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Why do i get just a number from console.log(Array.push()); in Javascript? [duplicate]

Are there any substantial reasons why modifying Array.push() to return the object pushed rather than the length of the new array might be a bad idea?
I don't know if this has already been proposed or asked before; Google searches returned only a myriad number of questions related to the current functionality of Array.push().
Here's an example implementation of this functionality, feel free to correct it:
;(function() {
var _push = Array.prototype.push;
Array.prototype.push = function() {
return this[_push.apply(this, arguments) - 1];
}
}());
You would then be able to do something like this:
var someArray = [],
value = "hello world";
function someFunction(value, obj) {
obj["someKey"] = value;
}
someFunction(value, someArray.push({}));
Where someFunction modifies the object passed in as the second parameter, for example. Now the contents of someArray are [{"someKey": "hello world"}].
Are there any drawbacks to this approach?

See my detailed answer here
TLDR;
You can get the return value of the mutated array, when you instead add an element using array.concat[].
concat is a way of "adding" or "joining" two arrays together. The awesome thing about this method, is that it has a return value of the resultant array, so it can be chained.
newArray = oldArray.concat[newItem];
This also allows you to chain functions together
updatedArray = oldArray.filter((item) => {
item.id !== updatedItem.id).concat[updatedItem]};
Where item = {id: someID, value: someUpdatedValue}
The main thing to notice is, that you need to pass an array to concat.
So make sure that you put your value to be "pushed" inside a couple of square brackets, and you're good to go.
This will give you the functionality you expected from push()
You can use the + operator to "add" two arrays together, or by passing the arrays to join as parameters to concat().
let arrayAB = arrayA + arrayB;
let arrayCD = concat(arrayC, arrayD);
Note that by using the concat method, you can take advantage of "chaining" commands before and after concat.

Are there any substantial reasons why modifying Array.push() to return the object pushed rather than the length of the new array might be a bad idea?
Of course there is one: Other code will expect Array::push to behave as defined in the specification, i.e. to return the new length. And other developers will find your code incomprehensible if you did redefine builtin functions to behave unexpectedly.
At least choose a different name for the method.
You would then be able to do something like this: someFunction(value, someArray.push({}));
Uh, what? Yeah, my second point already strikes :-)
However, even if you didn't use push this does not get across what you want to do. The composition that you should express is "add an object which consist of a key and a value to an array". With a more functional style, let someFunction return this object, and you can write
var someArray = [],
value = "hello world";
function someFunction(value, obj) {
obj["someKey"] = value;
return obj;
}
someArray.push(someFunction(value, {}));

Just as a historical note -- There was an older version of JavaScript -- JavaScript version 1.2 -- that handled a number of array functions quite differently.
In particular to this question, Array.push did return the item, not the length of the array.
That said, 1.2 has been not been used for decades now -- but some very old references might still refer to this behavior.
http://web.archive.org/web/20010408055419/developer.netscape.com/docs/manuals/communicator/jsguide/js1_2.htm

By the coming of ES6, it is recommended to extend array class in the proper way , then , override push method :
class XArray extends Array {
push() {
super.push(...arguments);
return (arguments.length === 1) ? arguments[0] : arguments;
}
}
//---- Application
let list = [1, 3, 7,5];
list = new XArray(...list);
console.log(
'Push one item : ',list.push(4)
);
console.log(
'Push multi-items :', list.push(-9, 2)
);
console.log(
'Check length :' , list.length
)

Method push() returns the last element added, which makes it very inconvenient when creating short functions/reducers. Also, push() - is a rather archaic stuff in JS. On ahother hand we have spread operator [...] which is faster and does what you needs: it exactly returns an array.
// to concat arrays
const a = [1,2,3];
const b = [...a, 4, 5];
console.log(b) // [1, 2, 3, 4, 5];
// to concat and get a length
const arrA = [1,2,3,4,5];
const arrB = [6,7,8];
console.log([0, ...arrA, ...arrB, 9].length); // 10
// to reduce
const arr = ["red", "green", "blue"];
const liArr = arr.reduce( (acc,cur) => [...acc, `<li style='color:${cur}'>${cur}</li>`],[]);
console.log(liArr);
//[ "<li style='color:red'>red</li>",
//"<li style='color:green'>green</li>",
//"<li style='color:blue'>blue</li>" ]

var arr = [];
var element = Math.random();
assert(element === arr[arr.push(element)-1]);

How about doing someArray[someArray.length]={} instead of someArray.push({})? The value of an assignment is the value being assigned.
var someArray = [],
value = "hello world";
function someFunction(value, obj) {
obj["someKey"] = value;
}
someFunction(value, someArray[someArray.length]={});
console.log(someArray)

How do I remove all the elements of an array using splice [duplicate]

This question's answers are a community effort. Edit existing answers to improve this post. It is not currently accepting new answers or interactions.
Is there a way to empty an array and if so possibly with .remove()?
For instance,
A = [1,2,3,4];
How can I empty that?

