Evaluate Strings JK🐉

Prompt cleaning, filename building, tag wrangling - ComfyUI is full of string chores, and the default kit handles them awkwardly. Evaluate Strings JK gives you Python string operations inside one node: bind a, b, c to three strings, write an expression like a.strip().title() + " " + b, and get the result out as a single STRING. Same SimpleEval engine as the rest of the Evaluate family, but with string functions added to the whitelist.

The defaults are instructive: a = "Hello", b = " World", c = "!", expression a + b + c → "Hello World!". It's a demo you can hit "Queue" on immediately and see work. From there, the examples node in the pack shows the real vocabulary: .upper(), .lower(), .title(), .strip(), .replace('old','new'), .split(' '), .isnumeric(), .startswith(), slicing a[:5], even a[::-1] to reverse. len() is also available, so len(a) + len(b) gives you a character count as a string.

The honest limitations: it's string expression evaluation, not a full scripting environment. No loops, no if statements - but you can fake a lot with ternary-style expressions, and the whitelist means everything is safe to run. The output is always a string, so if you want the length back as an integer you'd better end the expression with a value that converts cleanly (the node returns str(result)).

What you'd actually use it for: normalizing a messy prompt before it hits a text encoder, composing a filename prefix from parts (date + model name + seed string), stripping trailing commas after a random-prompt node, or standardizing casing so downstream tag matching works. If you're doing this more than once, this node beats a custom script.

Install. It's part of ComfyUI-JakeUpgrade:

cd ComfyUI/custom_nodes
git clone https://github.com/jakechai/ComfyUI-JakeUpgrade
cd ComfyUI-JakeUpgrade
pip install -r requirements.txt

Or ComfyUI Manager → "JakeUpgrade". Windows portable: install.bat or ../../../python_embeded/python.exe -s -m pip install -r requirements.txt. Restart. simpleeval comes with the pack - no model downloads.

Gotchas: the result is str(result), so a non-string result (like True from a.startswith('H')) comes back as the literal text "True" - usually fine, but know it. Remember each of a/b/c defaults to a non-empty string here, so an expression referencing a variable you never connected evaluates against the default rather than erroring - connect them all to be safe. And the pack constants: LOAD_DEPRECATED_NODES = True if JK nodes report missing, disable ComfyUI MultiGPU per the README if you hit CUDA errors.