feat: expose error length and raw error message on ParseError (#3820)

Add length and rawError as public parameters of ParseError to allow for richer error UIs in editors. This information was already there, but only encoded into the errors message. This change is purely additional, the existing parameters have not changed.

Author: mangerlahn

src/ParseError.js

@@ -12,15 +12,21 @@ import type {AnyParseNode} from "./parseNode";
  * about where in the source string the problem occurred.
  */
 class ParseError {
+    name: "ParseError";
     position: number | void;
-        // Error position based on passed-in Token or ParseNode.
+        // Error start position based on passed-in Token or ParseNode.
+    length: number | void;
+        // Length of affected text based on passed-in Token or ParseNode.
+    rawMessage: string | void;
+        // The underlying error message without any context added.
 
     constructor(
         message: string,               // The error message
         token?: ?Token | AnyParseNode, // An object providing position information
-    ): Error {
+    ): ParseError {
         let error = "KaTeX parse error: " + message;
         let start;
+        let end;
 
         const loc = token && token.loc;
         if (loc && loc.start <= loc.end) {
@@ -31,7 +37,7 @@ class ParseError {
 
             // Prepend some information
             start = loc.start;
-            const end = loc.end;
+            end = loc.end;
             if (start === input.length) {
                 error += " at end of input: ";
             } else {
@@ -60,12 +66,16 @@ class ParseError {
 
         // Some hackery to make ParseError a prototype of Error
         // See http://stackoverflow.com/a/8460753
-        const self = new Error(error);
+        // $FlowFixMe
+        const self: ParseError = new Error(error);
         self.name = "ParseError";
         // $FlowFixMe
         self.__proto__ = ParseError.prototype;
-        // $FlowFixMe
         self.position = start;
+        if (start != null && end != null) {
+            self.length = end - start;
+        }
+        self.rawMessage = message;
         return self;
     }
 }

test/katex-spec.js

@@ -5,6 +5,7 @@ import buildTree from "../src/buildTree";
 import katex from "../katex";
 import parseTree from "../src/parseTree";
 import Options from "../src/Options";
+import ParseError from "../src/ParseError";
 import Settings from "../src/Settings";
 import Style from "../src/Style";
 import {
@@ -3060,6 +3061,53 @@ describe("A parser that does not throw on unsupported commands", function() {
     });
 });
 
+describe("ParseError properties", function() {
+    it("should contain affected position and length information", function() {
+        try {
+            katex.renderToString("1 + \\fraq{}{}");
+
+            // Render is expected to throw, so this should not be called.
+            expect(true).toBe(false);
+        } catch (error) {
+            expect(error).toBeInstanceOf(ParseError);
+            expect(error.message).toBe("KaTeX parse error: Undefined control sequence: \\fraq at position 5: 1 + \\̲f̲r̲a̲q̲{}{}");
+            expect(error.rawMessage).toBe("Undefined control sequence: \\fraq");
+            expect(error.position).toBe(4);
+            expect(error.length).toBe(5);
+        }
+    });
+
+    it("should contain position and length information at end of input", function() {
+        try {
+            katex.renderToString("\\frac{}");
+
+            // Render is expected to throw, so this should not be called.
+            expect(true).toBe(false);
+        } catch (error) {
+            expect(error).toBeInstanceOf(ParseError);
+            expect(error.message).toBe("KaTeX parse error: Unexpected end of input in a macro argument, expected '}' at end of input: \\frac{}");
+            expect(error.rawMessage).toBe("Unexpected end of input in a macro argument, expected '}'");
+            expect(error.position).toBe(7);
+            expect(error.length).toBe(0);
+        }
+    });
+
+    it("should contain no position and length information if unavailable", function() {
+        try {
+            katex.renderToString("\\verb|hello\nworld|");
+
+            // Render is expected to throw, so this should not be called.
+            expect(true).toBe(false);
+        } catch (error) {
+            expect(error).toBeInstanceOf(ParseError);
+            expect(error.message).toBe("KaTeX parse error: \\verb ended by end of line instead of matching delimiter");
+            expect(error.rawMessage).toBe("\\verb ended by end of line instead of matching delimiter");
+            expect(error.position).toBeUndefined();
+            expect(error.length).toBeUndefined();
+        }
+    });
+});
+
 describe("The symbol table integrity", function() {
     it("should treat certain symbols as synonyms", function() {
         expect`<`.toBuildLike`\lt`;