Questions about Coffeescript/Javascript conditional results

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?

user1114 · Accepted Answer · 2011-08-28 12:55:05Z

3

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.

edited Aug 28, 2011 at 12:55

answered Aug 28, 2011 at 12:49

user1114

1

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1 Comment

Trevor Burnham Over a year ago

Exactly. Confusing is not with isnt in CoffeeScript is a common mistake.

Douglas · Accepted Answer · 2011-08-28 12:54:06Z

1

This is fiddly:

console.log(index === !0);

It gets treated in the same way as:

console.log(index === (!0));

0 is a falsey constant, so you could replace (!0) with true. The real code is then:

console.log(index === true);

So it will only log "true" when index is boolean true without type coercion.

answered Aug 28, 2011 at 12:54

Douglas

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