What's an "auto-casting bool"?

On the following answer to a previous question someone mentioned an "auto-casting bool"

I guess null has an auto-casting bool开发者_JAVA百科 that is false. What is it, and what does the code that makes it look like?

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The phrase "auto-casting bool" is a poor phrase someone used off hand. I believe what they mean is the internal ToBoolean operation

Of special note is the if statement which calls ToBoolean on the expression.

I don't know my way around the v8 source code but here is a search for ToBoolean on the v8 repo.

For reference v8 is the javascript implementation used by chrome and written in C++