javascript 数组 排除null, undefined, 和不存在的元素

The most common way to loop through the elements of an array is with a for loop: 

var o = [1,2,3,4,5]
var keys = Object.keys(o); 　　　　　　　　　　　　// Get an array of property names for object o
var values = [] 　　　　　　　　　　　　　　　　　　// Store matching property values in this array
for(var i = 0; i < keys.length; i++) {　　　　  　　// For each index in the array
　　var key = keys[i]; 　　　　　　　　　　　　　　// Get the key at that index
　　values[i] = o[key];　　　　　　　　　　　　　　// Store the value in the values array
}

In nested loops, or other contexts where performance is critical, you may sometimes
see this basic array iteration loop optimized so that the array length is only looked up
once rather than on each iteration:

for(var i = 0, len = keys.length; i < len; i++) {
// loop body remains the same
}

These examples assume that the array is dense and that all elements contain valid data.
If this is not the case, you should test the array elements before using them. If you want
to exclude null, undefined, and nonexistent elements, you can write this:

　　for(var i = 0; i < a.length; i++) {
　　　　if (!a[i]) continue; // Skip null, undefined, and nonexistent elements
　　　　　　// loop body here
　　}

If you only want to skip undefined and nonexistent elements, you might write:
　　for(var i = 0; i < a.length; i++) {
　　　　if (a[i] === undefined) continue; // Skip undefined + nonexistent elements
　　　　// loop body here
　　}

Finally, if you only want to skip indexes for which no array element exists but still want
to handle existing undefined elements, do this:
　　for(var i = 0; i < a.length; i++) {
　　　　if (!(i in a)) continue ; // Skip nonexistent elements
　　　　　　// loop body here
　　}

You can also use a for/in loop (§5.5.4) with sparse arrays. This loop assigns enumerable property names (including array indexes) to the loop variable one at a time.

Indexes that do not exist will not be iterated:

　　for(var index in sparseArray) {
　　　　var value = sparseArray[index];
　　　　// Now do something with index and value
　　}