Ways to clear an existing array A:
Method 1
(this was my original answer to the question)
A = [];
This code will set the variable A to a new empty array. This is perfect if you don't have references to the original array A anywhere else because this actually creates a brand new (empty) array. You should be careful with this method because if you have referenced this array from another variable or property, the original array will remain unchanged. Only use this if you only reference the array by its original variable A.
This is also the fastest solution.
This code sample shows the issue you can encounter when using this method:
var arr1 = ['a','b','c','d','e','f'];
var arr2 = arr1; // Reference arr1 by another variable
arr1 = [];
console.log(arr2); // Output ['a','b','c','d','e','f']
Method 2 (as suggested by Matthew Crumley)
A.length = 0
This will clear the existing array by setting its length to 0. It also works when using "strict mode" in ECMAScript 5 because the length property of an array is a read/write property.
Method 3 (as suggested by Anthony)
A.splice(0,A.length)
Using .splice() will work perfectly, but since the .splice() function will return an array with all the removed items, it will actually return a copy of the original array. Benchmarks suggest that this has no effect on performance whatsoever.
Method 4 (as suggested by tanguy_k)
while(A.length > 0) {
A.pop();
}
This solution is not very succinct, and it is also the slowest solution, contrary to earlier benchmarks referenced in the original answer.
Performance
Of all the methods of clearing an existing array, methods 2 and 3 are very similar in performance and are a lot faster than method 4. See this benchmark.
As pointed out by Diadistis in their answer below, the original benchmarks that were used to determine the performance of the four methods described above were flawed. The original benchmark reused the cleared array so the second iteration was clearing an array that was already empty.
The following benchmark fixes this flaw: http://jsben.ch/#/hyj65. It clearly shows that methods #2 (length property) and #3 (splice) are the fastest (not counting method #1 which doesn't change the original array).
This has been a hot topic and the cause of a lot of controversy. There are actually many correct answers and because this answer has been marked as the accepted answer for a very long time, I will include all of the methods here.

If you need to keep the original array because you have other references to it that should be updated too, you can clear it without creating a new array by setting its length to zero:
A.length = 0;

Here the fastest working implementation while keeping the same array ("mutable"):
function clearArray(array) {
while (array.length > 0) {
array.pop();
}
}
FYI it cannot be simplified to while (array.pop()): the tests will fail.
FYI Map and Set define clear(), it would have seem logical to have clear() for Array too.
TypeScript version:
function clearArray<T>(array: T[]) {
while (array.length > 0) {
array.pop();
}
}
The corresponding tests:
describe('clearArray()', () => {
test('clear regular array', () => {
const array = [1, 2, 3, 4, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
test('clear array that contains undefined and null', () => {
const array = [1, undefined, 3, null, 5];
clearArray(array);
expect(array.length).toEqual(0);
expect(array[0]).toEqual(undefined);
expect(array[4]).toEqual(undefined);
});
});
Here the updated jsPerf: http://jsperf.com/array-destroy/32 http://jsperf.com/array-destroy/152
jsPerf offline. Similar benchmark: https://jsben.ch/hyj65

A more cross-browser friendly and more optimal solution will be to use the splice method to empty the content of the array A as below:
A.splice(0, A.length);

