jest-snapshot: Distinguish empty string from external snapshot not written

CHANGELOG.md

@@ -17,6 +17,7 @@
 - `[jest-leak-detector]` [**BREAKING**] Use `weak-napi` instead of `weak` package ([#8686](https://github.com/facebook/jest/pull/8686))
 - `[jest-mock]` Fix for mockReturnValue overriding mockImplementationOnce ([#8398](https://github.com/facebook/jest/pull/8398))
 - `[jest-snapshot]` Remove only the added newlines in multiline snapshots ([#8859](https://github.com/facebook/jest/pull/8859))
+- `[jest-snapshot]` Distinguish empty string from external snapshot not written ([#8880](https://github.com/facebook/jest/pull/8880))
 
 ### Chore & Maintenance
 

packages/jest-snapshot/src/State.ts

@@ -185,7 +185,7 @@ export default class SnapshotState {
     // Do not mark the snapshot as "checked" if the snapshot is inline and
     // there's an external snapshot. This way the external snapshot can be
     // removed with `--updateSnapshot`.
-    if (!(isInline && this._snapshotData[key])) {
+    if (!(isInline && this._snapshotData[key] !== undefined)) {
       this._uncheckedKeys.delete(key);
     }
 
@@ -248,7 +248,7 @@ export default class SnapshotState {
         return {
           actual: unescape(receivedSerialized),
           count,
-          expected: expected ? unescape(expected) : null,
+          expected: expected !== undefined ? unescape(expected) : null,
           key,
           pass: false,
         };

packages/jest-snapshot/src/index.ts

@@ -340,7 +340,7 @@ const _toMatchSnapshot = ({
   let report: () => string;
   if (pass) {
     return {message: () => '', pass: true};
-  } else if (!expected) {
+  } else if (expected == null) {
     report = () =>
       `New snapshot was ${RECEIVED_COLOR('not written')}. The update flag ` +
       `must be explicitly passed to write a new snapshot.\n\n` +