I think the author may have gotten their TypeScript example wrong. Yes, TS is fine with passing more arguments to a callback that takes fewer. The callback can

I think the author may have gotten their TypeScript example wrong. Yes, TS is fine with passing more arguments to a callback that takes fewer. The callback cannot possibly use the additional arguments, so it doesn't matter what gets passed as it will not change the outcome. This is very different from passing the wrong kinds of arguments to functions that do read them and do something with them, like parseInt. Now, if you decide to pass a function with an optional second argument that matches the second argument that will get passed to the callback and expect that it will not be used because why would anyone pass additional arguments to a map callback - then yes, you will have the problem again. function addOneByDefault(num: number, addAmount = 1) { return num + addAmount } [1,2,3,4].map(addOneByDefault) // this typechecks but works poorly This extra example is missing in the article and might be helpful to add.