Instance of Array returning error

Question

I have a simple check to see if a particular set of keys is an array and if not create it but for some reason the if statement returns an error, this is what I am trying to do:

//test data
var i = 0;
var map = new Array(); 
var Data[i]['x'] = 6;
var Data[i]['y'] = 7;

if(!map[Data[i]['x']] instanceof Array){
   map[Data[i]['x']] = new Array();
}

if(!map[Data[i]['x']][Data[i]['y']] instanceof Array){ //error on this line
   map[Data[i]['x']][Data[i]['y']] = new Array();
}

The error is:

Uncaught TypeError: Cannot read property '6' of undefined

This error is occuring on the second IF statement. What is the mistake I am making here?

Can you post a demo to jsfiddle?

Michael Berkowski
– Michael Berkowski

2012-12-29 02:45:35 +00:00
Commented Dec 29, 2012 at 2:45 — Michael Berkowski
– Michael Berkowski, Commented Dec 29, 2012 at 2:45
You haven't declared i

Lee Taylor
– Lee Taylor

2012-12-29 02:47:08 +00:00
Commented Dec 29, 2012 at 2:47 — Lee Taylor
– Lee Taylor, Commented Dec 29, 2012 at 2:47
oops i forgot to paste i into the question :P

Sir
– Sir

2012-12-29 02:47:44 +00:00
Commented Dec 29, 2012 at 2:47 — Sir
– Sir, Commented Dec 29, 2012 at 2:47
@MichaelBerkowski jsfiddle.net/bH6aG/1

Sir
– Sir

2012-12-29 02:49:23 +00:00
Commented Dec 29, 2012 at 2:49 — Sir
– Sir, Commented Dec 29, 2012 at 2:49

Barmar · Accepted Answer · 2012-12-29 02:49:55Z

5

In the first if statement:

if(!map[Data[i]['x']] instanceof Array)

is being parsed as:

if((!map[Data[i]['x']]) instanceof Array)

At that point, map[6] is undefined, so !map[6] is true, and true is not an instance of Array. So it doesn't set it to new Array().

Change it to:

if (!(map[Data[i]['x']] instanceof Array))

answered Dec 29, 2012 at 2:49

Barmar

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