2.9. Expression Tool#

An expression tool is a type of Process that can be run by itself or as a Workflow step. It executes a pure JavaScript expression. It is meant to be used as a way to isolate complex JavaScript expressions that need to operate on input data and produce some result as output.

Similar to the command-line tool it requires inputs and outputs. But instead of baseCommand, it requires an expression attribute.

digraph G { compound=true; rankdir="LR"; fontname="Verdana"; fontsize="10"; graph [splines=ortho]; node [fontname="Verdana", fontsize="10", shape=box]; edge [fontname="Verdana", fontsize="10"]; subgraph cluster_0 { expression[style="filled" label="JavaScript"]; label="expression"; fill=gray; } inputs -> expression [lhead=cluster_0]; expression -> outputs [ltail=cluster_0]; }

CWL expression tool.#

cwlVersion: v1.2
class: ExpressionTool

  InlineJavascriptRequirement: {}

  message: string
  uppercase_message: string

expression: |
  ${ return {"uppercase_message": inputs.message.toUpperCase()}; }


We had to use an InlineJavascriptRequirement as our expression contains a JavaScript call in .toUpperCase(). This means to tools using the expression tool that JavaScript is a requirement.