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The point is, why use it at all if both values are equivalent to "missing"? I don't see a practical difference between undefined and null in the vast majority of cases, which is the the author is arguing.

Again: In the vast majority of cases. I'm sure you'll find your exceptions.

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I've worked with a ton of APIs where these two aren't equivalent.

Consider partial object updates. "null" means "explicitly set a field to null", whereas undefined means "don't change the value of the field". The distinction is meaningful.

In the first case I lost the information that the key exists and can contain a value.

The second form makes it explicit.

On the first case, the parser that's getting the data may want to calculate a sensible value. On the second one, it's clear the value is known.

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