Which side of a web app are you trying to fix?

A button that stops responding points you to the JavaScript running in the browser. A script that calculates a total on your own machine may run in Node.js. WebStorm can debug both. For its built-in browser debugger, use Chrome or another Chromium-based browser. Start where the unexpected result happens and pick the matching run configuration.

What should you copy from the project instructions?

Open the folder that contains the project files and check how the project starts. If it uses Node.js, check the required runtime version and read the project scripts before running them. Keep the project source, its dependency files and its start instructions. A different editor can use those same files.

Can you use the free WebStorm license for paid work?

JetBrains offers free WebStorm for qualifying non-commercial use such as individual learning. For paid work, check its paid licensing options and use the one that fits your situation. The buy page also offers a 30-day trial.

JetBrains says detailed code-related data sharing is on by default under the free non-commercial license and can be turned off. Check Data Sharing in settings before opening work with sensitive code.

How can you find why one click adds two?

Trace one click that takes a fictional counter from 2 to 4. A single click should make it 3. Use a local toy page or walk through the click on paper.

Set the starting value
In a local toy page, set a visible counter to 2. One click should make it 3.
Pause in the click handler
Watch the value before the increment. If the debugger pauses twice for one click, check whether the same action was attached twice. If it pauses once, step through the calculation.
Repeat from the same starting point
Reload, make one change and click once again. Look for 2 before the click and 3 after it.