异常的node.js行为

Question

Today i saw this code, that was running in node.js environment. (>node.exe test.js)

var param = (typeof module !== "undefined" && module.exports) || {};

(function(exports){

   console.log(exports === module.exports);

})(param);

And this log returned true.

Can anybody explain me such behavior?

Thanks in advance.

Answer 1

If module is not undefined (which it isn't since it is the default object) and module.exports is a truthy thing (which it is by default), then exports is assigned to param and passed to the function.

exports is then compared to module.exports , and they are the same because module.exports is where the object came from in the first place.

( exports wouldn't be the same as module.exports if it was running elsewhere (eg a browser where you get window , not module ) since {} would be assigned to param instead.)

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Update re comments on the question:

Hmm, maybe it's wrong, but i thought that ((typeof module ..) || {}) will return true, but not "exports" object

No. && will (working left to right) evaluate as the first falsey thing it tests or (if everything is truthy) the last truthy thing it tests.

typeof module !== "undefined" is true so it tests module.exports , which is also true so it returns module.exports .

(The || returns the first truthy or last falsey thing it tests, so it then returns module.exports )

Answer 2

var d = (a && b) || c

d evaluates to b if a is true. If a is false, d evaluates to c .