Easiest way to check subproperty exists

Question

I basically want to do var a = b.c.d || null

but the issue is, a might be undefined, b might be, or c might be

so short of ugly try{} catch{} or if statements what is the best way to do this? this is for constructing a quite large object (say 50 odd properties) from a 3rd party data source

This has been asked a couple of times already: [`[javascript] nested property exists`](http://stackoverflow.com/search?q=%5Bjavascript%5D+nested+property+exists). Does really none of those solve your issue? Why not? — Felix Kling, Jun 03 '15 at 08:29

score 2 · Accepted Answer · edited Jun 03 '15 at 08:45

2

var a = ( b && b.c && ('d' in b.c)) ? b.c.d : null;

edited Jun 03 '15 at 08:45

zerkms

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answered Jun 03 '15 at 08:31

dopeddude

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How about `b.c.d == 0`? – zerkms Jun 03 '15 at 08:31
@zerkms: Thanks, corrected :-) – dopeddude Jun 03 '15 at 08:39
How about `b.c.d == false`? – zerkms Jun 03 '15 at 08:40
@zerkms yea.... there would be another case like empty string... corrected to check the property existence on the parent object.. – dopeddude Jun 03 '15 at 08:44
@zerkms can you please bother to explain how the `let` construct used in this of your comment : `let { c: { d: a = null } = {} } = b;` work? – dopeddude Jun 03 '15 at 08:47
Sure: http://www.2ality.com/2015/01/es6-destructuring.html – zerkms Jun 03 '15 at 08:47

steo · Answer 2 · 2015-06-03T08:34:34.850

var a;
if(b !== undefined && b.c !== undefined && b.c.d !== undefined){
   a = b.c.d;
} else {
 a = null;
}

Shorthand

var a = b !== undefined && b.c !== undefined && b.c.d !== undefined ? b.c.d : null

Every condition will exit immediatly the statement if undefined as long as the logical operator is && This will work for undefined value of b b.c and b.c.d

I am wondering that you may try with a boolean casting, something like

var a = !!b && !!(b.c) && !!(b.c.d) ? b.c.d : null

This will fail for value 0 of b.c.d

score -1 · Answer 3 · answered Jun 03 '15 at 08:31

-1

In my opinion try catch in this case is not ugly: with a try I am asking the code to see if b.c.d exists and a ReferenceError in this case is some kind of "expected behaviour". Remember that an Exception is not necessary an error. There are many frameworks like TurboGears that uses Exception to change the program flow rapidly and not necessary when raising an Error.

answered Jun 03 '15 at 08:31

pinturic

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ok, now write 50 try ctaches and tell me its not ugly ;) – DrogoNevets Jun 03 '15 at 10:31
only one is needed: try { ... // all the code } catch { } – pinturic Jun 03 '15 at 10:46
and it fails on one attribute and everything after it will not get parsed, no this is not right – DrogoNevets Jun 04 '15 at 09:39
Yes it is : var a = null; try { a = b.c.d.e} catch {a = null}; . Very easy – pinturic Jun 04 '15 at 11:03
now put 50 into there like I said in the original post..... – DrogoNevets Jun 04 '15 at 13:19
var a = null; try { a = b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z} catch {a = null}; – pinturic Jun 04 '15 at 14:01
no, that is one very long one isn't it?! – DrogoNevets Jun 04 '15 at 14:07
Which is the shortest solution? – pinturic Jun 05 '15 at 07:42

Easiest way to check subproperty exists

3 Answers3