Javascript稀疏数组兼容性

Question

I'm in a situation where I'd like to add to the beginning of an array (instead of using .push() and adding to the end).

From what I can gather, using the .unshift() method is potentially not going to work in IE 8?

So I figured "that's ok.. I'll just asign values starting at the end of the array and working my way down"... but I'm wondering if that's ok in every browser?

Is it ok do this in all browsers?:

arr =[];
arr[2] = 'greg';

in chrome console that gives me an array of:

[undefined, undefined, 'greg']

Something about that just seems like it wouldn't work somewhere...

Answer 1

You get a sparsed array, which is iterable with the methods of Array, like Array#forEach

forEach() executes the provided callback once for each element present in the array in ascending order. It is not invoked for index properties that have been deleted or are uninitialized (ie on sparse arrays).

Caveat : The length property does not reflect the real count of items in the array. It is build upon the greatest index if the array.

The length property represents an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.

I suggest to have a look at this part Relationship between length and numerical properties as well for understanding arrays and the use of length and sparsed parts.

 var arr =[]; arr[2] = 'greg'; console.log(arr.length); // 3 arr.forEach(function (a, i) { console.log(i, a); // 2 greg });