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Master TypeScript 6.0's enhanced `--checkJs` to incrementally enforce strict mode on JavaScript files without breaking CI or forcing a big-bang migration.

Most TypeScript type errors come from misunderstanding when extends catches conflicts that & silently permits. Learn the declaration-time versus use-time distinction that determines whether your code compiles or fails at runtime.

Type guards return booleans and let execution continue. Assertion functions throw or validate and narrow on the same line. Most narrowing bugs stem from choosing the wrong pattern for your error handling strategy.