Coffee Existential Operator different compliations

Question

Similar to this topic: CoffeeScript Existential Operator and this

The coffeescript, I'm having a problem using the elvis operator:

foo = -> 
  y = foo()
  console.log 'y is null' unless y?
  console.log 'x is null' unless x?

Compiles to:

var foo, y;
foo = function() {};
y = foo();
if (y == null) {
  console.log('y is null');
}
if (typeof x === "undefined" || x === null) {
  console.log('x is null');
}

Output:

y is null
x is null

So the problem is that since y is assigned earlier that coffee takes the shortcut and assumes that y cannot be undefined. However, it is valid to return undefined from a function.

Is there a "safer" way to check that y is also not undefined?

UPDATED Clarified Examples and Explanation:

From the comments, in the first if statement (y == null) is using double equal instead of ( x === null) triple equal, as in the second if statement. Clever.

i think you're conflating three different things here: (1) does `y` exist / has it been declared? (2) does `y` have the value `undefined`? (3) does `y` have the value `null`? — flow, Oct 10 '13 at 14:06

score 3 · Accepted Answer · edited May 23 '17 at 11:50

3

? operator always checks if the value is neither null nor undefined.

y == null is absolutely correct way to check that the value of y is either null or undefined in JavaScript.

For example, the following CofeeScript code checks only for null values:

do_something() if y is null

which compiles to

if (y === null) do_something();

So, while y? (y == null in JS) checks for both null and undefined, y isnt null (y !== null in JS) checks only for null.

Check this answer for more detailed information.

See this answer for more info about equality checks in JavaScript.

edited May 23 '17 at 11:50

Community

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answered Oct 10 '13 at 12:00

Leonid Beschastny

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"? operator always checks if the value is neither null nor undefined." Thats what I thought, but in my example above it doesn't check for undefined only null. So y gets assigned to undefined, then fails the check against null. – bryce Oct 10 '13 at 14:12
2

@bryce look closely to your example, it checks for `y == null` which is actually `null` or `undefined`. Real `null` check is `y === null` and not `y == null`, see my example above. – Leonid Beschastny Oct 10 '13 at 14:46
@bryce See [this answer](http://stackoverflow.com/questions/359494/does-it-matter-which-equals-operator-vs-i-use-in-javascript-comparisons) for more info about equality checks in JavaScript. – Leonid Beschastny Oct 10 '13 at 14:48

Coffee Existential Operator different compliations

1 Answers1