遍历字符串数组会产生错误

Question

I pass in an array of error messages to parse. An example input would be:

"An item with this x Id already exists.
 An item with this y id already exists.
 An item with this barcode already exists.
"

That is, the string is literally each line above separated by a \\n, with a final \\n at the end.

function( msg )
{
  alert( "\"" + msg + "\"" );
  var aLines = msg.split( /\r?\n+/ );

  for ( var i in aLines )
  {
     if ( !aLines[i] ) { alert( "Error!" ); continue; }
     alert( i + ": \"" + aLines[i]  + "\"" );
  }
}

I split it into lines, and iterate over the lines. At index 3 there is no line and the first conditional triggers. Should that not be an empty line? eg ""

Then the loop actually goes one more element to 4, and shows a the contents of a function.

That is I get - five alerts:

0: "An item with this x Id already exists."
1: "An item with this y id already exists."
2: "An item with this barcode already exists."
Error!

The last one is most bizarre:

hasObject: "function(o) {
    var l = this.length + 1;
    ... more lines ...
}

I don't understand what is happening here. Why is it iterating over one more element? And why is the last element a function? And shouldn't offset 3 be an empty string? That is I should not be alerting "Error!" here.

Answer 1

Never loop over an array with for...in.

Iterates over the enumerable properties of an object , in arbitrary order.

Why is using "for...in" with array iteration a bad idea?

Answer 2

As jbabey said, using for .. in loops in Javascript is risky and undeterministic (random order--sometimes). You would most commonly use that for parsing through objects in associative arrays. But, if you insist on keeping the for .. in , wrap the inside of the for with an if block like such:

for (var i in aLines)
{
    if(aLines.hasOwnProperty(i))
    {
        // ... Do stuff here
    }
}

Otherwise, just change it to a classic incremental for loop to get rid of that error:

for (var i = 0; i < aLines.length; i++)
{
   if ( !aLines[i] ) { alert( "Error!" ); continue; }
   alert( i + ": \"" + aLines[i]  + "\"" );
}

Answer 3

You should use a regular for loop for arrays, because for-in will also return all the other property keys in the Array object. This is why you are seeing "hasObject" (and in my browser, I see a lot more after that): because your array has the function "hasObject", so when you enumerate all of Array's properties this comes up.

The correct for-loop:

  for ( var i = 0, ii = aLines.length; i<ii; i++ )
  {
    if ( !aLines[i] ) { alert( "Error!" ); continue; }
    alert( i + ": \"" + aLines[i]  + "\"" );
  }

Here is your code with the for-in loop replaced with a for loop, and it works as expected:

http://jsfiddle.net/SamFent/4rzTh/

Answer 4

You are getting Error! in end because the splitting is giving you empty line "" and when you check for it with if(!aLines[i]) it returns true because it is empty/null/nothing. You can check it here in a fiddle , you remove the empty line from end and it does not go over array 4 times.

I have also put in following code which shows alert:

    var a="";
    if(!a){
        alert("!a");
    }