Question

Given the following string:

var str = "one,two,three";

If I split the string on the commas, I normally get an array, as expected:

var arr = str.split(/\s*,\s*/);

Trouble is that in Google Chrome (for Mac), it appends extra properties to the array.

Output from Chrome’s debugger:

arr: Array
    0: one
    1: two
    2: three
    constructor: function Array()
    index: undefined
    input: undefined
    length: 3

So if I iterate over the array with a for/in loop, it iterates over the new properties. Specifically the input and index properties. Using hasOwnProperty doesn’t seem to help.

A fix would be to do a for loop based on the length of the Array. Still I’m wondering if anyone has insight into why Chrome behaves this way. Firefox and Safari don’t have this issue.

Answer 1

Don’t iterate over arrays using for...in loops!! This is one of the many pitfalls of Javascript (plug) – for...in loops are for iterating over object properties only.

Use normal for loops instead.

for (var i=0, max = arr.length; i < max; i++) { ... } 

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Firefox and Safari’s ECMAScript/Javascript engines make those particular properties non-enumerable ({DontEnum} attribute), so they would not be iterated over in a for...in loop. Still, for...in loops were not intended to iterate over array indexes.