diff --git a/lib/lbt/bundle/BundleWriter.js b/lib/lbt/bundle/BundleWriter.js
index 2862fb35a..8b63ba0ad 100644
--- a/lib/lbt/bundle/BundleWriter.js
+++ b/lib/lbt/bundle/BundleWriter.js
@@ -2,7 +2,8 @@
 
 
 const NL = "\n";
-const ENDS_WITH_NEW_LINE = /(^|\r\n|\r|\n)[ \t]*$/;
+const ENDS_WITH_NEW_LINE = /(\r\n|\r|\n)[ \t]*$/;
+const SPACES_OR_TABS_ONLY = /^[ \t]+$/;
 
 /**
  * A filtering writer that can count written chars and provides some convenience
@@ -20,11 +21,19 @@ class BundleWriter {
 		this.segments = [];
 		this.currentSegment = null;
 		this.currentSourceIndex = 0;
+		this.endsWithNewLine = true; // Initially we don't need a new line
 	}
 
 	write(...str) {
+		let writeBuf = "";
 		for ( let i = 0; i < str.length; i++ ) {
-			this.buf += str[i];
+			writeBuf += str[i];
+		}
+		if ( writeBuf.length >= 1 ) {
+			this.buf += writeBuf;
+			this.endsWithNewLine =
+				ENDS_WITH_NEW_LINE.test(writeBuf) ||
+				(this.endsWithNewLine && SPACES_OR_TABS_ONLY.test(writeBuf));
 		}
 	}
 
@@ -33,12 +42,13 @@ class BundleWriter {
 			this.buf += str[i];
 		}
 		this.buf += NL;
+		this.endsWithNewLine = true;
 	}
 
 	ensureNewLine() {
-		// TODO this regexp might be quite expensive (use of $ anchor on long strings)
-		if ( !ENDS_WITH_NEW_LINE.test(this.buf) ) {
+		if ( !this.endsWithNewLine ) {
 			this.buf += NL;
+			this.endsWithNewLine = true;
 		}
 	}
 
