added JS expressions episode

Author: Toby Hodges

_episodes/13-expressions.md

@@ -0,0 +1,70 @@
+---
+title: "JavaScript Expressions"
+teaching: 10
+exercises: 0
+questions:
+- "What do I do when I want to create values dynamically and CWL doesn't
+provide a built-in way of doing so?"
+objectives:
+- "Learn how to insert javaScript expressions into a CWL description."
+keypoints:
+- "If `InlineJavascriptRequirement` is specified, you can include JavaScript
+expressions that will be evaulated by the CWL runner."
+- "Expressions are only valid in certain fields."
+- "Expressions should only be used when no built in CWL solution exists."
+---
+If you need to manipulate input parameters, include the requirement
+`InlineJavascriptRequirement` and then anywhere a parameter reference is
+legal you can provide a fragment of Javascript that will be evaluated by
+the CWL runner.
+
+__Note: JavaScript expressions should only be used when absolutely necessary.
+When manipulating file names, extensions, paths etc, consider whether one of the
+[built in `File` properties][file-prop] like `basename`, `nameroot`, `nameext`,
+etc, could be used instead.
+See the [list of recommended practices][rec-practices].__
+
+*expression.cwl*
+
+```
+{% include cwl/expression.cwl %}
+```
+
+As this tool does not require any `inputs` we can run it with an (almost) empty job file:
+
+*empty.yml*
+
+```
+{% include cwl/empty.yml %}
+```
+
+We can then run `expression.cwl`:
+
+```
+$ cwl-runner expression.cwl empty.yml
+[job 140000594593168] /home/example$ echo -A 2 -B baz -C 10 9 8 7 6 5 4 3 2 1
+-A 2 -B baz -C 10 9 8 7 6 5 4 3 2 1
+Final process status is success
+{}
+```
+
+You can only use expressions in certain fields.  These are:
+
+- `filename`
+- `fileContent`
+- `envValue`
+- `valueFrom`
+- `glob`
+- `outputEval`
+- `stdin`
+- `stdout`
+- `coresMin`
+- `coresMax`
+- `ramMin`
+- `ramMax`
+- `tmpdirMin`
+- `tmpdirMax`
+- `outdirMin`
+- `outdirMax`
+
+[rec-practices]: rec-practices/