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I'm trying to make an array of empty arrays, the amount of which depends on N. Currently, I have this:
var seqList = [];
for(var i = 0; i < N; i++) {
seqList.push([]);
}
Is this the most efficient way of doing it? Or might there be another way?
Your method is the most efficient one. You can also use Array.fill method from ES6 but it is not the most efficient.
let n=10;
let array=Array.from({ length: n }, () => []);
console.log(array);
When you are using let array=[] you're telling the interpreter to create a new runtime array.
If you use:var a = new Array() you're telling the interpreterthat you want to call the constructor Array , generate an object and creating your array.
Array#fill is the cleanest
let count = 10;
let seqList = Array(count).fill([]);
Update:
After being schooled by #deceze I would rather use since the example above fails is a horribly subtle way.
let count = 10;
let seqList = Array(count).fill(0).map(_ => []);
Is it better than your attempt? Probably not. I've just become so used to reading map/reduce, I prefer it. YMMV
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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)
I am now practicing some methods in javascript, like shift(), unshift(), push(), pop(), and I want to write a little function that will take 3 arguments, an array, a start point, an end point. Which is used to cut a part of array out, and then return both the new array and the cut part. Not a useful thing or a big deal, just want to practice javascript.
But I encountered a strange thing, which I don't know why.
Here is the strange part
var a1 = [];
var a2 = [9,8,7,6,5,4,3,2,1];
for(var i=0; i<a2.length; i++){
a1.unshift(a2.shift())
}
So I wrote this code, and the expected result should be
a1 = [1,2,4,5,6,7,8,9]
a2 = []
However, when I run the code, this was what actually happened
a1 = [5,6,7,8,9]
a2 = [4,3,2,1]
It seems like the function was looped not enough time, so I tried to change the a2.length to a integer: 9
which make the code become
var a1 = [];
var a2 = [9,8,7,6,5,4,3,2,1];
for(var i=0; i<9; i++){
a1.unshift(a2.shift())
}
And that worked!
Then I change the approach to this
var a1 = [];
var a2 = [9,8,7,6,5,4,3,2,1]
var aLength = a2.length;
for(var i=0; i<aLength; i++){
a1.unshift(a2.shift())
}
And this worked too!!!
Could anyone tell me why is that ???
And how can I improve the loop?
Thank you very much, really appreciate for your time.
You could easily do this by using while loop
while (a2.length > 0)
{
a1.unshift(a2.shift());
}
The loop condition is re-evaluated for each iteration, and a2.length is constantly changing.
If you want to use its initial value, cache it beforehand:
var n = a2.length;
for (var i = 0; i < n; i++) {
a1.unshift(a2.shift());
}
Javascript Array: Shift() Method
The shift() method is like the pop() method, only it works at the
beginning of the array. The shift() method pulls the first element off
of the given array and returns it. This alters the array on which the
method was called.
So when you do a a2.shift() it will actually modify (decrease the number of elements in the array) the array and thus the effect
Do you think there is a big difference in for...in and for loops? What kind of "for" do you prefer to use and why?
Let's say we have an array of associative arrays:
var myArray = [{'key': 'value'}, {'key': 'value1'}];
So we can iterate:
for (var i = 0; i < myArray.length; i++)
And:
for (var i in myArray)
I don't see a big difference. Are there any performance issues?
The choice should be based on the which idiom is best understood.
An array is iterated using:
for (var i = 0; i < a.length; i++)
//do stuff with a[i]
An object being used as an associative array is iterated using:
for (var key in o)
//do stuff with o[key]
Unless you have earth shattering reasons, stick to the established pattern of usage.
Douglas Crockford recommends in JavaScript: The Good Parts (page 24) to avoid using the for in statement.
If you use for in to loop over property names in an object, the results are not ordered. Worse: You might get unexpected results; it includes members inherited from the prototype chain and the name of methods.
Everything but the properties can be filtered out with .hasOwnProperty. This code sample does what you probably wanted originally:
for (var name in obj) {
if (Object.prototype.hasOwnProperty.call(obj, name)) {
// DO STUFF
}
}
FYI - jQuery Users
jQuery's each(callback) method uses for( ; ; ) loop by default, and will use for( in ) only if the length is undefined.
Therefore, I would say it is safe to assume the correct order when using this function.
Example:
$(['a','b','c']).each(function() {
alert(this);
});
//Outputs "a" then "b" then "c"
The downside of using this is that if you're doing some non UI logic, your functions will be less portable to other frameworks. The each() function is probably best reserved for use with jQuery selectors and for( ; ; ) might be advisable otherwise.
there are performance differences depending on what kind of loop you use and on what browser.
