Debugging

This page shows how to inspect values while a script runs, how to read error messages, and how to find slow dataflow blocks.

Topics on this page:

• print
• printConsole
• Errors
• Profiling: startProfile, printProfile, stopProfile, profileModelTree

print(value1, value2, …)

Adds text to the print property of the block that runs the script.

• value1, value2, …: any values. Each value is converted to text and written directly after the previous one. print does not add spaces between values or a line break at the end. Add "\n" yourself if you want separate lines.
• Returns null.

Lists print as [1, 2, 3], objects as {a: 1}, and null as null.

The print property is updated once, when the script finishes. If the script stops with an error, the print property shows the error message instead, and the text printed before the error is lost.

print("2 + 2 = ", 2 + 2);
print("done");
return "ok";
// print property: 2 + 2 = 4done

var items = ["a", "b", "c"];
for (var i = 0; i < items.length; i++) {
  print(i, ": ", items[i], "\n");
}
return items.length;

print also works in the scripts of the Filter, Column Mapping, Edit Rows, and Remove Rows blocks. The text of all rows is collected in the print property of that block.

When a Script block is called as a function, print does nothing. Use printConsole instead.

printConsole(value1, value2, …)

Writes a line to the browser's developer console (open it with F12 in most browsers).

• value1, value2, …: any values. They are joined with '' , '' (space, comma, space).
• Returns null.

printConsole works everywhere, including scripts that are called as functions, so it is the best way to debug a reused script.

/* Prints the following lines to the browser console:
1
2
3
*/
var json = JSON.parse("[1,2,3]");
for (var i = 0; i < json.length; i++) {
  printConsole(json[i]);
}

printConsole("width", @parent.width);   // prints: width , 100

DG Script has no try/catch. An error stops the script.

When a Script block stops with an error:

• output becomes null;
• print shows the error message;
• the error is also written to the browser console.

When a Script block is called as a function and stops with an error, the call returns null and the error is written only to the browser console.

var x = null;
return x.length;
// output: null
// print: can not access length of null

If the script cannot be parsed, it does not run at all. print shows the error, and output keeps its previous value. The number in parentheses is the position of the problem, counted in characters from the start of the script (the first character is 0).

return 1 +;
// print: Unexpected token: SEMICOLON (10): ;

var list = [1, 2];
return list[5];
// print: can not access 5 of [1, 2]

Use these functions to find dataflow blocks that take a long time to run. Profiling measures every dataflow block that runs while profiling is on, including Script blocks, table blocks, and other logic blocks.

startProfile()

Starts recording how long each dataflow block takes to run. Returns null.

printProfile(count)

Returns a report of the count blocks that used the most total time since profiling started, and also writes it to the browser console.

• count: number of blocks to list. Required.
• Returns a string. The first line is profile result:. Each following line has the form t:<time> c:<runs> <path> (<type>), where <time> is the total time in microseconds, <runs> is the number of runs, and <path> is the page model path of the block. Blocks inside symbols start with the symbol name and ::. Returns null if count is not a number.

stopProfile()

Stops profiling and deletes the recorded data. Call printProfile before stopProfile. Returns null.

Create two Script blocks. In the first, start profiling:

startProfile();

Use the page for a while, then run the second:

var report = printProfile(10);
stopProfile();
return report;

A result might look like this:

profile result:
t:15300 c:12 page.dataflow.filter1 (tableFilter)
t:4200 c:40 page.dataflow.script1 (script)

profileModelTree(object)

Returns an object that describes the page model tree under object. Use it to find parts of a page that contain many objects.

• object: a page model object, such as @stage or @parent.
• Returns an object with:
  • @type: the type of the object;
  • @count: the number of objects in this subtree, including the object itself;
  • one key for each child object, holding the same kind of description. Children of repeaters and groups use the keys .0, .1, and so on.
• Returns null if object is not a page model object.

var tree = profileModelTree(@stage);
return JSON.stringify(tree);

var tree = profileModelTree(@);
return tree["@type"];   // "script"