For Loop
The For Loop block runs a chain of blocks again and again, counting from a start number up to an end number. Each round, the chain receives the current count.
When to use it
- Do something a fixed number of times, like retrying a call 3 times or building 10 rows of test data.
- Work through page numbers, like pages
1to5of an external API. - Don't use it to go through a list of items. For Each Loop hands you each item directly.
- Don't use it to run things at the same time. A loop runs one round after the other. For parallel work, use Orchestrator.
Inputs
| Field | Required | Default | What it does |
|---|---|---|---|
| Start | Yes | none | The number to start counting from, for example 0. |
| End | Yes | none | The number to stop at. The loop stops before reaching it. |
| Step | No | 1 | How much to add to the count after each round. |
Each of the three can be a plain number or a JS expression, for example js:input.length. A JS expression is worked out once, before the loop starts.
Outputs
| Handle | When it is used |
|---|---|
| Body (top) | Connect the first block of the chain to repeat. It runs once per round and receives the count as input. |
| Next (right) | Connect the block to run after the last round. |
The loop itself returns nothing. The block connected to Next receives an empty input, not the results of the rounds. To keep results, save them in a variable with Set Variable inside the loop.
Example
Fetch pages 1 to 3 of an external API and collect them.
| Field | Value |
|---|---|
| Start | 1 |
| End | 4 |
| Step | 1 |
The Body chain runs three times, with input set to 1, then 2, then 3. Use it in an HTTP Request URL like js: 'https://api.example.com/items?page=' + input. A Set Variable block at the end of the chain adds each page to a list.
How it behaves
- The end number is not included.
Start 0, End 3runs with0,1,2. To run with3as well, set End to4. - No rounds are a normal result. If Start is already at or past End, the chain never runs and the flow moves on to Next.
- A bad step can loop forever. A Step of
0, or a negative step with a Start below End, never reaches the end. The panel warns you when it spots this. - Rounds run in order. Round two starts only after round one has finished.
- An error stops the loop. If a block in the chain fails, the remaining rounds are skipped and the Error Handler runs.
- A Response ends everything. If the chain reaches a Response block, the whole route ends with that response, even in round one.
- Shared variables. Every round sees and changes the same variables, so what round one sets is still there in round two.
Related blocks
- For Each Loop: repeat once per item in a list.
- Set Variable: collect results across rounds.
- Orchestrator: run chains in parallel instead of one after another.