Account for extension length when truncating filename

filenamify.d.ts

@@ -11,6 +11,8 @@ export interface Options {
 	/**
 	Truncate the filename to the given length.
 
+	Only the base of the filename is truncated, preserving the extension. If the extension itself is longer than `maxLength`, you will get a string that is longer than `maxLength`, so you need to check for that if you allow arbitrary extensions.
+
 	Systems generally allow up to 255 characters, but we default to 100 for usability reasons.
 
 	@default 100

filenamify.js

@@ -35,9 +35,13 @@ export default function filenamify(string, options = {}) {
 	const allowedLength = typeof options.maxLength === 'number' ? options.maxLength : MAX_FILENAME_LENGTH;
 	if (string.length > allowedLength) {
 		const extensionIndex = string.lastIndexOf('.');
-		const filename = extensionIndex === -1 ? string : string.slice(0, extensionIndex);
-		const extension = extensionIndex === -1 ? '' : string.slice(extensionIndex);
-		string = filename.slice(0, Math.min(allowedLength, filename.length)) + extension;
+		if (extensionIndex === -1) {
+			string = string.slice(0, allowedLength);
+		} else {
+			const filename = string.slice(0, extensionIndex);
+			const extension = string.slice(extensionIndex);
+			string = filename.slice(0, Math.max(1, allowedLength - extension.length)) + extension;
+		}
 	}
 
 	return string;

readme.md

@@ -59,6 +59,8 @@ Default: `100`
 
 Truncate the filename to the given length.
 
+Only the base of the filename is truncated, preserving the extension. If the extension itself is longer than `maxLength`, you will get a string that is longer than `maxLength`, so you need to check for that if you allow arbitrary extensions.
+
 Systems generally allow up to 255 characters, but we default to 100 for usability reasons.
 
 ## Browser-only import

test.js

@@ -30,15 +30,18 @@ test('filnamify()', t => {
 test('filenamifyPath()', t => {
 	t.is(path.basename(filenamifyPath(path.join(directoryName, 'foo:bar'))), 'foo!bar');
 	t.is(path.basename(filenamifyPath(path.join(directoryName, 'This? This is very long filename that will lose its extension when passed into filenamify, which could cause issues.csv'))),
-		'This! This is very long filename that will lose its extension when passed into filenamify, which cou.csv');
+		'This! This is very long filename that will lose its extension when passed into filenamify, which.csv');
 });
 
 test('filenamify length', t => {
 	// Basename length: 152
 	const filename = 'this/is/a/very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_long_filename.txt';
-	t.is(filenamify(path.basename(filename)), 'very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_.txt');
+	t.is(filenamify(path.basename(filename)), 'very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_v.txt');
 	t.is(filenamify(path.basename(filename), {maxLength: 180}), 'very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_long_filename.txt');
 
+	// File extension longer than `maxLength`
+	t.is(filenamify('foo.asdfghjkl', {maxLength: 5}), 'f.asdfghjkl');
+
 	// Basename length: 148
 	const filenameNoExt = 'very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_long_filename';
 	t.is(filenamify(filenameNoExt), 'very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_');