Because TypeScript knows what types our objects are, it also knows what shapes our objects adhere to. An object’s shape describes, among other things, what properties and methods it does or doesn’t contain.

The built-in types in JavaScript each have known properties and methods that always exist. All strings, for example, are known to have a .length property and .toLowerCase() method.

"OH".length; // 2 "MY".toLowerCase(); // "my"

TypeScript’s tsc command will let you know if your code tries to access properties and methods that don’t exist:

"MY".toLowercase(); // Property 'toLowercase' does not exist on type '"MY"'. // Did you mean 'toLowerCase'?

Through this knowledge of type shapes, TypeScript helps us quickly locate bugs in our code.

Let’s try it!



More bugs! 🙀🙀🙀

We already compiled our index.ts file. Try running index.js by entering node index.js into the bash shell. The code throws an error instead of printing the following expected output:


Use TypeScript’s tsc command to see type errors in the index.ts file.


The TypeScript compiler is surfacing two errors. Using the error messages to guide you, fix both of the errors in the index.ts file.


Confirm that everything is now working properly. Run tsc again to compile your TypeScript code and then run node index.js to run the JavaScript program.

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