fix(helpers): keep fullform singular for negative one-magnitude values

decorateResult() captured numericValue (used to pick the singular vs
plural fullform suffix) after the negative sign was already prefixed
onto result[0], so filesize(-1, {fullform: true}) compared -1 === 1
and wrongly pluralized to "-1 bytes" instead of "-1 byte". Capture the
magnitude before the sign is applied instead.

src/helpers.js

@@ -402,6 +402,18 @@ export function decorateResult(
 	bits,
 	roundingFunc,
 ) {
+	// Capture the numeric value before the sign is prefixed and before
+	// formatting; a comma decimal separator (via separator or a locale such
+	// as de-DE) would otherwise make parseFloat read "1,5" as 1, and a
+	// negative sign would otherwise make -1 fail the "=== 1" singular check
+	// below, both wrongly selecting the plural unit name.
+	let numericValue;
+	if (typeof result[0] === "string") {
+		numericValue = parseFloat(result[0]);
+	} else {
+		numericValue = result[0];
+	}
+
 	if (neg) {
 		// `precision` leaves the value as a string from toPrecision (e.g. "1.50").
 		// Negating that arithmetically coerces it back to a number and drops the
@@ -413,16 +425,6 @@ export function decorateResult(
 		result[1] = symbols[result[1]];
 	}
 
-	// Capture the numeric value before formatting; a comma decimal separator
-	// (via separator or a locale such as de-DE) would otherwise make parseFloat
-	// read "1,5" as 1 and select the singular unit name.
-	let numericValue;
-	if (typeof result[0] === "string") {
-		numericValue = parseFloat(result[0]);
-	} else {
-		numericValue = result[0];
-	}
-
 	result[0] = applyNumberFormatting(
 		result[0],
 		locale,

tests/unit/filesize.test.js

@@ -268,6 +268,16 @@ describe("filesize", () => {
 			);
 		});
 
+		it("should keep the singular fullform name for negative one-magnitude values", () => {
+			assert.strictEqual(filesize(-1, { fullform: true }), "-1 byte");
+			assert.strictEqual(filesize(-0.125, { bits: true, fullform: true }), "-1 bit");
+		});
+
+		it("should keep the plural fullform name for negative values", () => {
+			assert.strictEqual(filesize(-2, { fullform: true }), "-2 bytes");
+			assert.strictEqual(filesize(-0.25, { bits: true, fullform: true }), "-2 bits");
+		});
+
 		it("should handle bits fullform singular", () => {
 			assert.strictEqual(filesize(0.125, { bits: true, fullform: true }), "1 bit");
 		});