Can I increment a counter with a Variables block inside the loop?

Not directly. Variables assignments substitute references but do not evaluate expressions. A value of + 1 becomes the text '0 + 1' and fails for a number variable. Put a Function block inside the loop that returns + 1, then assign to the variable.

Loop | SteelEngine Docs

The Loop block is a container that executes blocks repeatedly. Iterate over collections, repeat operations a fixed number of times, or continue while a condition is met.

Loop blocks are container nodes that hold other blocks inside them. The contained blocks execute multiple times based on your configuration.

Configuration Options

Loop Type

Choose between four types of loops:

For Loop (Iterations) - A numeric loop that executes a fixed number of times:

Use this when you need to repeat an operation a specific number of times.

Example: Run 5 times
- Iteration 1
- Iteration 2
- Iteration 3
- Iteration 4
- Iteration 5

A Variables block substitutes references but does not evaluate expressions, so i = <variable.i> + 1 fails with an invalid-number error. Compute the new value in a Function block and assign its result.

Condition Evaluation

While and Do While conditions are JavaScript expressions. Before each check, SteelEngine replaces every <block.field> and <variable.name> reference with its current value. Strings are quoted, numbers and booleans are inserted as-is, and objects are inserted as JSON. SteelEngine then evaluates the expression in an isolated sandbox with a 5-second timeout and coerces the result to a boolean.

<variable.i> < 10
<variable.status> !== "done"
<variable.remaining> > 0 && <variable.attempts> < 5

If the condition throws or times out, it evaluates to false and the loop exits. A While loop with no condition never runs. A Do While loop with no condition falls back to a fixed iteration count.

How to Use Loops

Creating a Loop

Drag a Loop block from the toolbar onto your canvas
Configure the loop type and parameters
Drag other blocks inside the loop container
Connect the blocks as needed

Referencing Loop Data

There's an important distinction between referencing loop data from inside vs outside the loop:

Inside the loop, use <loop.> references to access the current iteration context:

<loop.index>: Current iteration number (0-based)
<loop.currentItem>: Current item being processed (forEach only)
<loop.items>: Full collection being iterated (forEach only)

// Inside a Function block within the loop
const idx = <loop.index>;           // 0, 1, 2, ...
const item = <loop.currentItem>;    // Current item

<loop.iteration> and <loop.item> are accepted aliases for <loop.index> and <loop.currentItem>. You can also drill into the current item with a path, for example <loop.currentItem.email>.

These references are only available for blocks inside the loop container. They give you access to the current iteration's context.

Example Use Cases

Processing API Results - For Each loop processes customer records from an API

API (Fetch) → Loop (For Each) → Agent (Analyze) → Function (Store)

Iterative Content Generation - For loop generates multiple content variations

Loop (5x) → Agent (Generate) → Evaluator (Score) → Function (Select Best)

Counter with While Loop - While loop processes items with counter

Variables (i=0) → Loop (While i<10) → Agent (Process) → Function (i + 1) → Variables (i = function.result)

Advanced Features

Failure Handling

When a block inside the loop body fails, the loop does not stop. SteelEngine records the failure as a rejected cell for that iteration. It skips the blocks connected after the failed block for the rest of that iteration. The loop then continues with the next iteration (or the next condition check). After the loop exits, downstream blocks see the failure in <blockname.results> and can decide what to do with it.

Configuration errors are different. These fail the Loop block itself before any iteration runs:

An empty or unresolvable collection
A count above 1,000
A collection larger than 1,000 items

Limitations

Container blocks (Loops and Parallels) support nesting. You can place loops inside loops, parallels inside loops, and any combination of container blocks to build complex multi-dimensional workflows.

Each loop executes its own iterations sequentially. Separate loops inside different Parallel branches can make progress concurrently when nested parallel execution is enabled. See Parallel limitations.

Inputs and Outputs

Loop Type: Choose between 'for', 'forEach', 'while', or 'doWhile'
Iterations: Number of times to execute (for loops)
Collection: Array or object to iterate over (forEach loops)
Condition: Boolean expression to evaluate (while/do-while loops)

Best Practices

Set reasonable limits: Keep iteration counts reasonable to avoid long execution times
Use For Each for collections: When processing arrays or objects, use For Each instead of For loops
Handle errors gracefully: Consider adding error handling inside loops for robust workflows

Common Questions

For loops (fixed count) and For Each loops are limited to 1,000 iterations/items. Asking for more does not cap the loop. Instead, the Loop block fails before running with an error such as 'For loop iterations (1500) exceeds maximum allowed (1000). Execution blocked.' While loops and Do While loops with a condition have no hard iteration cap — they run until the condition evaluates to false. Do While loops without a condition fall back to a fixed iteration count, which is subject to the same 1,000 limit. Always ensure your While/Do While conditions will eventually become false to avoid infinite loops.

Loops execute all iterations sequentially, one after another. If you need concurrent execution across items, use the Parallel block instead. You can also nest a Parallel block inside a Loop if you need both iteration patterns.

Inside the loop, use <loop.currentItem> to get the current item being processed and <loop.index> for the zero-based iteration number. These references are only available to blocks placed inside the loop container — blocks outside the loop cannot access them.

After the loop completes, reference the loop block by its normalized name (lowercase, no spaces) using <blockname.results>. This returns one array per iteration, in order. Each array holds a settled cell per block that ran in that iteration, either { status: 'fulfilled', value } or { status: 'rejected', error }. For example, if your loop block is named 'Process Items', use <processitems.results>. Do not use <loop.> syntax outside the loop — that only works inside.

The loop continues. The failure becomes a { status: 'rejected', error } cell in that iteration's results. The remaining blocks in that iteration are skipped, and the next iteration starts as normal. Only configuration problems — an empty or unresolvable collection, or more than 1,000 iterations or items — fail the Loop block itself.

Not directly. Variables assignments substitute references but do not evaluate expressions. A value of <variable.i> + 1 becomes the text '0 + 1' and fails for a number variable. Put a Function block inside the loop that returns <variable.i> + 1, then assign <function.result> to the variable.

Yes. Container blocks (Loops and Parallels) fully support nesting. You can place loops inside loops, parallels inside loops, loops inside parallels, and any combination. Each nested container maintains its own scope and iteration context independently.

A While loop checks its condition before each iteration, so it may execute zero times if the condition is false initially. A Do While loop executes its body at least once, then checks the condition after each iteration to decide whether to continue. Use Do While when you need guaranteed first execution.

If the For Each loop's collection is empty, the loop body is skipped entirely and the loop outputs an empty results array. The workflow continues normally to any blocks connected after the loop.

Loop

Common Questions

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