fix: relax type checking in "fromFp" function

package.json

@@ -63,7 +63,7 @@
     "coverage": "yarn nyc --nycrc-path ./.nycrc.yaml mocha",
     "lint": "yarn lint:ts && yarn prettier:check && yarn typecheck",
     "lint:ts": "eslint --config ./.eslintrc.yaml --ignore-path ./.eslintignore --ext .ts .",
-    "_postinstall": "husky install",
+    "postinstall": "husky install",
     "postpublish": "pinst --enable",
     "prepack": "yarn build",
     "prepublishOnly": "pinst --disable",

src/fromBn.ts

@@ -4,8 +4,8 @@ import { BigNumber, FixedNumber } from "@ethersproject/bignumber";
  * Convert a big number with a custom number of decimals to a stringified fixed-point number.
  */
 export function fromBn(x: BigNumber, decimals: number = 18): string {
-  if (x === undefined || x instanceof BigNumber === false) {
-    throw new Error("Input must be an ethers BigNumber");
+  if (x === undefined) {
+    throw new Error("Input must not be undefined");
   }
 
   if (decimals < 1 || decimals > 77) {

test/fromBn.test.ts

@@ -5,15 +5,14 @@ import forEach from "mocha-each";
 import { fromBn } from "../src";
 
 describe("fromBn", function () {
-  describe("when x is not a big number", function () {
-    const testSets = [undefined, null, true, 2.71, Math.PI, "3.14", { x: 100 }, function () {}];
-
-    forEach(testSets).it("throws an error", function (x: any) {
-      expect(() => fromBn(x)).toThrow("Input must be an ethers BigNumber");
+  describe("when x is undefined", function () {
+    it("throws an error", function () {
+      const x: any = undefined;
+      expect(() => fromBn(x)).toThrow("Input must not be undefined");
     });
   });
 
-  describe("when x is a big number", function () {
+  describe("when x is not undefined", function () {
     describe("when the number of decimals is out of bounds", function () {
       const testSets = [-1729, 0, 78, 1729];