If you're just starting out learning javascript & don't want to spend time creating an entire webpage just for embedding test scripts, just open Dev Tools in a new tab in Chrome Browser, and click on Console
.
Then type out some test scripts: eg.
console.log('Aha!')
Then press Enter after every line (to submit for execution by Chrome)
or load your own "external script file":
document.createElement('script').src = 'http://example.com/file.js';
Then call your functions:
console.log(file.function('?????'))
Tested in Google Chrome 85.0.4183.121