I was walking on the web and came across this question:
What do you say we get out of here?
It's seems to me a tad slangy. I don't quite follow the structure of this sentence. It's like something is missing. For me, this sentence would sound more grammatically correct with "if". For example:
What do you say if we get out of here?
So, my question is: is it slang? When can we use "if" and when can we do without it?
It is indeed informal (I don't use the word slang).
Your paraphrase is grammatical, but I don't think it has the same meaning. I don't think if is what's omitted.
I think the full version would read something like:
What do you say to the suggestion that we get out of here?
A more normal version would be
What do you say about (us) getting out of here?
Used like this what do you say is an idiom. Like many idioms, it uses unusual syntax and like other idioms, you should not try to change its form (except as intentional wordplay).
We use the idiom what do you say I/we X to make a suggestion:
What do you say we go to the movies after dinner => Let's go to the movies after dinner
It's very similar in meaning and use to how would you like to X, how would you like it if we X, or how about we X. This is to say, like those "how would you like" expressions, what do you say we X is maybe slightly less pushy than let's do X.
Grammatically correct or not, the problem is that adding an if completely changes the meaning of the sentence.
"What do you say we get out of here?" is what you'd ask a companion if you don't like the place you're at, for instance it's a really boring party and you'd rather be somewhere else.
"What do you say IF we get out of here?" implies that there's some doubt about your ability to get out. Say you're trapped in the dungeon with a horde of hungry monsters on your trail, you might turn to your companion and say something like that.
What do you say we get out of here?
It's informal, but it is more a question of punctuation (which is omitted in informal speech of this sort).
For example:
What do you say: we get out of here?
Or:
What do you say, [should] we get out of here?
To be slightly more informal, with much the same meaning:
Waddya say? Let's get outta here!
When we can use "if" and when we can do without it?
You don't use "if" here because it completely changes the meaning.
The use of "if" might change the meaning to:
If we make it out of here, what would you say?
"Gosh that was lucky"
[should]
As @NickGammon mentions, inserting an "if" changes the tone, almost to the point of changing the meaning.
"What do you say ..." is used to offer up a suggestion, and asks for another person's opinion. So the implied connector is "should".
If you want to turn this into language that conforms to the simplest rules of grammar (at the expense of being less like the way most people talk), I'd suggest
What do you say? Should we get out of here?
or
What do you say - should we get out of here?
(although I use the dash, "-", much more often than is rigorously correct, and wouldn't be surprised if a copy editor objected to that second version.
"What do you say (that) we get out of here" is grammatically correct and standard. It uses the subjunctive, much like "I'd rather you didn't smoke".
If anything is elided, it is not if but the complementizer that.
