Only compute Position if not already in state (#9989)

* Only compute Position if not already in state

* Prioritize start locations

packages/babel-parser/src/parser/location.js

@@ -10,6 +10,17 @@ import CommentsParser from "./comments";
 // message.
 
 export default class LocationParser extends CommentsParser {
+  getLocationForPosition(pos: number): Position {
+    let loc;
+    if (pos === this.state.start) loc = this.state.startLoc;
+    else if (pos === this.state.lastTokStart) loc = this.state.lastTokStartLoc;
+    else if (pos === this.state.end) loc = this.state.endLoc;
+    else if (pos === this.state.lastTokEnd) loc = this.state.lastTokEndLoc;
+    else loc = getLineInfo(this.input, pos);
+
+    return loc;
+  }
+
   raise(
     pos: number,
     message: string,
@@ -21,7 +32,8 @@ export default class LocationParser extends CommentsParser {
       code?: string,
     } = {},
   ): empty {
-    const loc = getLineInfo(this.input, pos);
+    const loc = this.getLocationForPosition(pos);
+
     message += ` (${loc.line}:${loc.column})`;
     // $FlowIgnore
     const err: SyntaxError & { pos: number, loc: Position } = new SyntaxError(

packages/babel-parser/test/unit/util/location.js

@@ -0,0 +1,38 @@
+import { getLineInfo } from "../../../src/util/location";
+
+describe("getLineInfo", () => {
+  const input = "a\nb\nc\nd\ne\nf\ng\nh\ni";
+
+  it("reports correct position", () => {
+    expect(getLineInfo(input, 7)).toEqual({
+      column: 1,
+      line: 4,
+    });
+  });
+
+  it("reports correct position for first line", () => {
+    expect(getLineInfo(input, 0)).toEqual({
+      column: 0,
+      line: 1,
+    });
+  });
+
+  const inputArray = ["a", "b", "c", "d", "e", "f", "g", "h", "i"];
+  const singleCharLineEndings = ["\n", "\r", "\u2028", "\u2029"];
+
+  singleCharLineEndings.forEach(ending => {
+    it(`supports ${escape(ending)} line ending`, () => {
+      expect(getLineInfo(inputArray.join(ending), 7)).toEqual({
+        column: 1,
+        line: 4,
+      });
+    });
+  });
+
+  it(`supports ${escape("\r\n")} line ending`, () => {
+    expect(getLineInfo(inputArray.join("\r\n"), 7)).toEqual({
+      column: 1,
+      line: 3,
+    });
+  });
+});