When we declare a function in JavaScript, we often expect it to be invoked with arguments of a certain type. JavaScript does not share our expectations: its type flexibility often allows functions to be invoked with unexpected argument types. Even when this doesn’t result in thrown errors, there can be negative consequences:

function printLengthOfText(text) { console.log(text.length); } printLengthOfText(3); // Prints: undefined

JavaScript developers have found error-handling solutions to avoid such undesirable effects, but these techniques can be cumbersome:

function printLengthOfText(text) { if (typeof text !== 'string') { throw new Error('Argument is not a string!'); } console.log(text.length); } printLengthOfText(3); // Error: Argument is not a string!

In the code above, the function first checks the argument type. If it doesn’t match the expected type, an error is thrown; otherwise, it continues with its intended operation.

Before we explore how TypeScript handles this issue, let’s practice fixing some JavaScript type-related bugs. These are the kinds of problems that TypeScript helps us diagnose early on, before they become hard to spot.



This code looks pretty good! Let’s go ahead and run it using node index.js in the terminal. What could possibly go wrong?


AHHH! Error messages! They’re telling us that printOperations() should take in two numbers. It looks like it’s receiving two strings now. Change the function call (printOperations('6', '6')) to pass the right type of data. Then click “Run” to save your changes.


Try node index.js in the terminal one more time. No more error messages! But the function exclaim(name, count) still isn’t working properly. It is meant to print name a total of count times.

Do you spot the bug where it’s being called?

Fix the bug in exclaim()‘s function call so that the code prints 'Muriel!' six times.

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