_APPLY() construct has not been implemented in Saxon-JS
The fn:apply() function is not fully implemented in Saxon-JS, since Saxon-JS does not support the higher-order functions feature. However, it is allowed for cases where the function called is a map or array.
The error arises when a dynamic function call on a map or array has been translated (at compile time) to an apply call. Note that how this is done is slightly different for 9.7 and 9.8.
For example, see XSLT 3.0 test -s:maps -t:maps-001.
The SEF produced by 9.7 contains:
<fn name="_APPLY" dyn=".">
The SEF produced by 9.8 contains:
<ifCall xmlns:saxon="http://saxon.sf.net/" name="saxon:apply" type="item()*">
Neither of these constructs is implemented in Saxon-JS 0.9.
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