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If I say "no way", I mean it's impossible. E.g.

A: Can you help me?
B: No way.

My question is what the "opposite" of "no way" is.
Can person A say?

A: Way. (or)
A: Yes way.

Are these correct responses?

Thanks.
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"No way, José!" one of my teachers used to say.

Simon, I don't think there is an exact opposite of "no way". "No way" shows strong refusal or opposition: "There's no way I'm agreeing to that!"

The responses you posted as affirmative answers to "Can you help me?" do not exist in English.

There are many affirmative answers to the question you posted. these are only some of them:

Can you help me?
Sure.
Sure thing.
Yes, (I can).
I'd love to.
Yes, of course I can.

Miriam
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I think 'if so' is more commonly used in cases such as this.
Cheers
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Comments  
In some social groups, "way" and "yes way" can be used, but they aren't common responses these days and are used only in a very casual manner.

EDIT :

Example

Jake : I just won 200 bucks at the pokies
Mike : No way!
Jake : Yes way, dude
Thanks for your replies.

I also want to know the "opposite" of "if not". E.g.

A: Are you a smoker? If not, please turn right.

If I mean the opposite, which of the following is better:

A: Are you a smoker? If YES, please turn right. (or)
A: Are you a smoker? If SO, please turn right.

Thanks.
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 Mike in Japan's reply was promoted to an answer.
To emphasise willingness to help by using "yes way" is very playful. I imagine that a 2nd lang speaker saying it would be problematic - most listeners would just think you got it wrong!!

You could say:
"Certainly"
or "absolutely"
to emphasise willingness to agree or help.