Extract 'formatError' tests into separate file + improve coverage

Author: IvanGoncharov

src/error/__tests__/GraphQLError-test.js

@@ -10,7 +10,6 @@ import { Kind } from '../../language/kinds';
 import { parse } from '../../language/parser';
 import { Source } from '../../language/source';
 
-import { formatError } from '../formatError';
 import { GraphQLError, printError } from '../GraphQLError';
 
 const source = new Source(dedent`
@@ -132,34 +131,6 @@ describe('GraphQLError', () => {
       '{"message":"msg","path":["path",3,"to","field"]}',
     );
   });
-
-  it('default error formatter includes path', () => {
-    const e = new GraphQLError('msg', null, null, null, [
-      'path',
-      3,
-      'to',
-      'field',
-    ]);
-
-    expect(formatError(e)).to.deep.equal({
-      message: 'msg',
-      locations: undefined,
-      path: ['path', 3, 'to', 'field'],
-    });
-  });
-
-  it('default error formatter includes extension fields', () => {
-    const e = new GraphQLError('msg', null, null, null, null, null, {
-      foo: 'bar',
-    });
-
-    expect(formatError(e)).to.deep.equal({
-      message: 'msg',
-      locations: undefined,
-      path: undefined,
-      extensions: { foo: 'bar' },
-    });
-  });
 });
 
 describe('printError', () => {

src/error/__tests__/formatError-test.js

@@ -0,0 +1,60 @@
+// @flow strict
+
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import { formatError } from '../formatError';
+import { GraphQLError } from '../GraphQLError';
+
+describe('formatError: default error formatter', () => {
+  it('uses default message', () => {
+    // $DisableFlowOnNegativeTest
+    const e = new GraphQLError();
+
+    expect(formatError(e)).to.deep.equal({
+      message: 'An unknown error occurred.',
+      path: undefined,
+      locations: undefined,
+    });
+  });
+
+  it('includes path', () => {
+    const e = new GraphQLError('msg', null, null, null, [
+      'path',
+      3,
+      'to',
+      'field',
+    ]);
+
+    expect(formatError(e)).to.deep.equal({
+      message: 'msg',
+      locations: undefined,
+      path: ['path', 3, 'to', 'field'],
+    });
+  });
+
+  it('includes extension fields', () => {
+    const e = new GraphQLError('msg', null, null, null, null, null, {
+      foo: 'bar',
+    });
+
+    expect(formatError(e)).to.deep.equal({
+      message: 'msg',
+      locations: undefined,
+      path: undefined,
+      extensions: { foo: 'bar' },
+    });
+  });
+
+  it('rejects null and undefined errors', () => {
+    // $DisableFlowOnNegativeTest
+    expect(() => formatError(undefined)).to.throw(
+      'Received null or undefined error.',
+    );
+
+    // $DisableFlowOnNegativeTest
+    expect(() => formatError(null)).to.throw(
+      'Received null or undefined error.',
+    );
+  });
+});