The answers that have no less that 2739 upvotes by now are misleading and incorrect.
The question is: "How do you empty your existing array?" E.g. for A = [1,2,3,4].
Saying "A = [] is the answer" is ignorant and absolutely incorrect. [] == [] is false.
This is because these two arrays are two separate, individual objects, with their own two identities, taking up their own space in the digital world, each on its own.
Let's say your mother asks you to empty the trash can.
You don't bring in a new one as if you've done what you've been asked for.
Instead, you empty the trash can.
You don't replace the filled one with a new empty can, and you don't take the label "A" from the filled can and stick it to the new one as in A = [1,2,3,4]; A = [];
Emptying an array object is the easiest thing ever:
A.length = 0;
This way, the can under "A" is not only empty, but also as clean as new!
Furthermore, you are not required to remove the trash by hand until the can is empty! You were asked to empty the existing one, completely, in one turn, not to pick up the trash until the can gets empty, as in:
while(A.length > 0) {
A.pop();
}
Nor, to put your left hand at the bottom of the trash, holding it with your right at the top to be able to pull its content out as in:
A.splice(0, A.length);
No, you were asked to empty it:
A.length = 0;
This is the only code that correctly empties the contents of a given JavaScript array.

Performance test:
http://jsperf.com/array-clear-methods/3
a = []; // 37% slower
a.length = 0; // 89% slower
a.splice(0, a.length) // 97% slower
while (a.length > 0) {
a.pop();
} // Fastest

You can add this to your JavaScript file to allow your arrays to be "cleared":
Array.prototype.clear = function() {
this.splice(0, this.length);
};
Then you can use it like this:
var list = [1, 2, 3];
list.clear();
Or if you want to be sure you don't destroy something:
if (!Array.prototype.clear) {
Array.prototype.clear = function() {
this.splice(0, this.length);
};
}
Lots of people think you shouldn't modify native objects (like Array), and I'm inclined to agree. Please use caution in deciding how to handle this.

You can easily create a function to do that for you, change the length or even add it to native Array as remove() function for reuse.
Imagine you have this array:
var arr = [1, 2, 3, 4, 5]; //the array
OK, just simply run this:
arr.length = 0; //change the length
and the result is:
[] //result
easy way to empty an array...
Also using loop which is not necessary but just another way to do that:
/* could be arr.pop() or arr.splice(0)
don't need to return as main array get changed */
function remove(arr) {
while(arr.length) {
arr.shift();
}
}
There are also tricky way which you can think about, for example something like this:
arr.splice(0, arr.length); //[]
So if arr has 5 items, it will splice 5 items from 0, which means nothing will remain in the array.
Also other ways like simply reassign the array for example:
arr = []; //[]
If you look at the Array functions, there are many other ways to do this, but the most recommended one could be changing the length.
As I said in the first place, you can also prototype remove() as it's the answer to your question. you can simply choose one of the methods above and prototype it to Array object in JavaScript, something like:
Array.prototype.remove = Array.prototype.remove || function() {
this.splice(0, this.length);
};
and you can simply call it like this to empty any array in your javascript application:
arr.remove(); //[]

If you are using
a = [];
Then you are assigning new array reference to a, if reference in a is already assigned to any other variable, then it will not empty that array too and hence garbage collector will not collect that memory.
For ex.
var a=[1,2,3];
var b=a;
a=[];
console.log(b);// It will print [1,2,3];
or
a.length = 0;
When we specify a.length, we are just resetting boundaries of the array and memory for rest array elements will be connected by garbage collector.
Instead of these two solutions are better.
a.splice(0,a.length)
and
while(a.length > 0) {
a.pop();
}
As per previous answer by kenshou.html, second method is faster.

There is a lot of confusion and misinformation regarding the while;pop/shift performance both in answers and comments. The while/pop solution has (as expected) the worst performance. What's actually happening is that setup runs only once for each sample that runs the snippet in a loop. eg:
var arr = [];
for (var i = 0; i < 100; i++) {
arr.push(Math.random());
}
for (var j = 0; j < 1000; j++) {
while (arr.length > 0) {
arr.pop(); // this executes 100 times, not 100000
}
}
I have created a new test that works correctly :
http://jsperf.com/empty-javascript-array-redux
Warning: even in this version of the test you can't actually see the real difference because cloning the array takes up most of the test time. It still shows that splice is the fastest way to clear the array (not taking [] into consideration because while it is the fastest it's not actually clearing the existing array).