For instance:
for (var i = myArray.length-1; i >= 0; i--)
is almost twice as fast on some browsers than:
for (var i = 0; i < myArray.length; i++)
However unless your arrays are HUGE or you loop them constantly all are fast enough. I seriously doubt that array looping is a bottleneck in your project (or for any other project for that matter)
Note that the native Array.forEach method is now widely supported.
Updated answer for 2012 current version of all major browsers - Chrome, Firefox, IE9, Safari and Opera support ES5's native array.forEach.
Unless you have some reason to support IE8 natively (keeping in mind ES5-shim or Chrome frame can be provided to these users, which will provide a proper JS environment), it's cleaner to simply use the language's proper syntax:
myArray.forEach(function(item, index) {
console.log(item, index);
});
Full documentation for array.forEach() is at MDN.
The two are not the same when the array is sparse.
var array = [0, 1, 2, , , 5];
for (var k in array) {
// Not guaranteed by the language spec to iterate in order.
alert(k); // Outputs 0, 1, 2, 5.
// Behavior when loop body adds to the array is unclear.
}
for (var i = 0; i < array.length; ++i) {
// Iterates in order.
// i is a number, not a string.
alert(i); // Outputs 0, 1, 2, 3, 4, 5
// Behavior when loop body modifies array is clearer.
}
Using forEach to skip the prototype chain
Just a quick addendum to #nailer's answer above, using forEach with Object.keys means you can avoid iterating over the prototype chain without having to use hasOwnProperty.
var Base = function () {
this.coming = "hey";
};
var Sub = function () {
this.leaving = "bye";
};
Sub.prototype = new Base();
var tst = new Sub();
for (var i in tst) {
console.log(tst.hasOwnProperty(i) + i + tst[i]);
}
Object.keys(tst).forEach(function (val) {
console.log(val + tst[val]);
});
I second opinions that you should choose the iteration method according to your need. I would suggest you actually not to ever loop through native Array with for in structure. It is way slower and, as Chase Seibert pointed at the moment ago, not compatible with Prototype framework.
There is an excellent benchmark on different looping styles that you absolutely should take a look at if you work with JavaScript. Do not do early optimizations, but you should keep that stuff somewhere in the back of your head.
I would use for in to get all properties of an object, which is especially useful when debugging your scripts. For example, I like to have this line handy when I explore unfamiliar object:
l = ''; for (m in obj) { l += m + ' => ' + obj[m] + '\n' } console.log(l);
It dumps content of the whole object (together with method bodies) to my Firebug log. Very handy.
I'd use the different methods based on how I wanted to reference the items.
Use foreach if you just want the current item.
Use for if you need an indexer to do relative comparisons. (I.e. how does this compare to the previous/next item?)
I have never noticed a performance difference. I'd wait until having a performance issue before worrying about it.
here is something i did.
function foreach(o, f) {
for(var i = 0; i < o.length; i++) { // simple for loop
f(o[i], i); // execute a function and make the obj, objIndex available
}
}
this is how you would use it
this will work on arrays and objects( such as a list of HTML elements )
foreach(o, function(obj, i) { // for each obj in o
alert(obj); // obj
alert(i); // obj index
/*
say if you were dealing with an html element may be you have a collection of divs
*/
if(typeof obj == 'object') {
obj.style.marginLeft = '20px';
}
});
I just made this so I'm open to suggestions :)
With for (var i in myArray) you can loop over objects too, i will contain the key name and you can access the property via myArray[i]. Additionaly, any methods you will have added to the object will be included in the loop, too, i.e., if you use any external framework like jQuery or prototype, or if you add methods to object prototypes directly, at one point i will point to those methods.
Watch out!
If you have several script tags and your're searching an information in tag attributes for example, you have to use .length property with a for loop because it isn't a simple array but an HTMLCollection object.
https://developer.mozilla.org/en/DOM/HTMLCollection
If you use the foreach statement for(var i in yourList) it will return proterties and methods of the HTMLCollection in most browsers!
var scriptTags = document.getElementsByTagName("script");
for(var i = 0; i < scriptTags.length; i++)
alert(i); // Will print all your elements index (you can get src attribute value using scriptTags[i].attributes[0].value)
for(var i in scriptTags)
alert(i); // Will print "length", "item" and "namedItem" in addition to your elements!
