[
  {
    "value": {
      "_snaptype_datetime": "2018-04-27T20:03:58.453 UTC"
    },
    "sub-expression": {
      "0: Date": [
        {
          "_snaptype_scope": [
            "parse",
            "now",
            "UTC"
          ]
        }
      ],
      "1: (1,2,3)": [
        {
          "_snaptype_arguments": {
            "ignored": 3
          }
        }
      ],
      "2: .now": [
        {
          "_snaptype_datetime": "2018-04-27T20:03:58.453 UTC"
        }
      ]
    }
  }
]

The expression's evaluation happens in real time, as you start typing the expression SnapLogic starts referencing the expression library and show's suggestions accordingly in the Output Preview pop-up:

When an object has been typed, it then suggests the methods within that object:

When arguments are entered, the expression is validated and a response shown in the Output Preview pop-up. The Expression Builder window can be opened by clicking on the arrow icon in the Output Preview pop-up:

Example

The File Writer Snap provides the ability to toggle the filename into an expression by selecting the equals icon (left of the property box). The content of the text box is interpreted as an expression once the toggle is on.

An example for an expression would be: '/out' + Date.now() + '.json'
This would insert the current date as part of the filename.

Accessing Pipeline Parameters

Parameters allow a pipeline to be reused in multiple situations. For example, a File Writer Snap can be configured to write to a file path specified by a parameter, which allows the same pipeline to write to different files. The parameters for a pipeline can be defined by using the Edit Pipeline properties dialog. The name of each parameter must only contain alpha-numeric characters and the value will be converted to a string. The value for a parameter as defined in the pipeline properties dialog is treated as the default when running the pipeline in Designer. Parameters can also be passed to a pipeline by a Task or the ForEach Snap. Any parameters that are not passed down from the Task or Snap will use the defaults specified in the properties dialog.

To access a pipeline parameter from the expression language, you must prefix the parameter name with an underscore. For example, given the following parameters:

 The "firstName" parameter can then be accessed using _firstName, as in:

"Hello, " + _firstName  // result: Hello, Bob

Since the value of a parameter is always a string, you'll need to convert any string to numeric values before operating on them. For example, simply adding two to the "numValue" parameter will append the character "2" to "12" and yield "122":

_numValue + 2   // result: "122"

Instead, you need to use the parseInt/parseFloat functions to parse the string into a value and then add two to it:

parseInt(_numValue) + 2   // result: 14

If you need to parameterize your pipeline with an expression, you can use the eval() function to evaluate an expression stored in a string.  For example, to read the document field specified by the "path" parameter, you can use:

eval(_path)   // result: <the value of the "age" field in the current document>

Accessing Input View Variables as Part of Expressions

An input view schema attribute can be used as part of the expression by using the dollar sign ($) prefix.

Example

The REST Put Snap provides a URL. The URL can be toggled into an expression and the expressions could be created by dynamically substituting the variables from an input view, such as:

'http://someplace:someport/somepart/' + $inputvar + '/somemoreparts'