livid-mode: automatically evaluate JS as you type, as long as it's syntactically valid! https://github.com/pandeiro/livid-mode
Based on the browser-based tools that provide this facility.
miniblog.
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Visualisation in Markdown: https://visdown.amitkaps.com/ (elegant approach, especially considering MD has no explicit extension facility)
Today I learnt that GHC has an incredible facility to teach it optimisations applicable to your code: