Javascript 中的条件不起作用

Question

I'm about to get mad because of those conditions in JavaScript. Why this one doesn't work?

if (isNaN(first) || isNaN(second) && (act !== '-' || act !== '+'))

It should check is first is number, second is number and act is minus or plus. It's all right with numbers but it doesn't check act correct. What's wrong?

Eg first is 1, act is a, second is 2. It won't work.

PS

I've some knowledge in C++ and Java. I've never had any problems with it. But since the time I've started to learn Javascript, I get so many mistakes like this.

Is there different system of such expressions?

Answer 1

It should check is first is number, second is number and act is minus or plus. It's all right with numbers but it doesn't check act correct.

Thus for the requirement stated above the correct statement would be

if ( !isNaN(first) && !isNaN(second) && (act == '-' || act == '+'))

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If you want to invert the condition, that would be

if ( !(!isNaN(first) && !isNaN(second) && (act == '-' || act == '+')))

which is equivalent to

if ( isNaN(first) || isNaN(second) || (act != '-' && act != '+'))