Even if getElementsByTagName should return a NodeList, most browser are returning an HTMLCollection:
https://developer.mozilla.org/en/DOM/document.getElementsByTagName
For in loops on Arrays is not compatible with Prototype. If you think you might need to use that library in the future, it would make sense to stick to for loops.
http://www.prototypejs.org/api/array
I have seen problems with the "for each" using objects and prototype and arrays
my understanding is that the for each is for properties of objects and NOT arrays
If you really want to speed up your code, what about that?
for( var i=0,j=null; j=array[i++]; foo(j) );
it's kinda of having the while logic within the for statement and it's less redundant. Also firefox has Array.forEach and Array.filter
A shorter and best code according to jsperf is
keys = Object.keys(obj);
for (var i = keys.length; i--;){
value = obj[keys[i]];// or other action
}
for(;;) is for Arrays : [20,55,33]
for..in is for Objects : {x:20,y:55:z:33}
Use the Array().forEach loop to take advantage of parallelism
Be careful!!!
I am using Chrome 22.0 in Mac OS and I am having problem with the for each syntax.
I do not know if this is a browser issue, javascript issue or some error in the code, but it is VERY strange. Outside of the object it works perfectly.
var MyTest = {
a:string = "a",
b:string = "b"
};
myfunction = function(dicts) {
for (var dict in dicts) {
alert(dict);
alert(typeof dict); // print 'string' (incorrect)
}
for (var i = 0; i < dicts.length; i++) {
alert(dicts[i]);
alert(typeof dicts[i]); // print 'object' (correct, it must be {abc: "xyz"})
}
};
MyObj = function() {
this.aaa = function() {
myfunction([MyTest]);
};
};
new MyObj().aaa(); // This does not work
myfunction([MyTest]); // This works
There is an important difference between both. The for-in iterates over the properties of an object, so when the case is an array it will not only iterate over its elements but also over the "remove" function it has.
for (var i = 0; i < myArray.length; i++) {
console.log(i)
}
//Output
0
1
for (var i in myArray) {
console.log(i)
}
// Output
0
1
remove
You could use the for-in with an if(myArray.hasOwnProperty(i)). Still, when iterating over arrays I always prefer to avoid this and just use the for(;;) statement.
Although they both are very much alike there is a minor difference :
var array = ["a", "b", "c"];
array["abc"] = 123;
console.log("Standard for loop:");
for (var index = 0; index < array.length; index++)
{
console.log(" array[" + index + "] = " + array[index]); //Standard for loop
}
in this case the output is :
STANDARD FOR LOOP:
ARRAY[0] = A
ARRAY[1] = B
ARRAY[2] = C
console.log("For-in loop:");
for (var key in array)
{
console.log(" array[" + key + "] = " + array[key]); //For-in loop output
}
while in this case the output is:
FOR-IN LOOP:
ARRAY[1] = B
ARRAY[2] = C
ARRAY[10] = D
ARRAY[ABC] = 123
In the following code sample i get a strange behavior
var data = ['xxx', 'yyy'];
for (var i in data)
{
var a = i;
var b = data[i];
}
The two first iterations works just fine. I get index "0" and "1" in i, but then it loops one extra time and now the i is "sum". Is this by design or what is this extra iteration used for? The result in my case is always empty and it messes up my code. Is there a way to not do his extra loop?
BR
Andreas
It looks like you (or some other code you've included) have added extra properties onto the Array prototype. What you should be doing is checking to see whether the object you're iterating over actually has that property on itself, not on its prototype:
for (i in data) {
if (data.hasOwnProperty(i)) {
a = i;
b = data[i];
}
}
That said, you should never use for .. in on arrays. Use a regular for loop.
See here for more information: http://yuiblog.com/blog/2006/09/26/for-in-intrigue/
You are looping through an Array, not through an Object. For arrays it's better to use:
for (var i=0; i<data.length; i=i+1){
/* ... */
}
In your loop every property of the Array object is taken into account. That makes the for ... in loop for array less predictable. In your case it looks like sum is a property (method) that's added to Array.prototype elsewhere in your code.
There are more ways to loop through arrays. See for example this SO-question, or this one
Just for fun, a more esoteric way to loop an array:
Array.prototype.loop = function(fn){
var t = this;
return (function loop(fn,i){
return i ? loop(fn,i-1).concat(fn(t[i-1])) : [];
}(fn,t.length));
}
//e.g.
//add 1 to every value
var a = [1,2,3,4,5].loop(function(val){return val+1;});
alert(a); //=> [2,3,4,5,6]
//show every value in console
var b = [1,2,3,4,5].loop(function(val){return console.log(val), val;});
Here's a way to safely iterate.
var data = ['xxx', 'yyy'];
for (var i = 0; i < data.length; i++)
{
var a = i;
var b = data[i];
}
What you are getting is an method coming from extending the Array object, I guess you are using some library where is something like
Array.prototype.sum = function () {...};
Perhaps setting data like this would work better: var data = {0:'xxx', 1:'yyy'};
First of all data is an object. Try to add console.log(a); and console.log(b); inside your loop and you'll see.