4c55402..914bf511f 100644
--- a/packages/array-mean/index.d.ts
+++ b/packages/array-mean/index.d.ts
@@ -2,5 +2,5 @@
  * Finds the mean of a set of numbers
  * @throws {Error} if array is empty
  */
-export default function mean(arr: number[]): number;
-export default function mean(...arr: number[]): number;
+export default function mean(arr: readonly number[]): number;
+export default function mean(...arr: readonly number[]): number;
diff --git a/packages/array-mean/index.tests.ts b/packages/array-mean/index.tests.ts
index 1d52bb261..1404092f0 100644
--- a/packages/array-mean/index.tests.ts
+++ b/packages/array-mean/index.tests.ts
@@ -11,6 +11,10 @@ test1 = mean(4);
 mean([])
 mean()
 
+const readOnlyArr: readonly number[] = [1, 2, 3]
+mean(readOnlyArr)
+mean(...readOnlyArr)
+
 // Not OK
 
 // @ts-expect-error
diff --git a/packages/array-median/index.d.ts b/packages/array-median/index.d.ts
index 8a7903314..af0a7deac 100644
--- a/packages/array-median/index.d.ts
+++ b/packages/array-median/index.d.ts
@@ -2,5 +2,5 @@
  * Finds the median of a set of numbers
  * @throws {Error} when array.length is 0
  */
-export default function median(arr: number[]): number;
-export default function median(...arr: number[]): number;
+export default function median(arr: readonly number[]): number;
+export default function median(...arr: readonly number[]): number;
diff --git a/packages/array-median/index.tests.ts b/packages/array-median/index.tests.ts
index d4d3bf038..baefdf77f 100644
--- a/packages/array-median/index.tests.ts
+++ b/packages/array-median/index.tests.ts
@@ -8,6 +8,9 @@ median(1)
 median(1, 2)
 median(1, 2, 3)
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+median(readonlyArr)
+
 //OK but throws
 median()
 median([])
diff --git a/packages/array-mode/index.d.ts b/packages/array-mode/index.d.ts
index 19ee28694..2d1648d11 100644
--- a/packages/array-mode/index.d.ts
+++ b/packages/array-mode/index.d.ts
@@ -8,5 +8,5 @@
  * mode(1, 2, 1, 2, 3)
  * // => [1, 2]
  */
-export default function mode(arr: number[]): number | number[];
-export default function mode(...arr: number[]): number | number[];
+export default function mode(arr: readonly number[]): number | number[];
+export default function mode(...arr: readonly number[]): number | number[];
diff --git a/packages/array-mode/index.tests.ts b/packages/array-mode/index.tests.ts
index 96aae43a0..3271d5f1b 100644
--- a/packages/array-mode/index.tests.ts
+++ b/packages/array-mode/index.tests.ts
@@ -8,6 +8,10 @@ mode(1)
 mode(1, 2)
 mode(1, 2, 3)
 
+const readonlyArr: readonly number[] = [1, 2, 2]
+mode(readonlyArr)
+mode(...readonlyArr)
+
 // Kinda OK
 //Note: these will cause runtime errors, accepting the tradeoff to keep typings readable
 mode([])
diff --git a/packages/array-percentile/index.d.ts b/packages/array-percentile/index.d.ts
index 6ed71a48f..021333e62 100644
--- a/packages/array-percentile/index.d.ts
+++ b/packages/array-percentile/index.d.ts
@@ -11,4 +11,4 @@
  * percentile([1, 2, 3], 1.0); // => 3
  * percentile([15, 20, 35, 40, 50], .40); // => 26
  */
-export default function percentile(arr: number[], percentage: number): number
+export default function percentile(arr: readonly number[], percentage: number): number
diff --git a/packages/array-percentile/index.tests.ts b/packages/array-percentile/index.tests.ts
index a0a400cf1..89e1d95ee 100644
--- a/packages/array-percentile/index.tests.ts
+++ b/packages/array-percentile/index.tests.ts
@@ -7,12 +7,13 @@ percentile([1, 2, 3, 4, 5], 0.44)
 percentile([100, 101, 92, 4, 102, 66, 66], 0.65)
 percentile([100, 101, 92, 4, 102, 32], 0.50)
 
+const readonlyArr: readonly number[] = [1, 2, 2]
+percentile(readonlyArr, 0.5)
 
 //Ok but throws errors
 percentile([], 0.50)
 percentile([], 1.5)
 
