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Last active March 1, 2018 04:29
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Bad codegen
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bad-codegen.md

The following code should evaluate to true:

((or int? string?) 1)

But instead it evaluates to the int? function. In other words this evaluates to true:

(= int? ((or int? string?) 1))

What???

This is because the JavaScript generated for ((or int? string?) 1) is

(cljs.core.int_QMARK_) || (cljs.core.string_QMARK_).call(null,(1))

which short-circuits, evaluating to cljs.core.int_QMARK_. The fix is the addition of some needed parens.

This likewise affects and. But to see that you need a prefix operator.

If you evaluate this code

(js-delete (and int? string?) "bogus-property")

it should be the same as

(js-delete string? "bogus-property")

which is harmless. But the JavaScript generated for the original form is

delete (cljs.core.int_QMARK_) && (cljs.core.string_QMARK_)["bogus-property"]

and, poof!, the int? function is gone with the wind. This now evaluates to false.

(exists? int?)

Fixes for these landed in ClojureScript master.

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