Run Scripts

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)"
  }
]

For more information, see Dynamic Evaluation.

Parameters

• use

  String / Array of Strings / Object required

  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"
    ]
    

  That’s likely all you need to know about use, but you can view advanced use cases:

  › Advanced use cases

  • Step bundling. Some Robots can gather several Step results for a single invocation. For example, the /file/compress Robot would normally create one archive for each file passed to it. If you'd set bundle_steps to true, however, it will create one archive containing all the result files from all Steps you give it. To enable bundling, provide an object like the one below to the use parameter:

    "use": {
      "steps": [
        ":original",
        "encoded",
        "resized"
      ],
      "bundle_steps": true
    }
    

    This is also a crucial parameter for the /video/adaptive Robot, otherwise you'll generate 1 playlist for each viewing quality.
    Keep in mind that all input Steps must be present in your Template. If one of them is missing (for instance it is rejected by a filter), no result is generated because the Robot waits indefinitely for all input Steps to be finished.

    Here’s a demo that showcases Step bundling.

  • Group by original. Sticking with the /file/compress Robot example, you can set group_by_original to true, in order to create a separate archive for each of your uploaded or imported files, instead of creating one archive containing all originals (or one per resulting file). This is important for for the /media/playlist Robot where you'd typically set:

    "use": {
      "steps": [
        "segmented"
      ],
      "bundle_steps": true,
      "group_by_original": true
    }
    

  • Fields. You can be more discriminatory by only using files that match a field name by setting the fields property. When this array is specified, the corresponding Step will only be executed for files submitted through one of the given field names, which correspond with the strings in the name attribute of the HTML file input field tag for instance. When using a back-end SDK, it corresponds with myFieldName1 in e.g.: $transloadit->addFile('myFieldName1', './chameleon.jpg').

    This parameter is set to true by default, meaning all fields are accepted.

    Example:

    "use": {
      "steps": [ ":original" ],
      "fields": [ "myFieldName1" ]
    }
    

  • Use as. Sometimes Robots take several inputs. For instance, the /video/merge Robot can create a slideshow from audio and images. You can map different Steps to the appropriate inputs.

    Example:

    "use": {
      "steps": [
        { "name": "audio_encoded", "as": "audio" },
        { "name": "images_resized", "as": "image" }
      ]
    }
    

    Sometimes the ordering is important, for instance, with our concat Robots. In these cases, you can add an index that starts at 1. You can also optionally filter by the multipart field name. Like in this example, where all files are coming from the same source (end-user uploads), but with different <input> names:

    Example:

    "use": {
      "steps": [
        { "name": ":original", "fields": "myFirstVideo", "as": "video_1" },
        { "name": ":original", "fields": "mySecondVideo", "as": "video_2" },
        { "name": ":original", "fields": "myThirdVideo", "as": "video_3" }
      ]
    }
    

    For times when it is not apparent where we should put the file, you can use Assembly Variables to be specific. For instance, you may want to pass a text file to the /image/resize Robot to burn the text in an image, but you are burning multiple texts, so where do we put the text file? We specify it via ${use.text_1}, to indicate the first text file that was passed.

    Example:

    "watermarked": {
      "robot": "/image/resize",
      "use"  : {
        "steps": [
          { "name": "resized", "as": "base" },
          { "name": "transcribed", "as": "text" },
        ],
      },
      "text": [
        {
          "text"  : "Hi there",
          "valign": "top",
          "align" : "left",
        },
        {
          "text"    : "From the 'transcribed' Step: ${use.text_1}",
          "valign"  : "bottom",
          "align"   : "right",
          "x_offset": 16,
          "y_offset": -10,
        }
      ]
    }
    

• script

  String ⋅ example: "Math.max(min, file.meta.width)" required

  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.

Related blog posts

• Generate a preview of a website using Transloadit CDN May 11, 2022
• Introducing the /script/run Robot December 15, 2021