You are almost correct -- it's just that it's not a "safe" assumption. In your example, the JS interpreter will not throw an error when one provides a number (

You are almost correct -- it's just that it's not a "safe" assumption. In your example, the JS interpreter will not throw an error when one provides a number (for example). Your code could break things. But why would this happen? It's completely unrelated to TypeScript. A number could be provided by the data, which in all likelihood is JSON from the network (eg: HTTP request or message queue body). Strongly typed languages will give you strongly typed data to protect you from this, but a dynamic language like JS doesn't afford you the same protection. TypeScript doesn't mitigate this issue -- nor could it. TypeScript ensures your plumbing fits together, not that the sludge flowing through it won't cause a mess. A strongly typed language will throw an exception before unexpected behavior can occur, JS on the other-hand will attempt to continue. It rarely happens, but since it does you can't say it safely works properly.