Array.prototype.clear = function() {
this.length = 0;
};
And call it: array.clear();

In case you are interested in the memory allocation, you may compare each approach using something like this jsfiddle in conjunction with chrome dev tools' timeline tab. You will want to use the trash bin icon at the bottom to force a garbage collection after 'clearing' the array. This should give you a more definite answer for the browser of your choice. A lot of answers here are old and I wouldn't rely on them but rather test as in #tanguy_k's answer above.
(for an intro to the aforementioned tab you can check out here)
Stackoverflow forces me to copy the jsfiddle so here it is:
<html>
<script>
var size = 1000*100
window.onload = function() {
document.getElementById("quantifier").value = size
}
function scaffold()
{
console.log("processing Scaffold...");
a = new Array
}
function start()
{
size = document.getElementById("quantifier").value
console.log("Starting... quantifier is " + size);
console.log("starting test")
for (i=0; i<size; i++){
a[i]="something"
}
console.log("done...")
}
function tearDown()
{
console.log("processing teardown");
a.length=0
}
</script>
<body>
<span style="color:green;">Quantifier:</span>
<input id="quantifier" style="color:green;" type="text"></input>
<button onclick="scaffold()">Scaffold</button>
<button onclick="start()">Start</button>
<button onclick="tearDown()">Clean</button>
<br/>
</body>
</html>
And you should take note that it may depend on the type of the array elements, as javascript manages strings differently than other primitive types, not to mention arrays of objects. The type may affect what happens.

Use a modified version of Jan's initial suggestion:
var originalLength = A.length;
for (var i = originalLength; i > 0; i--) {
A.pop();
}
Terser:
for (let i = A.length; i > 0;A.pop(),i--) {}
Or here's another take:
while(!A[Symbol.iterator]().next().done)A.shift()

A.splice(0);
I just did this on some code I am working on. It cleared the array.

If you use constants then you have no choice:
const numbers = [1, 2, 3]
You can not reasign:
numbers = []
You can only truncate:
numbers.length = 0

To Empty a Current memory location of an array use: 'myArray.length = 0' or 'myArray.pop() UN-till its length is 0'
length : You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases.
pop() : The pop method removes the last element from an array and returns that returns the removed value.
shift() : The shift method removes the element at the zeroeth index and shifts the values at consecutive indexes down, then returns the removed value.
Example:
var arr = ['77'];
arr.length = 20;
console.log("Increasing : ", arr); // (20) ["77", empty × 19]
arr.length = 12;
console.log("Truncating : ", arr); // (12) ["77", empty × 11]
var mainArr = new Array();
mainArr = ['1', '2', '3', '4'];
var refArr = mainArr;
console.log('Current', mainArr, 'Refered', refArr);
refArr.length = 3;
console.log('Length: ~ Current', mainArr, 'Refered', refArr);
mainArr.push('0');
console.log('Push to the End of Current Array Memory Location \n~ Current', mainArr, 'Refered', refArr);
mainArr.poptill_length(0);
console.log('Empty Array \n~ Current', mainArr, 'Refered', refArr);
Array.prototype.poptill_length = function (e) {
while (this.length) {
if( this.length == e ) break;
console.log('removed last element:', this.pop());
}
};
new Array() | [] Create an Array with new memory location by using Array constructor or array literal.
mainArr = []; // a new empty array is addressed to mainArr.
var arr = new Array('10'); // Array constructor
arr.unshift('1'); // add to the front
arr.push('15'); // add to the end
console.log("After Adding : ", arr); // ["1", "10", "15"]
arr.pop(); // remove from the end
arr.shift(); // remove from the front
console.log("After Removing : ", arr); // ["10"]
var arrLit = ['14', '17'];
console.log("array literal « ", indexedItem( arrLit ) ); // {0,14}{1,17}
function indexedItem( arr ) {
var indexedStr = "";
arr.forEach(function(item, index, array) {
indexedStr += "{"+index+","+item+"}";
console.log(item, index);
});
return indexedStr;
}
slice() : By using slice function we get an shallow copy of elements from the original array, with new memory address, So that any modification on cloneArr will not affect to an actual|original array.
var shallowCopy = mainArr.slice(); // this is how to make a copy
var cloneArr = mainArr.slice(0, 3);
console.log('Main', mainArr, '\tCloned', cloneArr);
cloneArr.length = 0; // Clears current memory location of an array.
console.log('Main', mainArr, '\tCloned', cloneArr);

I'm surprised no one has suggested this yet:
let xs = [1,2,3,4];
for (let i in xs)
delete xs[i];
This yields an array in quite a different state from the other solutions. In a sense, the array has been 'emptied':
xs
=> Array [ <4 empty slots> ]
[...xs]
=> Array [ undefined, undefined, undefined, undefined ]
xs.length
=> 4
xs[0]
=> ReferenceError: reference to undefined property xs[0]
You can produce an equivalent array with [,,,,] or Array(4)

