I came across this, understood it but still did not know what javascript property it was:
var array = [];
array[1] = true;
array[5] = true;
console.log(array) #=> [true, true]
array[0] #=> undefined
array[1] #=> true
array[2] #=> undefined
array[5] #=> true
Can someone explain this for me? Thanks
Standard arrays in JavaScript aren't really arrays at all, and one effect of that is that they're inherently sparse. That is, an array can have empty slots in it.
That's what you're creating there. After your first three lines, you have an array with two entries in it, at indexes 1 and 5, and a bunch of completely empty slots (indexes 0, 2, 3, 4). Its length property would be 6. When you try to retrieve an element that doesn't exist from an array, you get the value undefined. (This is just a specific case of JavaScript's general behavior: If you try to retrieve an object property that doesn't exist, you get the value undefined.)
The output of console.log with a sparse array will vary depending on what the implementation of console.log does with them. The comments on the question suggest that there are various different ways the console may show the array. You might look at using console.log(array.join()) to get more consistent results. That would give you ,true,,,,true, because it shows blanks for array entries that don't exist (or that contain the value undefined, but in your case, they don't exist).
Related
This question already has answers here:
What is "undefined x 1" in JavaScript?
(5 answers)
Closed 4 years ago.
Some of the common things asked is , why Array appears to be zero even if there's content inside. My question now is a bit weird. Why am I seeing
(66) [empty × 65, {…}]
in chrome console even if the array has at least 1 array value inside ?
That is what you see when you have a sparse array - with some indicies defined, but some indicies lower than the defined indicies not defined (or ever assigned to), for example:
const arr = [];
arr[65] = 'foo';
console.log(arr);
(look at the results of these two snippets in the browser console, snippet console won't be visible)
Note that this is different from having literal undefined values in the first 65 indicies, which will be shown as undefined rather than empty:
const arr = new Array(65).fill(undefined);
arr[65] = 'foo';
console.log(arr);
Sparse arrays are almost never a good idea, though - if you ever see this, it's an indication that you should probably fix the code to avoid the sparse arrays. Consider another structure instead, perhaps an object with numeric keys:
const obj = {
'65': 'foo'
};
A programmer will expect an array to be an ordered collection of elements - what would it mean for a collection to not have any value at all at the top of an ordered collection (not just undefined, but not any value)? It just doesn't make much sense - while possible, such constructs are not what arrays are for.
As Kaiido notes, empty elements will not be iterated over with array methods like forEach:
const arr = [];
arr[65] = 'foo';
arr.forEach((item, i) => {
console.log(item, i);
});
Which makes a bit of sense, but it's still unintuitive (would you have been sure of this without trying the code yourself or looking up the spec?). One would usually expect the first iteration of a forEach callback to have an index of 0.
It's just 65 empty elements, it's can be if you create array like this:
const arr = Array(65);
arr.push(1);
console.log(arr)
or in new Array set new element on some index;
const arr = [];
arr[65] = 1;
I have an array like below
arr=[];
arr[0]={"zero": "apple"};
arr[1]={"one": "orange"};
arr["fancy"]="what?";
but i am getting length as 2 when i do console.log(arr.length) even though i am able to console all the values .
and not able to get all values while doing console.log(JSON.stringify(arr))
What is the issue here.
here is the link to fiddle fiddle
.length is a special property in Javascript arrays, which is defined as "the biggest numeric index in the array plus one" (or 2^32-1, whatever comes first). It's not "the number of elements", as the name might suggest.
When you iterate an array, either directly with for..of or map, or indirectly with e.g. JSON.stringify, JS just loops over all numbers from 0 to length - 1, and, if there's a property under this number, outputs/returns it. It doesn't look into other properties.
The length property don't work as one will expect on arrays that are hashtables or associative arrays. This property only works as one will expect on numeric indexed arrays (and normalized, i.e, without holes). But there exists a way for get the length of an associative array, first you have to get the list of keys from the associative array using Object.keys(arr) and then you can use the length property over this list (that is a normalized indexed array). Like on the next example:
arr=[];
arr[0]={"zero": "apple"};
arr[1]={"one": "orange"};
arr["fancy"]="what?";
console.log(Object.keys(arr).length);
And about this next question:
not able to get all values while doing console.log(JSON.stringify(arr))
Your arr element don't have the correct format to be a JSON. If you want it to be a JSON check the syntax on the next example:
jsonObj = {};
jsonObj[0] = {"zero": "apple"};
jsonObj[1] = {"one": "orange"};
jsonObj["fancy"] = "what?";
console.log(Object.keys(jsonObj).length);
console.log(JSON.stringify(jsonObj));
From MDN description on arrays, here, "Arrays cannot use strings as element indexes (as in an associative array) but must use integers."
