Accessing Output Downstream — Guide33_NodeOutputs

The $output Variable

Inside any expression field in a node's config (input mapping, conditions, message body, etc.), $output is a dictionary of all previously executed node outputs keyed by nodeId.

$output.{nodeId}           // full output object
$output.{nodeId}.field     // top-level field
$output.{nodeId}.nested.field   // nested field
$output.{nodeId}.items[0].name  // array element field

Concrete Examples

// Node "fetchUser" returned: { userId: "u-123", email: "alice@acme.com", roles: ["admin"] }

$output.fetchUser.userId          // → "u-123"
$output.fetchUser.email           // → "alice@acme.com"
$output.fetchUser.roles[0]        // → "admin"
$output.fetchUser.roles.length    // → 1

// Node "queryInvoices" returned: { items: [{id: "inv-1", amount: 500}], rowCount: 1 }
$output.queryInvoices.items[0].amount   // → 500
$output.queryInvoices.rowCount          // → 1

When Is Output Available?

A node's output becomes available in $output as soon as that node completes — whether it was in the same execution branch or a parallel branch. The engine always writes to ExecutionMemory before scheduling downstream nodes, so any node that executes after a given node can safely read its output.

Parallel branches: If NodeA and NodeB run in parallel (fan-out from a parent), NodeA cannot read $output.nodeB because NodeB hasn't completed yet. Only nodes that run after both complete (e.g., after a join/merge gateway) can safely read both outputs.

Null-Safe Access

If a node has not yet executed (or was skipped), $output.{nodeId} returns null. Use null-coalescing in expressions to guard against this:

// Safe access with fallback
$output.optionalNode?.amount ?? 0

// Check before use
$output.lookupNode != null ? $output.lookupNode.userId : $context.actorId

Full Expression Context

The expression engine provides several root variables alongside $output:

Variable	Description
`$output`	Map of all completed node outputs by nodeId
`$json`	The workflow's trigger input data
`$context`	Execution context (actorId, tenantId, executionId, timestamp)
`$env`	Environment variables and platform config values

TypeScript — Accessing Output in the Designer

In the React designer, the expression autocomplete is aware of node output schemas. When you type $output., the editor suggests available node IDs. This is powered by the WorkflowNodeOutputSchema type registered for each node type:

// packages/flow-studio-designer/src/stores/expressionContext.ts
export function buildExpressionContext(
  nodes: WorkflowNode[],
  memory: ExecutionMemorySnapshot
): ExpressionContext {
  const output: Record<string, unknown> = {};
  for (const node of nodes) {
    if (memory.nodeOutputs[node.id] !== undefined) {
      output[node.id] = memory.nodeOutputs[node.id];
    }
  }
  return { $output: output, $json: memory.triggerData, $context: memory.context };
}

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