According to ES10.0, If an object does not define or inherit @@hasInstance it uses the default instanceof semantics. When changing the value of __proto__ of an object into String, the algorithm to be used will be Function.prototype [ @@hasInstance ] instead of the default semantics. So the output of the testcase below is false as other engines do. This may be an issue of chakra.

version

v1.11.19

command

 chakra/cc_linux_x64_1_11_19/ChakraCoreFiles/bin/ch testcase.js

testcase

var  obj = {__proto__:String};
var result = "hello" instanceof obj;
print(result);

output

TypeError: obj is not a function. (evaluating '"hello" instanceof obj')

expected output

false

contributor:@YuanWangC