feat(data): create function to check if a member exists

re #321

packages/data/README.md

@@ -118,6 +118,12 @@ const group = await semaphoreSubgraph.getGroup("42")
 const { members, verifiedProofs } = semaphoreSubgraph.getGroup("42", { members: true, verifiedProofs: true })
 ```
 
+\# **isGroupMember**(groupId: _string_, member: _string_): _Promise\<boolean>_
+
+```typescript
+await semaphoreSubgraph.isGroupMember("42", "16948514235341957898454876473214737047419402240398321289450170535251226167324")
+```
+
 \# **new Ethers**(networkOrEthereumURL: Network | string = "goerli", options: EthersOptions = {}): _SemaphoreEthers_
 
 ```typescript

packages/data/src/subgraph.test.ts

@@ -338,4 +338,31 @@ describe("SemaphoreSubgraph", () => {
             })
         })
     })
+
+    describe("# isGroupMember", () => {
+        it("Should throw an error if the member parameter type is wrong", async () => {
+            const fun = () => semaphore.isGroupMember("1", 1 as any)
+
+            await expect(fun).rejects.toThrow("Parameter 'member' is not a string")
+        })
+
+        it("Should return true if a group member exist", async () => {
+            requestMocked.mockImplementationOnce(() =>
+                Promise.resolve({
+                    groups: [
+                        {
+                            id: "1"
+                        }
+                    ]
+                })
+            )
+
+            const expectedValue = await semaphore.isGroupMember(
+                "1",
+                "19759682999141591121829027463339362582441675980174830329241909768001406603165"
+            )
+
+            expect(expectedValue).toBeTruthy()
+        })
+    })
 })

packages/data/src/subgraph.ts

@@ -195,4 +195,30 @@ export default class SemaphoreSubgraph {
 
         return groups[0]
     }
+
+    /**
+     * Returns true if a member is part of group, and false otherwise.
+     * @param groupId Group id
+     * @param member Group member.
+     * @returns True if the member is part of the group, false otherwise.
+     */
+    async isGroupMember(groupId: string, member: string): Promise<boolean> {
+        checkParameter(groupId, "groupId", "string")
+        checkParameter(member, "member", "string")
+
+        const config: AxiosRequestConfig = {
+            method: "post",
+            data: JSON.stringify({
+                query: `{
+                    groups(where: { id: "${groupId}", members_: { identityCommitment: "${member}" } }) {
+                        id
+                    }
+                }`
+            })
+        }
+
+        const { groups } = await request(this._url, config)
+
+        return groups.length !== 0
+    }
 }