diff --git a/__snapshots__/format-comment.test.js.snap b/__snapshots__/format-comment.test.js.snap
deleted file mode 100644
index 94da2c2..0000000
--- a/__snapshots__/format-comment.test.js.snap
+++ /dev/null
@@ -1,123 +0,0 @@
-// Jest Snapshot v1, https://goo.gl/fbAQLP
-
-exports[`Formats multiple code blocks 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-foo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());
-
--->
-\`\`\`js
-foo(
-  reallyLongArg(),
-  omgSoManyParameters(),
-  IShouldRefactorThis(),
-  isThereSeriouslyAnotherOne()
-);
-\`\`\`
-<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-foo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());
-
--->
-\`\`\`js
-foo(
-  reallyLongArg(),
-  omgSoManyParameters(),
-  IShouldRefactorThis(),
-  isThereSeriouslyAnotherOne()
-);
-\`\`\`"
-`;
-
-exports[`Overrides notice if the comment has been formatted previously 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub\` at https://github.com/jgierer12/prettier-github
-
-foo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());
-
--->
-<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-bar()
-
--->
-\`\`\`js
-bar();
-\`\`\`"
-`;
-
-exports[`languages Formats a CSS code block 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-.foo {color: red;}
-
--->
-\`\`\`css
-.foo {
-  color: red;
-}
-\`\`\`"
-`;
-
-exports[`languages Formats a GraphQL code block 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-{language(name: \\"GraphQL\\") {isSupported}}
-
--->
-\`\`\`graphql
-{
-  language(name: \\"GraphQL\\") {
-    isSupported
-  }
-}
-\`\`\`"
-`;
-
-exports[`languages Formats a JSON code block 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-{\\"supportJSON\\": true}
-
--->
-\`\`\`json
-{ \\"supportJSON\\": true }
-\`\`\`"
-`;
-
-exports[`languages Formats a JavaScript code block 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-foo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());
-
--->
-\`\`\`js
-foo(
-  reallyLongArg(),
-  omgSoManyParameters(),
-  IShouldRefactorThis(),
-  isThereSeriouslyAnotherOne()
-);
-\`\`\`"
-`;
-
-exports[`languages Formats a TypeScript code block 1`] = `
-"<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to \`prettier-github disable\`. A copy of your original code block is included below in case you want to restore it.
-Learn more about Prettier GitHub at https://github.com/jgierer12/prettier-github
-
-interface ITypeScript {supported: boolean}
-
--->
-\`\`\`ts
-interface ITypeScript {
-  supported: boolean;
-}
-\`\`\`"
-`;
diff --git a/__tests__/format-comment.test.js b/__tests__/format-comment.test.js
new file mode 100644
index 0000000..c35baba
--- /dev/null
+++ b/__tests__/format-comment.test.js
@@ -0,0 +1,126 @@
+const m = '../format-comment.js';
+
+beforeEach(() => {
+	jest.resetModules();
+});
+
+test('Runs Prettier', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	formatComment(comment);
+	expect(format).toHaveBeenCalled();
+});
+
+test('Passes code block to Prettier', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	formatComment(comment);
+	expect(format.mock.calls[0][0]).toBe('__CODE__');
+});
+
+test('Passes language to Prettier via `filepath`', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	formatComment(comment);
+	expect(format.mock.calls[0][1].filepath).toBe('.js');
+});
+
+test('Outputs formatted code', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('__CODE_1__');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	expect(formatComment(comment)).toMatch(/```js\n__CODE_1__[\s\S]*\n```$/);
+});
+
+test('Trims trailing newline', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('__CODE_1__\n');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	expect(formatComment(comment)).toMatch(/```js\n__CODE_1__\n```$/);
+});
+
+test('Outputs notice comment', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	expect(formatComment(comment)).toMatch(/^<!--[\s\S]*-->/);
+});
+
+test('Outputs old code in notice comment', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	expect(formatComment(comment)).toMatch(/^<!--[\s\S]*__CODE__[\s\S]*-->/);
+});
+
+test('Overrides existing notice comments', () => {
+	const comment = '```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValueOnce('__CODE_1__').mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	const processedComment = formatComment(formatComment(comment));
+	expect(processedComment).toMatch(/^<!--[\s\S]*__CODE_1__[\s\S]*-->/);
+	expect(processedComment).not.toMatch(/<!--[\s\S]*__CODE__[\s\S]*-->/);
+});
+
+test('Ignores comments without code blocks', () => {
+	const comment = 'stuff and things';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	const processedComment = formatComment(comment);
+	expect(format).not.toHaveBeenCalled();
+	expect(processedComment).toBe(comment);
+});
+
+test('Ignores incompatible code blocks', () => {
+	const comment = '```sh\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	const processedComment = formatComment(comment);
+	expect(format).not.toHaveBeenCalled();
+	expect(processedComment).toBe(comment);
+});
+
+test('Ignores comments containing `<!-- prettier-github disable -->`', () => {
+	const comment = '<!-- prettier-github disable -->\n```js\n__CODE__\n```';
+
+	const format = jest.fn().mockReturnValue('');
+	jest.doMock('prettier', () => ({format}));
+	const formatComment = require(m);
+
+	const processedComment = formatComment(comment);
+	expect(format).not.toHaveBeenCalled();
+	expect(processedComment).toBe(comment);
+});
diff --git a/format-comment.test.js b/format-comment.test.js
deleted file mode 100644
index c4eefa7..0000000
--- a/format-comment.test.js
+++ /dev/null
@@ -1,90 +0,0 @@
-const formatComment = require('./format-comment.js');
-
-describe('languages', () => {
-	test('Formats a JavaScript code block', () => {
-		const comment = '```js\nfoo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());\n```';
-
-		expect(formatComment(comment)).toMatchSnapshot();
-	});
-
-	test('Formats a CSS code block', () => {
-		const comment = '```css\n.foo {color: red;}\n```';
-
-		expect(formatComment(comment)).toMatchSnapshot();
-	});
-
-	test('Formats a TypeScript code block', () => {
-		const comment = '```ts\ninterface ITypeScript {supported: boolean}\n```';
-
-		expect(formatComment(comment)).toMatchSnapshot();
-	});
-
-	test('Formats a JSON code block', () => {
-		const comment = '```json\n{"supportJSON": true}\n```';
-
-		expect(formatComment(comment)).toMatchSnapshot();
-	});
-
-	test('Formats a GraphQL code block', () => {
-		const comment = '```graphql\n{language(name: "GraphQL") {isSupported}}\n```';
-
-		expect(formatComment(comment)).toMatchSnapshot();
-	});
-});
-
-test('Formats multiple code blocks', () => {
-	const comment = '```js\nfoo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());\n```\n```js\nfoo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());\n```';
-
-	expect(formatComment(comment)).toMatchSnapshot();
-});
-
-test('Overrides notice if the comment has been formatted previously', () => {
-	const comment = '<!-- The following code block was formatted with Prettier. If this is not desired, please change this comment to `prettier-github disable`. A copy of your original code block is included below in case you want to restore it.\nLearn more about Prettier GitHub` at https://github.com/jgierer12/prettier-github\n\nfoo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());\n\n-->\n```js\nbar()\n```';
-
-	expect(formatComment(comment)).toMatchSnapshot();
-});
-
-test('Ignores no code blocks', () => {
-	const comment = 'Nothing to format here\n';
-
-	expect(formatComment(comment)).toBe(comment);
-});
-
-test('Ignores incompatible code blocks', () => {
-	const comment = '```sh\necho "Nothing to format here"\n```';
-
-	expect(formatComment(comment)).toBe(comment);
-});
-
-test('Ignores correctly formatted code blocks', () => {
-	const comment = '```js\nconst all = "good";\n```';
-
-	expect(formatComment(comment)).toBe(comment);
-});
-
-test('Ignores comments containing `<!-- prettier-github disable -->`', () => {
-	const comment = '<!-- prettier-github disable -->\n\n```js\nfoo(reallyLongArg(), omgSoManyParameters(), IShouldRefactorThis(), isThereSeriouslyAnotherOne());\n```';
-
-	expect(formatComment(comment)).toBe(comment);
-});
-
-describe('invalid code blocks', () => {
-	beforeEach(() => {
-		global.console = {
-			warn: jest.fn()
-		};
-	});
-
-	test('Warns about invalid code blocks', () => {
-		const comment = '```js\ninvalid javascript\n```';
-		formatComment(comment);
-
-		expect(global.console.warn).toHaveBeenCalled();
-	});
-
-	test('Ignores invalid code blocks', () => {
-		const comment = '```js\ninvalid javascript\n```';
-
-		expect(formatComment(comment)).toBe(comment);
-	});
-});