Monitoring for transactions: start chaining code

content/_code-samples/monitor-payments-websocket/chaining.js

@@ -0,0 +1,58 @@
+const ADDRESS = '' // address to watch for gaps
+
+async function resetStartingPoint() {
+  // using the api_request function defined in earlier examples
+  const resp = await api_request({
+    "command": "account_info",
+    "account": ADDRESS
+  })
+  if (resp.status === "success") {
+    return resp.result.account_data.PreviousTxnID
+  } else {
+    console.error("Couldn't get account_info for address "+address+" because of error: "+JSON.stringify(resp))
+  }
+}
+
+let knownPreviousTxnID = undefined
+// Using the socket from earlier examples
+socket.addEventListener('open', (event) => {
+  knownPreviousTxnID = resetStartingPoint()
+})
+
+function hasGaps(affected_nodes) {
+  if (typeof knownPreviousTxnID === "undefined") {
+    console.warn("Can't detect gaps without a known PreviousTxnID...")
+    return undefined
+  }
+
+  for (let i=0; i<affected_nodes.length; i++) {
+    if ((affected_nodes[i].hasOwnProperty("ModifiedNode"))) {
+      const ledger_entry = affected_nodes[i].ModifiedNode
+      if (ledger_entry.LedgerEntryType === "AccountRoot" &&
+          ledger_entry.FinalFields.Account === ADDRESS) {
+        if (ledger_entry.PreviousTxnID === knownPreviousTxnID) {
+          return false
+        } else {
+          console.warn("Gap detected...")
+          return true
+        }
+      }
+    }
+  }
+  console.log("This transaction did not directly modify account "+address)
+  return false
+}
+
+const lookOutForGaps = function(data) {
+  const gap_status = hasGaps(data.meta.AffectedNodes)
+  if (gap_status === false) {
+    console.log("Chained successfully. New tx:")
+  } else if (gap_status === true) {
+    // fill gap
+  } else if (gap_status === undefined) {
+    console.log("")
+  }
+
+
+}
+WS_HANDLERS['transaction'] = watchForGaps

content/tutorials/use-simple-xrp-payments/monitor-incoming-payments-with-websocket.md

@@ -470,7 +470,7 @@ To look for gaps that might affect a transaction's XRP balance, you can use this
     - If you haven't filled the gap by the time you reach a the transaction that created the object (where the account appears as a `CreatedNode` entry instead of a `ModifiedNode` entry), something has gone wrong and there is a bug somewhere.
 
 ```js
-TODO: code sample of history chaining
+{% include '_code-samples/monitor-payments-websocket/chaining.js' %}
 ```
 
 ***TODO: interactive gap detector (maybe you can force it to have gaps by temporarily switching your browser to offline mode?)***