javascript - How do assertion libraries such as Chai work without forcing a call to a function? -

in chai, can stuff following:

expect({}).to.exist; 

exist not function call, still works in testing frameworks. opposite (expect({}).to.not.exist) causes tests fail, again, exist not function call.

how these assertions work without making me call function? in fact, if try expect({}).to.exist() test fails because exist not function.

i figured out (or @ least, figured out a method). use javascript getters:

var throws = {   a() {     throw new error('a');   },   b() {     throw new error('b');   },   c() {     throw new error('c');   } }; 

when doing throws.a, throws.b, or throws.c, appropriate error thrown.

from point rather easy build assertions contained within chai.