prefer-math-trunc: Improve fix logic (#1670)

Author: fisker

rules/prefer-math-trunc.js

@@ -61,15 +61,15 @@ const create = context => {
 
 			if (!isAssignment || !hasSideEffect(left, sourceCode)) {
 				const fix = function * (fixer) {
-					let fixed = mathTruncFunctionCall(left);
+					const fixed = mathTruncFunctionCall(left);
 					if (isAssignment) {
-						// TODO[@fisker]: Improve this fix, don't touch left
-						fixed = `${sourceCode.getText(left)} = ${fixed}`;
+						const operatorToken = sourceCode.getTokenAfter(left, token => token.type === 'Punctuator' && token.value === operator);
+						yield fixer.replaceText(operatorToken, '=');
+						yield fixer.replaceText(right, fixed);
 					} else {
 						yield * fixSpaceAroundKeyword(fixer, node, sourceCode);
+						yield fixer.replaceText(node, fixed);
 					}
-
-					yield fixer.replaceText(node, fixed);
 				};
 
 				if (operator === '|') {

test/snapshots/prefer-math-trunc.mjs.md

@@ -683,7 +683,7 @@ Generated by [AVA](https://avajs.dev).
 
     `␊
       1 | const foo = {a: {b: 3.4}};␊
-      2 | foo /* Comment 1 */ .a /* Comment 2 */ . /* Comment 3 */ b = Math.trunc(foo /* Comment 1 */ .a /* Comment 2 */ . /* Comment 3 */ b) /* Comment 5 */;␊
+      2 | foo /* Comment 1 */ .a /* Comment 2 */ . /* Comment 3 */ b = /* Comment 4 */ Math.trunc(foo /* Comment 1 */ .a /* Comment 2 */ . /* Comment 3 */ b) /* Comment 5 */;␊
     `
 
 > Error 1/1