Remove all elements from array of strings that do not contain "IN"

I need to remove all elements in an array that do not contain "IN" in uppercase exactly like that.
How I thought of doing this was to traverse the array with a for loop and write all values that contain IN to another array.
Is there a way I can do it without writing to a new array and just removing those items that don't match from the current array?
Here is the code for how I was planning on doing it:
arrTwo = [];
for(var i = 0; i<arr.length; i++){
if(arr[i].indexOf('IN') > -1) arrTwo.push[arr[i]];
}

You can use ES5 filter method:
arr = arr.filter(function(s){
return ~s.indexOf("IN");
});
And using ES6 arrow functions, it can be simplified to:
arr = arr.filter(s=>~s.indexOf("IN"));

Here's a really good thread that has a couple of ways to accomplish this. If you do not delete the element of the array in the correct manner, you that element will be undefined rather than actually deleted. The .spilce() method is what you want to look into.
Deleting array elements in JavaScript - delete vs splice

I would do it using the splice() method:
var testArray = [ 'this one contains IN', 'this one does not' ];
function filterArray ( arr ) {
var i = arr.length;
//-- Loop through the array in reverse order since we are modifying the array.
while (i--) {
if (arr[i].indexOf('IN') < 0) {
//-- splice will remove the non-matching element
arr.splice(i, 1);
}
}
}
filterArray( testArray );
document.body.innerText = JSON.stringify(testArray);
JSFiddle: http://jsfiddle.net/5DW8L/1/

How to create a list of unique items in JavaScript? [duplicate]

This question already has answers here:
Get all unique values in a JavaScript array (remove duplicates)
(91 answers)
Closed 1 year ago.
In my CouchDB reduce function I need to reduce a list of items to the unique ones.
Note: In that case it's ok to have a list, it will be a small number of items of string type.
My current way is to set keys of a object, then return the keys of that object
since the place the code can't use things like _.uniq for example.
I'd like to find a more elegant way to spell it than this.
function(keys, values, rereduce) {
// values is a Array of Arrays
values = Array.concat.apply(null, values);
var uniq = {};
values.forEach(function(item) { uniq[item] = true; });
return Object.keys(uniq);
}

The best method seem to be using ES6 and Set. Single line and faster* than above according to fiddle
const myList = [1,4,5,1,2,4,5,6,7];
const unique = [...new Set(myList)];
console.log(unique);
*tested in safari

2021 answer:
const unique = (arr) => [...new Set(arr)];
unique([1, 2, 2, 3, 4, 4, 5, 1]); // [1, 2, 3, 4, 5]
Here you just create a set from the given array and then convert it back to the array.
I measured performance and it's almost twice faster now than the approach proposed in the old answer I posted before. Also, it's just a one-liner.
Updated fiddle
Old answer just for the record:
Commonly, the approach you used is a good idea.
But I could propose a solution that will make the algorithm a lot faster.
function unique(arr) {
var u = {}, a = [];
for(var i = 0, l = arr.length; i < l; ++i){
if(!u.hasOwnProperty(arr[i])) {
a.push(arr[i]);
u[arr[i]] = 1;
}
}
return a;
}
As you can see we have only one loop here.
I've made an example that is testing both your and my solutions. Try to play with it.

An alternative that's suitable for small lists would be to ape the Unix command line approach of sort | uniq:
function unique(a) {
return a.sort().filter(function(value, index, array) {
return (index === 0) || (value !== array[index-1]);
});
}
This function sorts the argument, and then filters the result to omit any items that are equal to their predecessor.
The keys-based approach is fine, and will have better performance characteristics for large numbers of items (O(n) for inserting n items into a hashtable, compared to O(n log n) for sorting the array). However, this is unlikely to be noticeable on small lists. Moreover, with this version you could modify it to use a different sorting or equality function if necessary; with hash keys you're stuck with JavaScripts notion of key equality.