-
 // @ts-expect-error
 percentile(1, 2, 3, 4, 5, 0.50);
 
diff --git a/packages/array-permutations/index.d.ts b/packages/array-permutations/index.d.ts
index de924c122..3a05e788a 100644
--- a/packages/array-permutations/index.d.ts
+++ b/packages/array-permutations/index.d.ts
@@ -1 +1 @@
-export default function permutations<T>(arr: T[]): T[][]; // for empty arrays
+export default function permutations<T>(arr: readonly T[]): T[][]; // for empty arrays
diff --git a/packages/array-permutations/index.tests.ts b/packages/array-permutations/index.tests.ts
index 79cc22c23..6546f3c04 100644
--- a/packages/array-permutations/index.tests.ts
+++ b/packages/array-permutations/index.tests.ts
@@ -6,7 +6,7 @@ const test2: number[][] = permutations([1, 2, 3, 4, 5]);
 const test3: (string | boolean)[][] = permutations(['a', true])
 const test4: (string | boolean | number)[][] = permutations([true, 'a', 1, false])
 
-const numbers: number[] = []
+const numbers: readonly number[] = []
 let test5: number[][] = permutations(numbers)
 
 // Not OK
diff --git a/packages/array-remove/index.d.ts b/packages/array-remove/index.d.ts
index 9756f942e..dc3eefaa7 100644
--- a/packages/array-remove/index.d.ts
+++ b/packages/array-remove/index.d.ts
@@ -1 +1 @@
-export default function remove<T, U, V extends T>(arr1: T[], arr2: U[]): (V | Exclude<T, U>)[];
\ No newline at end of file
+export default function remove<T, U, V extends T>(arr1: readonly T[], arr2: readonly U[]): (V | Exclude<T, U>)[];
diff --git a/packages/array-remove/index.tests.ts b/packages/array-remove/index.tests.ts
index 50e4ee870..f1ae8e57a 100644
--- a/packages/array-remove/index.tests.ts
+++ b/packages/array-remove/index.tests.ts
@@ -9,6 +9,9 @@ const test5: (number | true)[] = remove([1, 'a', true, false], [2, 'a', false])
 remove([], [])
 remove([], [1, 2, 3])
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+remove(readonlyArr, readonlyArr)
+
 // Not OK
 // @ts-expect-error
 remove()
@@ -25,4 +28,4 @@ remove({})
 // @ts-expect-error
 remove("abc", "a")
 // @ts-expect-error
-const test6: string[] = remove([1, 2, 'a'], ['a'])
\ No newline at end of file
+const test6: string[] = remove([1, 2, 'a'], ['a'])
diff --git a/packages/array-shuffle/index.d.ts b/packages/array-shuffle/index.d.ts
index b0233a33d..ed319fa60 100644
--- a/packages/array-shuffle/index.d.ts
+++ b/packages/array-shuffle/index.d.ts
@@ -2,4 +2,4 @@ type Options = {
   shuffleAll: boolean
 }
 
-export default function shuffle<T>(arr: T[], options?: Options): T[]
\ No newline at end of file
+export default function shuffle<T>(arr: readonly T[], options?: Options): T[]
diff --git a/packages/array-shuffle/index.tests.ts b/packages/array-shuffle/index.tests.ts
index 109afbd41..0911803c1 100644
--- a/packages/array-shuffle/index.tests.ts
+++ b/packages/array-shuffle/index.tests.ts
@@ -7,6 +7,9 @@ shuffle([true, false, 1])
 shuffle([1, 2, 3], { shuffleAll: true })
 shuffle([1, 2, 3], { shuffleAll: false })
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+shuffle(readonlyArr)
+
 // Not OK
 // @ts-expect-error
 shuffle()
@@ -27,4 +30,4 @@ shuffle([], { option: true })
 // @ts-expect-error
 shuffle([1, 2, 3], { shuffleAll: undefined })
 // @ts-expect-error
-shuffle([], { shuffleAll: 1 })
\ No newline at end of file
+shuffle([], { shuffleAll: 1 })
diff --git a/packages/array-skewness/index.d.ts b/packages/array-skewness/index.d.ts
index 817bf97ec..25fc4ffb2 100644
--- a/packages/array-skewness/index.d.ts
+++ b/packages/array-skewness/index.d.ts
@@ -1,3 +1,3 @@
-declare function skewness(arr: number[]): number
-declare function skewness(...arr: number[]): number
+declare function skewness(arr: readonly number[]): number
+declare function skewness(...arr: readonly number[]): number
 export default skewness
diff --git a/packages/array-skewness/index.tests.ts b/packages/array-skewness/index.tests.ts
index 181e2863a..f9a14c00f 100644
--- a/packages/array-skewness/index.tests.ts
+++ b/packages/array-skewness/index.tests.ts
@@ -9,7 +9,11 @@ skewness([1, 2, 3, 4, 9]);
 
 skewness([4]);
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+skewness(readonlyArr)
+skewness(...readonlyArr)
 
