Run scripts in Assemblies

This Robot allows you to run arbitrary JavaScript as part of the Assembly execution process. The Robot is invoked automatically when there are Assembly Instructions containing ${...}:

{
  "robot": "/image/resize",
  "width": "${Math.max(file.meta.width, file.meta.height)}"
}

You can also invoke this Robot directly, leaving out the ${...}:

{
  "robot": "/script/run",
  "script": "Math.max(file.meta.width, file.meta.height)"
}

When accessing arrays, the syntax is the same as in any JavaScript program:

{
  "robot": "/image/resize",
  "width": "${file.meta.faces[0].width * 2}"
}

Compared to only accessing an Assembly Variable:

{
  "robot": "/image/resize",
  "width": "${file.meta.faces[0].width}"
}

For more information, see Dynamic Evaluation.

Usage example

{
  "steps": {
    "evaluate": {
      "robot": "/script/run",
      "use": ":original",
      "script": "({ width: file.meta.width, height: file.meta.height, area: file.meta.width * file.meta.height })"
    }
  }
}

Parameters

• interpolate

  boolean | Record<string, boolean>

  Controls whether Assembly Variables are interpolated for individual instruction fields.

  By default, most Robot instruction fields interpolate Assembly Variables. Set this to false to treat every instruction field as literal text, or set an individual field path to false to treat only that field as literal text. For Robot-specific fields that are literal by default, set this to true or set that field path to true to opt back into interpolation.

  Use field names such as path, or dotted paths such as ffmpeg.vf for nested objects.

• output_meta

  Record<string, boolean> | boolean | Array<string>

  Allows you to specify a set of metadata that is more expensive on CPU power to calculate, and thus is disabled by default to keep your Assemblies processing fast.

  For images, you can add "has_transparency": true in this object to extract if the image contains transparent parts and "dominant_colors": true to extract an array of hexadecimal color codes from the image.

  For images, you can also add "blurhash": true to extract a BlurHash string — a compact representation of a placeholder for the image, useful for showing a blurred preview while the full image loads.

  For videos, you can add the "colorspace: true" parameter to extract the colorspace of the output video.

  For videos, you can also add "interlaced": true to detect whether the video is interlaced. This combines the cheap ffprobe field_order flag with a bounded idet sampling pass over the first frames of the source, exposing interlaced, field_order, and a diagnostic interlace_detection object under file.meta. This is computationally expensive and billed accordingly.

  For audio, you can add "mean_volume": true to get a single value representing the mean average volume of the audio file.

  You can also set this to false to skip metadata extraction and speed up transcoding.

• result

  boolean (default: false)

  Whether the results of this Step should be present in the Assembly Status JSON

• queue

  batch

  Setting the queue to 'batch', manually downgrades the priority of jobs for this step to avoid consuming Priority job slots for jobs that don't need zero queue waiting times

• force_accept

  boolean (default: false)

  Force a Robot to accept a file type it would have ignored.

  By default, Robots ignore files they are not familiar with. 🤖/video/encode, for example, will happily ignore input images.

  With the force_accept parameter set to true, you can force Robots to accept all files thrown at them. This will typically lead to errors and should only be used for debugging or combatting edge cases.

• ignore_errors

  boolean | Array<meta | execute> (default: [])

  Ignore errors during specific phases of processing.

  Setting this to ["meta"] will cause the Robot to ignore errors during metadata extraction.

  Setting this to ["execute"] will cause the Robot to ignore errors during the main execution phase.

  Setting this to true is equivalent to ["meta", "execute"] and will ignore errors in both phases.

• use

  string | Array<string> | Array<object> | object

  Specifies which Step(s) to use as input.

  • You can pick any names for Steps except ":original" (reserved for user uploads handled by Transloadit)
  • You can provide several Steps as input with arrays:
    

    {
      "use": [
        ":original",
        "encoded",
        "resized"
      ]
    }
    

  • You can also tag an input Step with as to pass semantic intent to robots:
  • You can also tag input Steps with as to pass semantic intent to robots:
    

    {
      "use": [
        {
          "name": ":original",
          "as": "image"
        },
        {
          "name": ":original",
          "as": "mask"
        }
      ]
    }
    

  Tip

  That's likely all you need to know about use, but you can view Advanced use cases.

• script — required

  string

  A string of JavaScript to evaluate. It has access to all JavaScript features available in a modern browser environment.

  The script is expected to return a JSON.stringify-able value in the same tick, so no await or callbacks are allowed (yet).

  If the script does not finish within 1000ms it times out with an error. The return value or error is exported as file.meta.result. If there was an error, file.meta.isError is true. Note that the Assembly will not crash in this case. If you need it to crash, you can check this value with a 🤖/file/filter Step, setting error_on_decline to true.

  You can check whether evaluating this script was free by inspecting file.meta.isFree. It is recommended to do this during development as to not see sudden unexpected costs in production.

Demos

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