You'll always be able to type in JavaScript code in the free play modding section.
And LearnToMod has recently added JavaScript badge tutorials - not too many yet, but their numbers are growing.
How to learn JavaScript after finishing the badges:
1) Try creating your own mini-game (create your 'main function and go from there!)
2) Follow our advanced concepts tutorial videos here: https://www.youtube.com/playlist?list=PLyXVSS9DX9ZLySTUFDQuhhx6stA4p2xJB
3) Try translating existing mods to JavaScript, by using the toggle JavaScript button that is on all tutorials
Some more explanation on #3 -- try coding with two browser tabs open. One tab will have your JavaScript mod, and the other with a blockly mod.
Drag the block onto the block workspace, and click the grey JS button located on all badge lessons to see what's generated. Then type the code into your JavaScript tab's editor. This way you can see the JavaScript that's rendered by a given block (We recommend actually typing the code; don't copy and paste. This way your muscle memory will pick up on the syntax.)
This technique allows you to take the semantic expertise you've acquired with blockly, and translate it to some syntactic expertise in JavaScript. Though you may have questions about operators, syntax and such afterwards.