diff --git a/src/compiler/preprocess/index.ts b/src/compiler/preprocess/index.ts
index b51b67bb2345..264fbbdbcd42 100644
--- a/src/compiler/preprocess/index.ts
+++ b/src/compiler/preprocess/index.ts
@@ -39,6 +39,10 @@ function parse_attributes(str: string) {
 	return attrs;
 }
 
+function get_file_basename(filename: string) {
+	return filename.split(/[/\\]/).pop();
+}
+
 interface Replacement {
 	offset: number;
 	length: number;
@@ -46,7 +50,7 @@ interface Replacement {
 }
 
 async function replace_async(
-	filename: string,
+	file_basename: string,
 	source: string,
 	get_location: ReturnType<typeof getLocator>,
 	re: RegExp,
@@ -73,13 +77,13 @@ async function replace_async(
 	)) {
 		// content = unchanged source characters before the replaced segment
 		const content = StringWithSourcemap.from_source(
-			filename, source.slice(last_end, offset), get_location(last_end));
+			file_basename, source.slice(last_end, offset), get_location(last_end));
 		out.concat(content).concat(replacement);
 		last_end = offset + length;
 	}
 	// final_content = unchanged source characters after last replaced segment
 	const final_content = StringWithSourcemap.from_source(
-		filename, source.slice(last_end), get_location(last_end));
+		file_basename, source.slice(last_end), get_location(last_end));
 	return out.concat(final_content);
 }
 
@@ -160,7 +164,7 @@ function decoded_sourcemap_from_generator(generator: any) {
  * Convert a preprocessor output and its leading prefix and trailing suffix into StringWithSourceMap
  */
 function get_replacement(
-	filename: string,
+	file_basename: string,
 	offset: number,
 	get_location: ReturnType<typeof getLocator>,
 	original: string,
@@ -171,9 +175,9 @@ function get_replacement(
 
 	// Convert the unchanged prefix and suffix to StringWithSourcemap
 	const prefix_with_map = StringWithSourcemap.from_source(
-		filename, prefix, get_location(offset));
+		file_basename, prefix, get_location(offset));
 	const suffix_with_map = StringWithSourcemap.from_source(
-		filename, suffix, get_location(offset + prefix.length + original.length));
+		file_basename, suffix, get_location(offset + prefix.length + original.length));
 
 	// Convert the preprocessed code and its sourcemap to a StringWithSourcemap
 	let decoded_map: DecodedSourceMap;
@@ -186,7 +190,11 @@ function get_replacement(
 			// import decoded sourcemap from mozilla/source-map/SourceMapGenerator
 			decoded_map = decoded_sourcemap_from_generator(decoded_map);
 		}
-		sourcemap_add_offset(decoded_map, get_location(offset + prefix.length));
+		// offset only segments pointing at original component source
+		const source_index = decoded_map.sources.indexOf(file_basename);
+		if (source_index !== -1) {
+			sourcemap_add_offset(decoded_map, get_location(offset + prefix.length), source_index);
+		}
 	}
 	const processed_with_map = StringWithSourcemap.from_processed(processed.code, decoded_map);
 
@@ -203,6 +211,9 @@ export default async function preprocess(
 	const filename = (options && options.filename) || preprocessor.filename; // legacy
 	const dependencies = [];
 
+	// preprocess source must be relative to itself or equal null
+	const file_basename = filename == null ? null : get_file_basename(filename);
+
 	const preprocessors = preprocessor
 		? Array.isArray(preprocessor) ? preprocessor : [preprocessor]
 		: [];
@@ -246,13 +257,13 @@ export default async function preprocess(
 			: /<!--[^]*?-->|<script(\s[^]*?)?(?:>([^]*?)<\/script>|\/>)/gi;
 
 		const res = await replace_async(
-			filename,
+			file_basename,
 			source,
 			get_location,
 			tag_regex,
 			async (match, attributes = '', content = '', offset) => {
 				const no_change = () => StringWithSourcemap.from_source(
-					filename, match, get_location(offset));
+					file_basename, match, get_location(offset));
 				if (!attributes && !content) {
 					return no_change();
 				}
@@ -265,10 +276,13 @@ export default async function preprocess(
 					attributes: parse_attributes(attributes),
 					filename
 				});
-
-				if (!processed) return no_change();
-				if (processed.dependencies) dependencies.push(...processed.dependencies);
-				return get_replacement(filename, offset, get_location, content, processed, `<${tag_name}${attributes}>`, `</${tag_name}>`);
+				if (processed && processed.dependencies) {
+					dependencies.push(...processed.dependencies);
+				}
+				if (!processed || !processed.map && processed.code === content) {
+					return no_change();
+				}
+				return get_replacement(file_basename, offset, get_location, content, processed, `<${tag_name}${attributes}>`, `</${tag_name}>`);
 			}
 		);
 		source = res.string;
@@ -285,7 +299,7 @@ export default async function preprocess(
 
 	// Combine all the source maps for each preprocessor function into one
 	const map: RawSourceMap = combine_sourcemaps(
-		filename,
+		file_basename,
 		sourcemap_list
 	);
 
diff --git a/src/compiler/utils/string_with_sourcemap.ts b/src/compiler/utils/string_with_sourcemap.ts
index 421a0c1fbd97..1766a0db069a 100644
--- a/src/compiler/utils/string_with_sourcemap.ts
+++ b/src/compiler/utils/string_with_sourcemap.ts
@@ -13,21 +13,22 @@ function last_line_length(s: string) {
 
 // mutate map in-place
 export function sourcemap_add_offset(
-	map: DecodedSourceMap, offset: SourceLocation
+	map: DecodedSourceMap, offset: SourceLocation, source_index: number
 ) {
-	if (map.mappings.length == 0) return map;
-	// shift columns in first line
-	const segment_list = map.mappings[0];
-	for (let segment = 0; segment < segment_list.length; segment++) {
-		const seg = segment_list[segment];
-		if (seg[3]) seg[3] += offset.column;
-	}
-	// shift lines
+	if (map.mappings.length == 0) return;
 	for (let line = 0; line < map.mappings.length; line++) {
 		const segment_list = map.mappings[line];
 		for (let segment = 0; segment < segment_list.length; segment++) {
 			const seg = segment_list[segment];
-			if (seg[2]) seg[2] += offset.line;
+			// shift only segments that belong to component source file
+			if (seg[1] === source_index) { // also ensures that seg.length >= 4
+				// shift column if it points at the first line
+				if (seg[2] === 0) {
+					seg[3] += offset.column;
+				}
+				// shift line
+				seg[2] += offset.line;
+			}
 		}
 	}
 }
@@ -97,6 +98,9 @@ export class StringWithSourcemap {
 			return this;
 		}
 
+		// compute last line length before mutating
+		const column_offset = last_line_length(this.string);
+
 		this.string += other.string;
 
 		const m1 = this.map;
@@ -117,8 +121,8 @@ export class StringWithSourcemap {
 				const segment_list = m2.mappings[line];
 				for (let segment = 0; segment < segment_list.length; segment++) {
 					const seg = segment_list[segment];
-					if (seg[1]) seg[1] = new_source_idx[seg[1]];
-					if (seg[4]) seg[4] = new_name_idx[seg[4]];
+					if (seg[1] >= 0) seg[1] = new_source_idx[seg[1]];
+					if (seg[4] >= 0) seg[4] = new_name_idx[seg[4]];
 				}
 			}
 		} else if (sources_idx_changed) {
@@ -126,7 +130,7 @@ export class StringWithSourcemap {
 				const segment_list = m2.mappings[line];
 				for (let segment = 0; segment < segment_list.length; segment++) {
 					const seg = segment_list[segment];
-					if (seg[1]) seg[1] = new_source_idx[seg[1]];
+					if (seg[1] >= 0) seg[1] = new_source_idx[seg[1]];
 				}
 			}
 		} else if (names_idx_changed) {
@@ -134,7 +138,7 @@ export class StringWithSourcemap {
 				const segment_list = m2.mappings[line];
 				for (let segment = 0; segment < segment_list.length; segment++) {
 					const seg = segment_list[segment];
-					if (seg[4]) seg[4] = new_name_idx[seg[4]];
+					if (seg[4] >= 0) seg[4] = new_name_idx[seg[4]];
 				}
 			}
 		}
@@ -146,7 +150,6 @@ export class StringWithSourcemap {
 		// 2. first line of second map
 		// columns of 2 must be shifted
 
-		const column_offset = last_line_length(this.string);
 		if (m2.mappings.length > 0 && column_offset > 0) {
 			const first_line = m2.mappings[0];
 			for (let i = 0; i < first_line.length; i++) {
@@ -164,12 +167,23 @@ export class StringWithSourcemap {
 	}
 
 	static from_processed(string: string, map?: DecodedSourceMap): StringWithSourcemap {
-		if (map) return new StringWithSourcemap(string, map);
+		const line_count = string.split('\n').length;
+
+		if (map) {
+			// ensure that count of source map mappings lines 
+			// is equal to count of generated code lines
+			// (some tools may produce less)
+			const missing_lines = line_count - map.mappings.length;
+			for (let i = 0; i < missing_lines; i++) {
+				map.mappings.push([]);
+			}
+			return new StringWithSourcemap(string, map);
+		}
+
 		if (string == '') return new StringWithSourcemap();
 		map = { version: 3, names: [], sources: [], mappings: [] };
 
 		// add empty SourceMapSegment[] for every line
-		const line_count = (string.match(/\n/g) || '').length;
 		for (let i = 0; i < line_count; i++) map.mappings.push([]);
 		return new StringWithSourcemap(string, map);
 	}
diff --git a/test/sourcemaps/helpers.ts b/test/sourcemaps/helpers.ts
index d0bea310e623..f546566a95ce 100644
--- a/test/sourcemaps/helpers.ts
+++ b/test/sourcemaps/helpers.ts
@@ -1,4 +1,111 @@
-import MagicString from 'magic-string';
+import * as assert from 'assert';
+import { getLocator } from 'locate-character';
+import MagicString, { Bundle } from 'magic-string';
+
+type AssertMappedParameters = {
+	code: string;
+	filename?: string;
+	input: string | ReturnType<typeof getLocator>;
+	input_code?: string;
+	preprocessed: any;
+};
+
+export function assert_mapped(
+	{ code, filename, input, input_code, preprocessed }: AssertMappedParameters
+) {
+	const locate_input = typeof input === 'function' ? input : getLocator(input);
+	if (filename === undefined) filename = 'input.svelte';
+	if (input_code === undefined) input_code = code;	
+
+	const source_loc = locate_input(input_code);
+	assert.notEqual(
+		source_loc,
+		undefined,
+		`failed to locate "${input_code}" in "${filename}"`
+	);
+
+	const transformed_loc = preprocessed.locate_1(code);
+	assert.notEqual(
+		transformed_loc,
+		undefined,
+		`failed to locate "${code}" in transformed "${filename}"`
+	);
+
+	assert.deepEqual(
+		preprocessed.mapConsumer.originalPositionFor(transformed_loc),
+		{
+			source: filename,
+			name: null,
+			line: source_loc.line + 1,
+			column: source_loc.column
+		},
+		`incorrect mappings for "${input_code}" in "${filename}"`
+	);
+}
+
+type AssertNotMappedParameters = {
+	code: string;
+	filename?: string;
+	preprocessed: any;
+};
+
+export function assert_not_mapped(
+	{ code, filename, preprocessed }: AssertNotMappedParameters
+) {
+	if (filename === undefined) filename = 'input.svelte';
+
+	const transformed_loc = preprocessed.locate_1(code);
+	assert.notEqual(
+		transformed_loc,
+		undefined,
+		`failed to locate "${code}" in transformed "${filename}"`
+	);
+
+	assert.deepEqual(
+		preprocessed.mapConsumer.originalPositionFor(transformed_loc),
+		{
+			source: null,
+			name: null,
+			line: null,
+			column: null
+		},
+		`incorrect mappings for "${code}" in "${filename}"`
+	);
+}
+
+export function assert_not_located(
+	code: string,
+	locate: ReturnType<typeof getLocator>,
+	filename = 'input.svelte'
+) {
+	assert.equal(
+		locate(code),
+		undefined,
+		`located "${code}" that should be removed from ${filename}`
+	);
+}
+
+export function magic_string_bundle(
+	inputs: Array<{ code: string | MagicString, filename: string }>,
+	filename = 'bundle.js',
+	separator = '\n'
+) {
+	const bundle = new Bundle({ separator });
+	inputs.forEach(({ code, filename }) => {
+		bundle.addSource({
+			filename,
+			content: typeof code === 'string' ? new MagicString(code) : code
+		});
+	});
+	return {
+		code: bundle.toString(),
+		map: bundle.generateMap({
+			source: filename,
+			hires: true,
+			includeContent: false
+		})
+	};
+}
 
