fix(mergeDeep): handle undefined sources correctly

Author: ardatan

.changeset/chilly-icons-guess.md

@@ -0,0 +1,11 @@
+---
+'@graphql-tools/utils': patch
+---
+
+Fix the bug in `mergeDeep`;
+
+The following inputs and outputs are corrected;
+
+- `mergeDeep([{a:2} undefined])` - Any nullish values should be ignored so it should return `{a:2}`
+- `mergeDeep([])` - no sources should return `undefined`
+- `mergeDeep([undefined])` - no sources should return `undefined`

packages/utils/src/mergeDeep.ts

@@ -12,6 +12,12 @@ export function mergeDeep<S extends any[]>(
   respectArrays = false,
   respectArrayLength = false,
 ): UnboxIntersection<UnionToIntersection<BoxedTupleTypes<S>>> & any {
+  if (sources.length === 0) {
+    return;
+  }
+  if (sources.length === 1) {
+    return sources[0];
+  }
   let expectedLength: number | undefined;
   let allArrays = true;
   const areArraysInTheSameLength = sources.every(source => {
@@ -54,6 +60,9 @@ export function mergeDeep<S extends any[]>(
     }
   }
   for (const source of sources) {
+    if (source == null) {
+      continue;
+    }
     if (isObject(source)) {
       if (firstObjectSource) {
         const outputPrototype = Object.getPrototypeOf(output);

packages/utils/tests/mergeDeep.test.ts

@@ -72,4 +72,16 @@ describe('mergeDeep', () => {
     const b = { options: ['$A', '$B'] };
     expect(mergeDeep([a, b], undefined, true, true)).toEqual({ options: ['$A', '$B'] });
   });
+
+  it('filters nullish values', () => {
+    expect(mergeDeep([{ a: 'dsa' }, { a: 'dd', b: 1 }, undefined])).toEqual({ a: 'dd', b: 1 });
+  });
+
+  it('respects empty objects', () => {
+    expect(mergeDeep([{}])).toEqual({});
+  });
+
+  it('returns undefined when an empty sources array passed', () => {
+    expect(mergeDeep([])).toEqual(undefined);
+  });
 });