javascript：令人惊讶的操作顺序

Question

I recently wrote code that didnt work as i would expect, it was:

message = 'Thank You';
type = 'success';

message = message || type == 'success' ? 'Success' : 'Error';

It was news to me that at the end of that message was set to 'Success' .

I would think that since the truthy value of message is true , the right side of the or would not evaluate.

Parenthesis around the right side of the OR solved this, but i still dont understand why the right side was evaluated at all

Answer 1

Your code is equivalent to

message = ( message || type == 'success' ) ? 'Success' : 'Error';

That's why. :)

Answer 2

The value of message doesn't end up as "success" but "Success" .

The ? operator has lower precedence than the || operator, so the code is evaluated as:

message = (message || type == 'success') ? 'Success' : 'Error';

The result of message || type == 'success' message || type == 'success' will be "Thank You" , and when that is evaluated as a boolean for the ? operator, the result is true .