 export function magic_string_preprocessor_result(filename: string, src: MagicString) {
 	return {
diff --git a/test/sourcemaps/samples/external/_config.js b/test/sourcemaps/samples/external/_config.js
new file mode 100644
index 000000000000..400def20ac63
--- /dev/null
+++ b/test/sourcemaps/samples/external/_config.js
@@ -0,0 +1,24 @@
+import { magic_string_bundle } from '../../helpers';
+
+export const COMMON = ':global(html) { height: 100%; }\n';
+
+// TODO: removing '\n' breaks test
+// - _actual.svelte.map looks correct
+// - _actual.css.map adds reference to </style> on input.svelte
+// - Most probably caused by bug in current magic-string version (fixed in 0.25.7)
+export const STYLES = '.awesome { color: orange; }\n';
+
+export default {
+	css_map_sources: ['common.scss', 'styles.scss'],
+	js_map_sources: [],
+	preprocess: [
+		{
+			style: () => {
+				return magic_string_bundle([
+					{ filename: 'common.scss', code: COMMON },
+					{ filename: 'styles.scss', code: STYLES }
+				]);
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/external/input.svelte b/test/sourcemaps/samples/external/input.svelte
new file mode 100644
index 000000000000..94d895e4a724
--- /dev/null
+++ b/test/sourcemaps/samples/external/input.svelte
@@ -0,0 +1,3 @@
+<style lang="scss" src="./styles.scss"></style>
+
+<div class="awesome">Divs ftw!</div>
\ No newline at end of file
diff --git a/test/sourcemaps/samples/external/test.js b/test/sourcemaps/samples/external/test.js
new file mode 100644
index 000000000000..5193b62ba1d4
--- /dev/null
+++ b/test/sourcemaps/samples/external/test.js
@@ -0,0 +1,26 @@
+import { assert_mapped } from '../../helpers';
+import { COMMON, STYLES } from './_config';
+
+export function test({ input, preprocessed }) {
+	// Transformed script, main file
+	assert_mapped({
+		filename: 'input.svelte',
+		code: 'Divs ftw!',
+		input: input.locate,
+		preprocessed
+	});
+
+	// External files
+	assert_mapped({
+		filename: 'common.scss',
+		code: 'height: 100%;',
+		input: COMMON,
+		preprocessed
+	});
+	assert_mapped({
+		filename: 'styles.scss',
+		code: 'color: orange;',
+		input: STYLES,
+		preprocessed
+	});
+}
diff --git a/test/sourcemaps/samples/preprocessed-no-map/_config.js b/test/sourcemaps/samples/preprocessed-no-map/_config.js
new file mode 100644
index 000000000000..9888b56ddf8f
--- /dev/null
+++ b/test/sourcemaps/samples/preprocessed-no-map/_config.js
@@ -0,0 +1,15 @@
+export default {
+	css_map_sources: [],
+	preprocess: [
+		{
+			style: ({ content }) => {
+				// Modified without source map
+				return { code: content + ' ' };
+			},
+			script: ({ content }) => {
+				// Not modified
+				return { code: content };
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/preprocessed-no-map/input.svelte b/test/sourcemaps/samples/preprocessed-no-map/input.svelte
new file mode 100644
index 000000000000..febfa5191b4b
--- /dev/null
+++ b/test/sourcemaps/samples/preprocessed-no-map/input.svelte
@@ -0,0 +1,13 @@
+<script>
+	export let name;
+
+	console.log(name);
+</script>
+
+<main>
+	<div>{name}</div>
+</main>
+
+<style>
+	main { font-weight: bold; }
+</style>
diff --git a/test/sourcemaps/samples/preprocessed-no-map/test.js b/test/sourcemaps/samples/preprocessed-no-map/test.js
new file mode 100644
index 000000000000..22286ae282b1
--- /dev/null
+++ b/test/sourcemaps/samples/preprocessed-no-map/test.js
@@ -0,0 +1,37 @@
+import { assert_mapped, assert_not_mapped } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	// markup (start)
+	assert_mapped({
+		code: '<script>',
+		input: input.locate,
+		preprocessed
+	});
+
+	// script content (preprocessed without map, content not changed)
+	assert_mapped({
+		code: 'console.log(name);',
+		input: input.locate,
+		preprocessed
+	});
+	
+	// markup (middle)
+	assert_mapped({
+		code: '<div>{name}</div>',
+		input: input.locate,
+		preprocessed
+	});
+	
+	// style content (preprocessed without map, content changed)
