datemath: Fixes state mutation by parseDateMath function (#74180)

.betterer.results

@@ -101,7 +101,8 @@ exports[`better eslint`] = {
     ],
     "packages/grafana-data/src/datetime/datemath.ts:5381": [
       [0, 0, 0, "Unexpected any. Specify a different type.", "0"],
-      [0, 0, 0, "Unexpected any. Specify a different type.", "1"]
+      [0, 0, 0, "Do not use any type assertions.", "1"],
+      [0, 0, 0, "Unexpected any. Specify a different type.", "2"]
     ],
     "packages/grafana-data/src/datetime/durationutil.ts:5381": [
       [0, 0, 0, "Do not use any type assertions.", "0"]

packages/grafana-data/src/datetime/datemath.test.ts

@@ -181,6 +181,12 @@ describe('DateMath', () => {
       const date = dateMath.parseDateMath(' - 2d', dateTime([2014, 1, 5]));
       expect(date!.valueOf()).toEqual(dateTime([2014, 1, 3]).valueOf());
     });
+
+    it('should not mutate dateTime passed in', () => {
+      const dateInput = dateTime([2014, 1, 5]);
+      dateMath.parseDateMath(' - 2d', dateInput);
+      expect(dateInput.valueOf()).toEqual(dateTime([2014, 1, 5]).valueOf());
+    });
   });
 
   describe('isMathString', () => {

packages/grafana-data/src/datetime/datemath.ts

@@ -110,7 +110,7 @@ export function parseDateMath(
   fiscalYearStartMonth = 0
 ): DateTime | undefined {
   const strippedMathString = mathString.replace(/\s/g, '');
-  const dateTime = time;
+  const result = dateTime(time);
   let i = 0;
   const len = strippedMathString.length;
 
@@ -118,7 +118,7 @@ export function parseDateMath(
     const c = strippedMathString.charAt(i++);
     let type;
     let num;
-    let unit;
+    let unitString: string;
     let isFiscal = false;
 
     if (c === '/') {
@@ -152,34 +152,37 @@ export function parseDateMath(
         return undefined;
       }
     }
-    unit = strippedMathString.charAt(i++);
 
-    if (unit === 'f') {
-      unit = strippedMathString.charAt(i++);
+    unitString = strippedMathString.charAt(i++);
+
+    if (unitString === 'f') {
+      unitString = strippedMathString.charAt(i++);
       isFiscal = true;
     }
 
+    const unit = unitString as DurationUnit;
+
     if (!includes(units, unit)) {
       return undefined;
     } else {
       if (type === 0) {
         if (isFiscal) {
-          roundToFiscal(fiscalYearStartMonth, dateTime, unit, roundUp);
+          roundToFiscal(fiscalYearStartMonth, result, unit, roundUp);
         } else {
           if (roundUp) {
-            dateTime.endOf(unit);
+            result.endOf(unit);
           } else {
-            dateTime.startOf(unit);
+            result.startOf(unit);
           }
         }
       } else if (type === 1) {
-        dateTime.add(num, unit);
+        result.add(num, unit);
       } else if (type === 2) {
-        dateTime.subtract(num, unit);
+        result.subtract(num, unit);
       }
     }
   }
-  return dateTime;
+  return result;
 }
 
 export function roundToFiscal(fyStartMonth: number, dateTime: any, unit: string, roundUp: boolean | undefined) {