refactor(fake-timers): don't setup fakeTimerAPIs unless needed (#10551)

tgriesser · web-flow · commit ecb31a71f03e · 2020-10-19T13:12:07.000+02:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -6,6 +6,7 @@
 
 ### Fixes
 
+- `[jest-fake-timers]` Lazily instantiate mock timers ([#10551](https://github.com/facebook/jest/pull/10551))
 - `[jest-runtime]` `require.main` is no longer `undefined` when using `jest.resetModules` ([#10626](https://github.com/facebook/jest/pull/10626))
 - `[@jest/types]` Add missing values for `timers` ([#10632](https://github.com/facebook/jest/pull/10632))
 
diff --git a/packages/jest-fake-timers/src/legacyFakeTimers.ts b/packages/jest-fake-timers/src/legacyFakeTimers.ts
@@ -94,7 +94,6 @@ export default class FakeTimers<TimerRef> {
     };
 
     this.reset();
-    this._createMocks();
   }
 
   clearAllTimers(): void {
@@ -349,7 +348,7 @@ export default class FakeTimers<TimerRef> {
   }
 
   private _checkFakeTimers() {
-    if (this._global.setTimeout !== this._fakeTimerAPIs.setTimeout) {
+    if (this._global.setTimeout !== this._fakeTimerAPIs?.setTimeout) {
       this._global.console.warn(
         `A function to advance timers was called but the timers API is not ` +
           `mocked with fake timers. Call \`jest.useFakeTimers()\` in this ` +

Original file line number	Diff line number	Diff line change
`@@ -94,7 +94,6 @@ export default class FakeTimers<TimerRef> {`
`94`	`94`	`};`
`95`	`95`
`96`	`96`	`this.reset();`
`97`		`- this._createMocks();`
`98`	`97`	`}`
`99`	`98`
`100`	`99`	`clearAllTimers(): void {`
`@@ -349,7 +348,7 @@ export default class FakeTimers<TimerRef> {`
`349`	`348`	`}`
`350`	`349`
`351`	`350`	`private _checkFakeTimers() {`
`352`		`- if (this._global.setTimeout !== this._fakeTimerAPIs.setTimeout) {`
	`351`	`+ if (this._global.setTimeout !== this._fakeTimerAPIs?.setTimeout) {`
`353`	`352`	`this._global.console.warn(`
`354`	`353`	`A function to advance timers was called but the timers API is not ` +
`355`	`354`	`mocked with fake timers. Call \`jest.useFakeTimers()\` in this ` +