+	assert_not_mapped({
+		code: 'font-weight: bold;',
+		preprocessed
+	});
+
+	// markup (end)
+	assert_mapped({
+		code: '</style>',
+		input: input.locate,
+		preprocessed
+	});
+}
diff --git a/test/sourcemaps/samples/sourcemap-basename/_config.js b/test/sourcemaps/samples/sourcemap-basename/_config.js
new file mode 100644
index 000000000000..d50dd882a685
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-basename/_config.js
@@ -0,0 +1,33 @@
+import { magic_string_bundle } from '../../helpers';
+
+export const component_filepath = 'src/input.svelte';
+
+export const component_file_basename = 'input.svelte';
+
+// as output by external tool for src/external_code.css (relative to src/input.svelte)
+export const external_relative_filename = 'external_code.css';
+
+const external_code = `
+span {
+	--external_code-var: 1px;
+}
+`;
+
+export default {
+	css_map_sources: [external_relative_filename],
+	js_map_sources: [],
+	preprocess: [
+		{
+			style: ({ content, filename }) => {
+				const external =`/* Filename from preprocess: ${filename} */` + external_code;
+				return magic_string_bundle([
+					{ code: external, filename: external_relative_filename },
+					{ code: content, filename }
+				]);
+			}
+		}
+	],
+	options: {
+		filename: component_filepath
+	}
+};
diff --git a/test/sourcemaps/samples/sourcemap-basename/input.svelte b/test/sourcemaps/samples/sourcemap-basename/input.svelte
new file mode 100644
index 000000000000..72e524a30834
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-basename/input.svelte
@@ -0,0 +1,3 @@
+<style src="external_code.css"></style>
+
+<span>Hello world</span>
diff --git a/test/sourcemaps/samples/sourcemap-basename/test.js b/test/sourcemaps/samples/sourcemap-basename/test.js
new file mode 100644
index 000000000000..34f650472254
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-basename/test.js
@@ -0,0 +1,15 @@
+import { component_filepath, component_file_basename, external_relative_filename } from './_config';
+
+export function test({ assert, preprocessed }) {
+	assert.notEqual(
+		preprocessed.locate(`/* Filename from preprocess: ${component_filepath} */`),
+		undefined,
+		'Preprocessor should receive same value for filename as passed to preprocess function'
+	);
+	
+	assert.deepEqual(
+		preprocessed.map.sources.slice().sort(),
+		[external_relative_filename, component_file_basename].sort(),
+		'Preprocessed map should contain sources relative to filepath'
+	);
+}
diff --git a/test/sourcemaps/samples/sourcemap-concat/_config.js b/test/sourcemaps/samples/sourcemap-concat/_config.js
new file mode 100644
index 000000000000..835b3c9e48cf
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-concat/_config.js
@@ -0,0 +1,15 @@
+import MagicString from 'magic-string';
+import { magic_string_preprocessor_result } from '../../helpers';
+
+export default {
+	js_map_sources: ['input.svelte'],
+	preprocess: [
+		{
+			script: ({ content }) => {
+				const src = new MagicString(content);
+				src.prepend('console.log("Injected first line");\n');
+				return magic_string_preprocessor_result('input.svelte', src);
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/sourcemap-concat/input.svelte b/test/sourcemaps/samples/sourcemap-concat/input.svelte
new file mode 100644
index 000000000000..18a1ac775f4d
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-concat/input.svelte
@@ -0,0 +1,3 @@
+<script>console.log('Target')</script>
+
+<h1>Hello</h1>
\ No newline at end of file
diff --git a/test/sourcemaps/samples/sourcemap-concat/test.js b/test/sourcemaps/samples/sourcemap-concat/test.js
new file mode 100644
index 000000000000..c68d7175dc23
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-concat/test.js
@@ -0,0 +1,9 @@
+import { assert_mapped } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	assert_mapped({
+		code: 'Target',
+		input: input.locate,
+		preprocessed
+	});
+}
