are there browsers in which I can directly evaluate TypeScript (rather than JavaScript) expressions directly in the console
No
do current browsers have APIs that could be used to develop plugins or extensions to do just that — integrating smoothly with the built-in console
Not the built in console. But you can create a new TypeScript console
. This would be similar to the coffescript console : https://developers.google.com/chrome-developer-tools/docs/sample-extensions#coffeescript_console
Should be simple enough to twist that to a TypeScript
console if you want to. Its basically this line : https://github.com/snookca/CoffeeConsole/blob/master/src/coffee-console.html#L89 that needs to point to something like TypeString
: https://www.npmjs.org/package/typestring. Syntax highlighting is a much more involved problem.