New: printError()

Lifted from / inspired by a similar change in #722, this creates a new function `printError()` (and uses it as the implementation for `GraphQLError#toString()`) which prints location information in the context of an error.

This is moved from the syntax error where it used to be hard-coded, so it may now be used to format validation errors, value coercion errors, or any other error which may be associated with a location.

src/error/GraphQLError.js

@@ -7,15 +7,12 @@
  * @flow
  */
 
+import { printError } from './printError';
 import { getLocation } from '../language/location';
+import type { SourceLocation } from '../language/location';
 import type { ASTNode } from '../language/ast';
 import type { Source } from '../language/source';
 
-export type GraphQLErrorLocation = {|
-  +line: number,
-  +column: number,
-|};
-
 /**
  * A GraphQLError describes an Error found during the parse, validate, or
  * execute phases of performing a GraphQL operation. In addition to a message
@@ -52,7 +49,7 @@ declare class GraphQLError extends Error {
    *
    * Enumerable, and appears in the result of JSON.stringify().
    */
-  +locations: $ReadOnlyArray<GraphQLErrorLocation> | void;
+  +locations: $ReadOnlyArray<SourceLocation> | void;
 
   /**
    * An array describing the JSON-path into the execution response which
@@ -194,4 +191,9 @@ export function GraphQLError( // eslint-disable-line no-redeclare
 (GraphQLError: any).prototype = Object.create(Error.prototype, {
   constructor: { value: GraphQLError },
   name: { value: 'GraphQLError' },
+  toString: {
+    value: function toString() {
+      return printError(this);
+    },
+  },
 });

src/error/formatError.js

@@ -8,7 +8,8 @@
  */
 
 import invariant from '../jsutils/invariant';
-import type { GraphQLError, GraphQLErrorLocation } from './GraphQLError';
+import type { GraphQLError } from './GraphQLError';
+import type { SourceLocation } from '../language/location';
 
 /**
  * Given a GraphQLError, format it according to the rules described by the
@@ -26,7 +27,7 @@ export function formatError(error: GraphQLError): GraphQLFormattedError {
 
 export type GraphQLFormattedError = {
   +message: string,
-  +locations: $ReadOnlyArray<GraphQLErrorLocation> | void,
+  +locations: $ReadOnlyArray<SourceLocation> | void,
   +path: $ReadOnlyArray<string | number> | void,
   // Extensions
   +[key: string]: mixed,

src/error/index.js

@@ -10,7 +10,7 @@
 export { GraphQLError } from './GraphQLError';
 export { syntaxError } from './syntaxError';
 export { locatedError } from './locatedError';
+export { printError } from './printError';
 export { formatError } from './formatError';
 
-export type { GraphQLErrorLocation } from './GraphQLError';
 export type { GraphQLFormattedError } from './formatError';

src/error/printError.js

@@ -0,0 +1,76 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import type { SourceLocation } from '../language/location';
+import type { Source } from '../language/source';
+import { GraphQLError } from './GraphQLError';
+
+/**
+ * Prints a GraphQLError to a string, representing useful location information
+ * about the error's position in the source.
+ */
+export function printError(error: GraphQLError): string {
+  const source = error.source;
+  const locations = error.locations || [];
+  const printedLocations = locations.map(
+    location =>
+      source
+        ? highlightSourceAtLocation(source, location)
+        : ` (${location.line}:${location.column})`,
+  );
+  return error.message + printedLocations.join('');
+}
+
+/**
+ * Render a helpful description of the location of the error in the GraphQL
+ * Source document.
+ */
+function highlightSourceAtLocation(
+  source: Source,
+  location: SourceLocation,
+): string {
+  const line = location.line;
+  const lineOffset = source.locationOffset.line - 1;
+  const columnOffset = getColumnOffset(source, location);
+  const contextLine = line + lineOffset;
+  const contextColumn = location.column + columnOffset;
+  const prevLineNum = (contextLine - 1).toString();
+  const lineNum = contextLine.toString();
+  const nextLineNum = (contextLine + 1).toString();
+  const padLen = nextLineNum.length;
+  const lines = source.body.split(/\r\n|[\n\r]/g);
+  lines[0] = whitespace(source.locationOffset.column - 1) + lines[0];
+  return (
+    `\n\n${source.name} (${contextLine}:${contextColumn})\n` +
+    (line >= 2
+      ? lpad(padLen, prevLineNum) + ': ' + lines[line - 2] + '\n'
+      : '') +
+    lpad(padLen, lineNum) +
+    ': ' +
+    lines[line - 1] +
+    '\n' +
+    whitespace(2 + padLen + contextColumn - 1) +
+    '^\n' +
+    (line < lines.length
+      ? lpad(padLen, nextLineNum) + ': ' + lines[line] + '\n'
+      : '')
+  );
+}
+
+function getColumnOffset(source: Source, location: SourceLocation): number {
+  return location.line === 1 ? source.locationOffset.column - 1 : 0;
+}
+
+function whitespace(len: number): string {
+  return Array(len + 1).join(' ');
+}
+
+function lpad(len: number, str: string): string {
+  return whitespace(len - str.length) + str;
+}