diff --git a/test/sourcemaps/samples/sourcemap-offsets/_config.js b/test/sourcemaps/samples/sourcemap-offsets/_config.js
new file mode 100644
index 000000000000..392fb68a434f
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-offsets/_config.js
@@ -0,0 +1,18 @@
+import { magic_string_bundle } from '../../helpers';
+
+export const EXTERNAL = 'span { --external-var: 1px; }';
+
+export default {
+	js_map_sources: [],
+	css_map_sources: ['input.svelte', 'external.css'],
+	preprocess: [
+		{
+			style: ({ content, filename }) => {
+				return magic_string_bundle([
+					{ code: EXTERNAL, filename: 'external.css' },
+					{ code: content, filename }
+				]);
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/sourcemap-offsets/input.svelte b/test/sourcemaps/samples/sourcemap-offsets/input.svelte
new file mode 100644
index 000000000000..51f7762e2ab7
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-offsets/input.svelte
@@ -0,0 +1,9 @@
+<script>
+	// This block is here to offset style block
+</script>
+
+<style>
+	div { --component-var: 2px; }
+</style>
+
+<div><span>Text</span></div>
\ No newline at end of file
diff --git a/test/sourcemaps/samples/sourcemap-offsets/test.js b/test/sourcemaps/samples/sourcemap-offsets/test.js
new file mode 100644
index 000000000000..a6c328770c0e
--- /dev/null
+++ b/test/sourcemaps/samples/sourcemap-offsets/test.js
@@ -0,0 +1,19 @@
+import { assert_mapped } from '../../helpers';
+import { EXTERNAL } from './_config';
+
+export function test({ input, preprocessed }) {
+	// Part from component, should be with offset
+	assert_mapped({
+		code: '--component-var',
+		input: input.locate,
+		preprocessed
+	});
+	
+	// Part from external file, should be without offset
+	assert_mapped({
+		filename: 'external.css',
+		code: '--external-var',
+		input: EXTERNAL,
+		preprocessed
+	});
+}
diff --git a/test/sourcemaps/samples/typescript/_config.js b/test/sourcemaps/samples/typescript/_config.js
new file mode 100644
index 000000000000..c8a955dfbdf4
--- /dev/null
+++ b/test/sourcemaps/samples/typescript/_config.js
@@ -0,0 +1,26 @@
+import * as ts from 'typescript';
+
+export default {
+	js_map_sources: [
+		'input.svelte'
+	],
+	preprocess: [
+		{
+			script: ({ content, filename }) => {
+				const { outputText, sourceMapText } = ts.transpileModule(content, {
+					fileName: filename,
+					compilerOptions: {
+						target: ts.ScriptTarget.ES2015,
+						module: ts.ModuleKind.ES2015,
+						sourceMap: true
+					}
+				});
+
+				return {
+					code: outputText,
+					map: sourceMapText
+				};
+			}
+		}
+	]
+};
diff --git a/test/sourcemaps/samples/typescript/input.svelte b/test/sourcemaps/samples/typescript/input.svelte
new file mode 100644
index 000000000000..93639544b66e
--- /dev/null
+++ b/test/sourcemaps/samples/typescript/input.svelte
@@ -0,0 +1,18 @@
+<script lang="ts">
+	import { onMount } from 'svelte';
+
+	let count: number = 0;
+
+	interface ITimeoutDestroyer {
+		(): void; // send timeout to the void!
+	}
+
+	onMount(() => {
+		const id = setInterval(() => count++, 1000);
+		const clear: ITimeoutDestroyer = () => clearInterval(id);
+		return clear;
+	});
+</script>
+
+<h1>Hello world!</h1>
+<div>Counter value: {count}</div>
diff --git a/test/sourcemaps/samples/typescript/test.js b/test/sourcemaps/samples/typescript/test.js
new file mode 100644
index 000000000000..bec397e33c49
--- /dev/null
+++ b/test/sourcemaps/samples/typescript/test.js
@@ -0,0 +1,21 @@
+import { assert_mapped, assert_not_located } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	// TS => JS code
+	assert_mapped({
+		code: 'let count = 0;',
+		input_code: 'let count: number = 0;',
+		input: input.locate,
+		preprocessed
+	});
+
+	// Markup, not touched
+	assert_mapped({
+		code: '<h1>Hello world!</h1>',
+		input: input.locate,
+		preprocessed
+	});
+	
+	// TS types, removed
+	assert_not_located('ITimeoutDestroyer', preprocessed.locate_1);
+}