Question

I have this Coffeescript:

console.log 'TEST'
console.log index
console.log (index is not 0)
console.log (index > 0)
unless index is 0
    console.log "passed test"

This is the compiled Javascript:

console.log('TEST');
console.log(index);
console.log(index === !0);
console.log(index > 0);
_results.push(index !== 0 ? console.log("passed test") : void 0);

This is the console output

TEST
0
false
false
passed test
TEST
1
false
true
passed test

Question 1) Why does (index is not 0) return false when index is 1? (index > 0) returns true for 1, so why doesn’t (index is not 0)?

Question 2) Why does the unless index is 0 test get passed when index is 0?

Answer 1

Why does (index is not 0) return false when index is 1? (index > 0) returns true for 1, so why doesn’t (index is not 0)?

CoffeeScript doesn’t use is not for inequality, it uses != and isnt. By looking at the compiled code, we can see that it’s actually interpreting (index is not 0) as (index is (not 0)).

Why does the unless index is 0 test get passed when index is 0?

When I tried it myself the test did not pass. This behaviour is probably being caused by something in your testing code that you haven’t included in your post.