Use 'extends' keyword to narrow the value of a generic
The extends keyword is very powerful in TypeScript. Here, I use it to narrow the value of a generic to enable some beautiful autocomplete/inference.
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You can use the extends keyword to narrow the scope of a generic to make it more useful. Here, for instance, we want to get a deep value from an object, and getDeepValue is going to take in the object and then take in the two keys that it's going to use to grab the value.
We can use extends here to say the FirstKey needs to be keyof Obj, which is our object. Now, we get some nice autocompletes on this deep value, which is either foo or bar. Let's say it's bar. This second key is now going to extends keyof Obj, FirstKey. That means that we get this bar. When we select bar, this is going to be C or D. When we select foo, this is going to be A or B.
Let's say it's A, for instance. We now need to work out what this result is going to be. It's currently typed as any. Here, we can say Obj, FirstKey, SecondKey, just like a property access is JavaScript. The result is a Boolean. If I change that to B, then it becomes a number.