@@ -75,4 +85,3 @@ class BundleWriter {
 }
 
 module.exports = BundleWriter;
-
diff --git a/test/lib/lbt/bundle/BundleWriter.js b/test/lib/lbt/bundle/BundleWriter.js
new file mode 100644
index 000000000..202453959
--- /dev/null
+++ b/test/lib/lbt/bundle/BundleWriter.js
@@ -0,0 +1,219 @@
+const test = require("ava");
+
+const BundleWriter = require("../../../../lib/lbt/bundle/BundleWriter");
+
+test("Constructor", (t) => {
+	const w = new BundleWriter();
+	t.is(w.buf, "", "Buffer should be an empty string");
+	t.deepEqual(w.segments, [], "Segments should be empty");
+	t.is(w.currentSegment, null, "No initial current segment");
+	t.is(w.currentSourceIndex, 0, "Source index is initially at 0");
+	t.is(w.endsWithNewLine, true, "Initially endsWithNewLine is true as buffer is empty");
+});
+
+test("write", (t) => {
+	const w = new BundleWriter();
+	t.is(w.toString(), "", "Output should be initially empty");
+	w.write("");
+	t.is(w.toString(), "", "Output should still be empty when writing an empty string");
+	w.write("foo");
+	t.is(w.toString(), "foo");
+	w.write("   ");
+	t.is(w.toString(), "foo   ");
+	w.write("bar");
+	t.is(w.toString(), "foo   bar");
+	w.write("");
+	t.is(w.toString(), "foo   bar");
+});
+
+test("write (endsWithNewLine)", (t) => {
+	const w = new BundleWriter();
+	t.is(w.endsWithNewLine, true, "Initially endsWithNewLine is true as buffer is empty");
+
+	w.write("");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after empty string");
+	w.write("   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces only");
+	w.write("\t\t\t");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing tabs only");
+	w.write(" \t \t\t   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces and tabs only");
+
+	w.write("foo");
+	t.is(w.endsWithNewLine, false, "endsWithNewLine should be false after writing 'foo'");
+	w.write("   ");
+	t.is(w.endsWithNewLine, false, "endsWithNewLine should still be false after writing spaces only");
+	w.write("\t\t\t");
+	t.is(w.endsWithNewLine, false, "endsWithNewLine should still be false after writing tabs only");
+	w.write(" \t \t\t   ");
+	t.is(w.endsWithNewLine, false, "endsWithNewLine should still be false after writing spaces and tabs only");
+
+	w.write("foo\n");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should be true after write with new-line");
+	w.write("   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces only");
+	w.write("\t\t\t");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing tabs only");
+	w.write(" \t \t\t   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces and tabs only");
+
+	w.write("foo\nbar");
+	t.is(w.endsWithNewLine, false,
+		"endsWithNewLine should be false after write that includes but not ends with new-line");
+
+	w.write("foo\n \t    \t ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should be true after write with new-line and tabs/spaces");
+	w.write("   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces only");
+	w.write("\t\t\t");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing tabs only");
+	w.write(" \t \t\t   ");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should still be true after writing spaces and tabs only");
+});
+
+test("writeln", (t) => {
+	const w = new BundleWriter();
+	t.is(w.toString(), "", "Output should be initially empty");
+	w.writeln("");
+	t.is(w.toString(), "\n", "Output should only contain a new-line");
+	w.writeln("foo");
+	t.is(w.toString(), "\nfoo\n");
+	w.writeln("   ");
+	t.is(w.toString(), "\nfoo\n   \n");
+	w.writeln("bar");
+	t.is(w.toString(), "\nfoo\n   \nbar\n");
+	w.writeln("");
+	t.is(w.toString(), "\nfoo\n   \nbar\n\n");
+});
+
+test("writeln (endsWithNewLine)", (t) => {
+	const w = new BundleWriter();
+
+	w.endsWithNewLine = false;
+
+	w.writeln("");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should be true after writeln with empty string");
+
+	w.endsWithNewLine = false;
+
+	w.writeln("c");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should be true again after writeln with 'c'");
+});
+
+test("ensureNewLine", (t) => {
+	const w = new BundleWriter();
+	t.is(w.toString(), "", "Output should be initially empty");
+	t.is(w.endsWithNewLine, true, "Initially endsWithNewLine is true as buffer is empty");
+
+	w.ensureNewLine();
+	t.is(w.toString(), "", "Output should still be empty as no new-line is needed");
+
+	w.endsWithNewLine = false;
+
+	w.ensureNewLine();
+	t.is(w.toString(), "\n", "Output should contain a new-line as 'endsWithNewLine' was false");
+	t.is(w.endsWithNewLine, true, "endsWithNewLine should be set to true");
+});
+
+test("toString", (t) => {
+	const w = new BundleWriter();
+	w.buf = "some string";
+	t.is(w.toString(), "some string", "toString returns internal 'buf' property");
+});
+
+test("length", (t) => {
+	const w = new BundleWriter();
+	w.buf = "some string";
+	t.is(w.length, "some string".length, "length returns internal 'buf' length");
+});
+
+test("startSegment / endSegment", (t) => {
+	const w = new BundleWriter();
+
+	const module1 = {test: 1};
+
+	w.startSegment(module1);
+
+	t.deepEqual(w.currentSegment, {module: {test: 1}, startIndex: 0});
+	t.is(w.currentSegment.module, module1);
+	t.is(w.currentSourceIndex, 0);
+	t.deepEqual(w.segments, []);
+
+	w.write("foo");
+
+	t.deepEqual(w.currentSegment, {module: {test: 1}, startIndex: 0});
+	t.is(w.currentSegment.module, module1);
+	t.is(w.currentSourceIndex, 0);
+	t.deepEqual(w.segments, []);
+
+	const targetSize1 = w.endSegment();
+
+	t.is(targetSize1, 3);
+	t.is(w.currentSegment, null);
+	t.is(w.currentSourceIndex, -1);
+	t.deepEqual(w.segments, [{
+		module: {test: 1},
+		startIndex: 0,
+		endIndex: 3
+	}]);
+
+	const module2 = {test: 2};
+
+	w.startSegment(module2);
+
+	t.deepEqual(w.currentSegment, {module: {test: 2}, startIndex: 3});
+	t.is(w.currentSegment.module, module2);
+	t.is(w.currentSourceIndex, 1);
+	t.deepEqual(w.segments, [{
+		module: {test: 1},
+		startIndex: 0,
+		endIndex: 3
+	}]);
+
+	w.write("bar!");
+
+	t.deepEqual(w.currentSegment, {module: {test: 2}, startIndex: 3});
+	t.is(w.currentSegment.module, module2);
+	t.is(w.currentSourceIndex, 1);
+	t.deepEqual(w.segments, [{
+		module: {test: 1},
+		startIndex: 0,
+		endIndex: 3
+	}]);
+
+	const targetSize2 = w.endSegment();
+
+	t.is(targetSize2, 4);
+	t.is(w.currentSegment, null);
+	t.is(w.currentSourceIndex, -1);
+	t.deepEqual(w.segments, [{
+		module: {test: 1},
+		startIndex: 0,
+		endIndex: 3
+	}, {
+		module: {test: 2},
+		startIndex: 3,
+		endIndex: 7
+	}]);
+});
+
+test("startSegment (Error handling)", (t) => {
+	const w = new BundleWriter();
+	w.startSegment({});
+
+	t.throws(() => {
+		w.startSegment({});
+	}, {
+		message: "trying to start a segment while another segment is still open"
+	});
+});
+
+test("endSegment (Error handling)", (t) => {
+	const w = new BundleWriter();
+
+	t.throws(() => {
+		w.endSegment({});
+	}, {
+		message: "trying to end a segment while no segment is open"
+	});
+});