+// NG
 // @ts-expect-error
 skewness(["3", 2]); // throws
 // @ts-expect-error
diff --git a/packages/array-split-at/index.d.ts b/packages/array-split-at/index.d.ts
index 041b04f2f..5f5a2ce0c 100644
--- a/packages/array-split-at/index.d.ts
+++ b/packages/array-split-at/index.d.ts
@@ -1 +1 @@
-export default function splitAt<T>(arr: T[], n?: number | null): [T[], T[]]
\ No newline at end of file
+export default function splitAt<T>(arr: readonly T[], n?: number | null): [T[], T[]]
diff --git a/packages/array-split-at/index.tests.ts b/packages/array-split-at/index.tests.ts
index a05d82f41..3f3f5c8cd 100644
--- a/packages/array-split-at/index.tests.ts
+++ b/packages/array-split-at/index.tests.ts
@@ -6,6 +6,9 @@ const test2: [number[], number[]] = splitAt([1, 2, 3, 4, 5], -1); // [[1, 2, 3,
 const test3: [unknown[], unknown[]] = splitAt([], 3); // [[], []]
 const test4: [boolean[], boolean[]] = splitAt([true, false]); // [[], [true, false]]
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+splitAt(readonlyArr, 1)
+
 // Not OK
 // @ts-expect-error
 splitAt()
@@ -18,4 +21,4 @@ splitAt([], true)
 // @ts-expect-error
 splitAt([], [])
 // @ts-expect-error
-splitAt([], 2, 3)
\ No newline at end of file
+splitAt([], 2, 3)
diff --git a/packages/array-standard-deviation/index.d.ts b/packages/array-standard-deviation/index.d.ts
index 7df544a24..e47b30205 100644
--- a/packages/array-standard-deviation/index.d.ts
+++ b/packages/array-standard-deviation/index.d.ts
@@ -1,3 +1,3 @@
-declare function stdev(arr: number[]): number
-declare function stdev(...arr: number[]): number
+declare function stdev(arr: readonly number[]): number
+declare function stdev(...arr: readonly number[]): number
 export default stdev
diff --git a/packages/array-standard-deviation/index.tests.ts b/packages/array-standard-deviation/index.tests.ts
index 2e2b329c6..e7fec0af5 100644
--- a/packages/array-standard-deviation/index.tests.ts
+++ b/packages/array-standard-deviation/index.tests.ts
@@ -6,6 +6,10 @@ standardDeviation([1, 2, 3, 2, 4, 1]);
 standardDeviation(1, 2, 3, 4, 5, -6);
 standardDeviation([1, 2, 3, 4, 9]);
 
+const readonlyArr: readonly number[] = [1, 2, 3]
+standardDeviation(readonlyArr)
+standardDeviation(...readonlyArr)
+
 //OK But throws
 standardDeviation([4]);
 standardDeviation([]);
diff --git a/packages/array-variance/index.d.ts b/packages/array-variance/index.d.ts
index 2be72dba3..bece0ab6d 100644
--- a/packages/array-variance/index.d.ts
+++ b/packages/array-variance/index.d.ts
@@ -1,3 +1,3 @@
-declare function variance(arr: number[]): number
-declare function variance(...arr: number[]): number
+declare function variance(arr: readonly number[]): number
+declare function variance(...arr: readonly number[]): number
 export default variance
diff --git a/packages/array-variance/index.tests.ts b/packages/array-variance/index.tests.ts
index 0d635b6ad..a9815f434 100644
--- a/packages/array-variance/index.tests.ts
+++ b/packages/array-variance/index.tests.ts
@@ -8,6 +8,11 @@ variance([1, 2, 3, 4, 9]);
 
 variance([4]);
 
+const readonlyArr: readonly number[] = [1, 2, 3];
+variance(readonlyArr);
+variance(...readonlyArr);
+
+// NG
 // @ts-expect-error
 variance(["3", 2]); // throws
 // @ts-expect-error