From e0ac63cb9d8cc361171bf9f7cb176f5f3d88a980 Mon Sep 17 00:00:00 2001
From: Alexander Kireev <ak.chatman.media@gmail.com>
Date: Fri, 3 Jul 2026 08:53:05 +0700
Subject: [PATCH] fix(helpers): clamp negative forced exponent to 0

calculateExponent only clamps e > 8, but any exponent below -1 (the
documented auto sentinel) skips that too and indexes DECIMAL_POWERS /
BINARY_POWERS out of bounds, so e.g. filesize(1024, {exponent: -3})
returns "NaN undefined" instead of a sane string, and object output
silently drops symbol/unit. Clamp e < 0 to 0 the same way e > 8 is
already clamped to 8.
---
 src/helpers.js              |  5 +++++
 tests/unit/filesize.test.js | 17 +++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/src/helpers.js b/src/helpers.js
index 5102a83..56ad7ea 100644
--- a/src/helpers.js
+++ b/src/helpers.js
@@ -289,6 +289,11 @@ export function calculateExponent(num, e, exponent, isDecimal, precision) {
 		if (e < 0) {
 			e = 0;
 		}
+	} else if (e < 0) {
+		// A forced exponent below the auto sentinel (-1) has no meaning and
+		// would otherwise index the power-of-ten/two lookup tables out of
+		// bounds (producing NaN). Clamp to 0, mirroring the e > 8 clamp below.
+		e = 0;
 	}
 
 	if (e > 8) {
diff --git a/tests/unit/filesize.test.js b/tests/unit/filesize.test.js
index 035cb46..3e8ac6c 100644
--- a/tests/unit/filesize.test.js
+++ b/tests/unit/filesize.test.js
@@ -615,6 +615,23 @@ describe("filesize", () => {
 			const result = filesize(1024, { exponent: NaN, standard: "iec" });
 			assert.strictEqual(result, "1 KiB");
 		});
+
+		it("should clamp an out-of-range negative exponent to 0 instead of returning NaN", () => {
+			// Only -1 is the documented "auto" sentinel. Any other negative exponent
+			// indexed the power lookup tables out of bounds and produced "NaN undefined".
+			assert.strictEqual(filesize(1024, { exponent: -2 }), "1024 B");
+			assert.strictEqual(filesize(1024, { exponent: -100 }), "1024 B");
+		});
+
+		it("should clamp a negative exponent to 0 for object output", () => {
+			const result = filesize(1024, { exponent: -3, output: "object" });
+			assert.deepStrictEqual(result, { value: 1024, symbol: "B", exponent: 0, unit: "B" });
+		});
+
+		it("should clamp a negative exponent to 0 for array output", () => {
+			const result = filesize(1024, { exponent: -3, output: "array" });
+			assert.deepStrictEqual(result, [1024, "B"]);
+		});
 	});
 
 	describe("Error handling", () => {