This should work with anything, not just strings:
export const getUniqueList = (a: Array<any>) : Array<any> => {
const set = new Set<any>();
for(let v of a){
set.add(v);
}
return Array.from(set);
};
the above can just be reduced to:
export const getUniqueValues = (a: Array<any>) => {
return Array.from(new Set(a));
};
:)

To get unique objects, you can use JSON.stringify and JSON.parse:
const arr = [{test: "a"}, {test: "a"}];
const unique = Array.from(new Set(arr.map(JSON.stringify))).map(JSON.parse);
console.log(unique);

Using Object.keys will give you strings if you put in integer arguments (uniq([1,2,3]) => ['1','2','3']. Here's one with Array.reduce:
function uniq(list) {
return list.reduce((acc, d) => acc.includes(d) ? acc : acc.concat(d), []);
}

This is an old question, I know. However, it is at the top of some google searches, so I wanted to add that you can combine the answers from #RobHague and #EugeneNaydenov using the following:
function unique(arr) {
const u = {};
return arr.filter((v) => {
return u[v] = !u.hasOwnProperty(v);
});
};
You can also ignore undefined values (often handy) by adding:
function unique(arr) {
const u = {};
return arr.filter((v) => {
return u[v] = (v !== undefined && !u.hasOwnProperty(v));
});
};
You can play with this solution here: https://jsfiddle.net/s8d14v5n/

I find the other answers to be rather complicated for no gain that I can see.
We can use the indexOf method of the Array to verify if an item exists in it before pushing:
const duplicated_values = ['one', 'one', 'one', 'one', 'two', 'three', 'three', 'four'];
const unique_list = [];
duplicated_values.forEach(value => {
if (unique_list.indexOf(value) === -1) {
unique_list.push(value);
}
});
console.log(unique_list);
That will work with any type of variable as well, even objects (given the identifier actually reference the same entity, merely equivalent objects are not seen as the same).

what about
function unique(list) {
for (i = 0; i<list.length; i++) {
for (j=i+1; j<list.length; j++) {
if (list[i] == list[j]) {
list.splice(j, 1);
}
}
}
}

Get first element of a sparse JavaScript array

I have an array of objects in javascript. I use jquery.
How do i get the first element in the array? I cant use the array index - as I assign each elements index when I am adding the objects to the array. So the indexes arent 0, 1, 2 etc.
Just need to get the first element of the array?

If you don't use sequentially numbered elements, you'll have to loop through until you hit the first one:
var firstIndex = 0;
while (firstIndex < myarray.length && myarray[firstIndex] === undefined) {
firstIndex++;
}
if (firstIndex < myarray.length) {
var firstElement = myarray[firstIndex];
} else {
// no elements.
}
or some equivalently silly construction. This gets you the first item's index, which you might or might not care about it.
If this is something you need to do often, you should keep a lookaside reference to the current first valid index, so this becomes an O(1) operation instead of O(n) every time. If you're frequently needing to iterate through a truly sparse array, consider another data structure, like keeping an object alongside it that back-maps ordinal results to indexes, or something that fits your data.

The filter method works with sparse arrays.
var first = array.filter(x => true)[0];

Have you considered:
function getFirstIndex(array){
var result;
if(array instanceof Array){
for(var i in array){
result = i;
break;
}
} else {
return null;
}
return result;
}
?
And as a way to get the last element in the array:
function getLastIndex(array){
var result;
if(array instanceof Array){
result = array.push("");
array.pop;
}
} else {
return null;
}
return result;
}
Neither of these uses jquery.

Object.keys(array)[0] returns the index (in String form) of the first element in the sparse array.
var array = [];
array[2] = true;
array[5] = undefined;
var keys = Object.keys(array); // => ["2", "5"]
var first = Number(keys[0]); // => 2
var last = Number(keys[keys.length - 1]); // => 5

I was also facing a similar problem and was surprised that no one has considered the following:
var testArray = [];
testArray [1245]= 31;
testArray[2045] = 45;
for(index in testArray){
console.log(index+','+testArray[index])
}
The above will produce
1245,31
2045,45
If needed you could exist after the first iteration if all that was required but generally we need to know where in the array to begin.

This is a proposal with ES5 method with Array#some.
The code gets the first nonsparse element and the index. The iteration stops immediately with returning true in the callback:
var a = [, , 22, 33],
value,
index;
a.some(function (v, i) {
value = v;
index = i;
return true;
});
console.log(index, value);

If you find yourself needing to do manipulation of arrays a lot, you might be interested in the Underscore library. It provides utility methods for manipulating arrays, for example compact:
var yourArray = [];
yourArray[10] = "foo";
var firstValue = _.compact(yourArray)[0];
However, it does sound like you are doing something strange when you are constructing your array. Perhaps Array.push would help you out?