src/error/syntaxError.js

@@ -7,12 +7,9 @@
  * @flow
  */
 
-import { getLocation } from '../language/location';
 import type { Source } from '../language/source';
 import { GraphQLError } from './GraphQLError';
 
-import type { SourceLocation } from '../language/location';
-
 /**
  * Produces a GraphQLError representing a syntax error, containing useful
  * descriptive information about the syntax error's position in the source.
@@ -22,60 +19,7 @@ export function syntaxError(
   position: number,
   description: string,
 ): GraphQLError {
-  const location = getLocation(source, position);
-  const line = location.line + source.locationOffset.line - 1;
-  const columnOffset = getColumnOffset(source, location);
-  const column = location.column + columnOffset;
-  const error = new GraphQLError(
-    `Syntax Error ${source.name} (${line}:${column}) ${description}` +
-      '\n\n' +
-      highlightSourceAtLocation(source, location),
-    undefined,
-    source,
-    [position],
-  );
-  return error;
-}
-
-/**
- * Render a helpful description of the location of the error in the GraphQL
- * Source document.
- */
-function highlightSourceAtLocation(source, location) {
-  const line = location.line;
-  const lineOffset = source.locationOffset.line - 1;
-  const columnOffset = getColumnOffset(source, location);
-  const contextLine = line + lineOffset;
-  const prevLineNum = (contextLine - 1).toString();
-  const lineNum = contextLine.toString();
-  const nextLineNum = (contextLine + 1).toString();
-  const padLen = nextLineNum.length;
-  const lines = source.body.split(/\r\n|[\n\r]/g);
-  lines[0] = whitespace(source.locationOffset.column - 1) + lines[0];
-  return (
-    (line >= 2
-      ? lpad(padLen, prevLineNum) + ': ' + lines[line - 2] + '\n'
-      : '') +
-    lpad(padLen, lineNum) +
-    ': ' +
-    lines[line - 1] +
-    '\n' +
-    whitespace(2 + padLen + location.column - 1 + columnOffset) +
-    '^\n' +
-    (line < lines.length
-      ? lpad(padLen, nextLineNum) + ': ' + lines[line] + '\n'
-      : '')
-  );
-}
-
-function getColumnOffset(source: Source, location: SourceLocation): number {
-  return location.line === 1 ? source.locationOffset.column - 1 : 0;
-}
-
-function whitespace(len) {
-  return Array(len + 1).join(' ');
-}
-
-function lpad(len, str) {
-  return whitespace(len - str.length) + str;
+  return new GraphQLError(`Syntax Error: ${description}`, undefined, source, [
+    position,
+  ]);
 }

src/execution/__tests__/sync-test.js

@@ -84,10 +84,7 @@ describe('Execute: synchronously when possible', () => {
       expect(result).to.containSubset({
         errors: [
           {
-            message:
-              'Syntax Error GraphQL request (1:29) Expected Name, found {\n\n' +
-              '1: fragment Example on Query { { { syncField }\n' +
-              '                               ^\n',
+            message: 'Syntax Error: Expected Name, found {',
             locations: [{ line: 1, column: 29 }],
           },
         ],

src/index.js

@@ -145,6 +145,7 @@ export type {
 // Parse and operate on GraphQL language source files.
 export {
   Source,
+  SourceLocation,
   getLocation,
   // Parse
   parse,
@@ -265,10 +266,10 @@ export {
   VariablesInAllowedPositionRule,
 } from './validation';
 
-// Create and format GraphQL errors.
-export { GraphQLError, formatError } from './error';
+// Create, format, and print GraphQL errors.
+export { GraphQLError, formatError, printError } from './error';
 
-export type { GraphQLFormattedError, GraphQLErrorLocation } from './error';
+export type { GraphQLFormattedError } from './error';
 
 // Utilities for operating on GraphQL type schema and parsed sources.
 export {