In other words, this is not Javascript array syntax
arr["fancy"]="what?";
Which leads to the error in .length.
I'm doing some small experiments based on this blog entry.
I am doing this research in Google Chrome's debugger and here comes the hard part.
I get the fact that I can't delete local variables (since they are not object attributes). I get that I can 'read out' all of the parameters passed to a function from the array called 'arguments'. I even get it that I can't delete and array's element, only achieve to have array[0] have a value of undefined.
Can somebody explain to me what undefined x 1 means on the embedded image?
And when I overwrite the function foo to return the arguments[0], then I get the usual and 'normal' undefined.
This is only an experiment, but seems interresting, does anybody know what undefined x 1 refers to?
That seems to be Chrome's new way of displaying uninitialized indexes in arrays (and array-like objects):
> Array(100)
[undefined × 100]
Which is certainly better than printing [undefined, undefined, undefined,...] or however it was before.
Although, if there is only one undefined value, they could drop the x 1.
Newer versions of Chrome use empty instead of undefined to make the difference a bit clearer.
For example, entering [,,undefined,,] in the DevTools console results in:
▼(4) [empty × 2, undefined, empty]
2: undefined
length: 4
▶__proto__: Array(0)
Google Chrome seems to choose to display sparse array using this undefined x n notation. It will show [undefined, undefined] if it is not a sparse array:
var arr = new Array(2);
console.log(arr);
var arr2 = [];
arr2[3] = 123;
console.log(arr2);
var arr3 = [,,,];
console.log(arr3)
var arr4 = [,];
console.log(arr4)
var arr5 = [undefined, undefined]
console.log(arr5)
var arr6 = [undefined]
console.log(arr6)
arr1 to arr4 are all sparse arrays, while arr5 and arr6 are not. Chrome will show them as:
[undefined × 2]
[undefined × 3, 123]
[undefined × 3]
[undefined × 1]
[undefined, undefined]
[undefined]
note the [undefined x 1] for the sparse array.
Since you deleted an element, it follows that: If you delete an element from an array, the array becomes sparse.
Here is how it looks, Type this code into chrome devtools console:-
arr = new Array(4);
arr[2] = "adf";
console.log(arr); // [undefined × 2, "adf", undefined × 1]
See the first two consecutive "undefined" that are represented by *2, to show that there are two consecutive undefined there
It just happened to me too. Open the same thing in another browser and output/log the array to the console. As long as it works in both crome and in firefox the same way (as in accessing the elements works fine even if the output has the undefinedx1), I consider it some kind of bug in Chrome not refreshing something internally. You can actually test it this way:
I was adding elements to an array, and firing console.log() to log the element, to check if it was undefined or not. They were all defined.
After push() I logged the array and it seemed that when this happened, it was always the last one being undefined x 1 on Chrome.
Then I tried logging it more times and also later, but with the same result.
However when I accessed (for the sake of output), the element by its index, it returned the proper value (funny).
After that I logged the array again and it said the last index was undefined x 1 (the very one I just accessed directly with success!).
Then I did a for loop on the array, logging or using each element, all showed up fine.
Then another log on the array, still buggy.
The code I'm originally using has checks for undefined and they didn't fire, but I was seeing all these undefineds in the console and it was bugging me (no pun intended).
Also worth reading: Unitialized variables in an array when using only Array.push? I'm using pop() too in my code and it's a reasonable explanation that the console log actually represents a different state in time (when the code is 'halted').
I have an order form and a JavaScript which should calculate grand total from textbox (quantity*price), checkbox and radio, but I'm missing something because i receive a NaN error. Could somebody help me fix it?
Thanks a lot.
DEMO
Like many have commented, there are some serious short-comings to your code. I won't re-iterate these concerns, however, they are very important and you should learn as you are getting more acquainted with the technologies you are using.
The reason you are getting a NaN is this. Your fields object is an array. Your code populates the fields array at an arbitrary indices, i.e. fields[f] = 0;
When you populate the array for index, say 5, your code does this fields[5] = 0;. With this statement you went from having an empty array [], to having an array with 6 entries with the first 5 entries being defaulted to undefined [undefined, undefined, undefined, undefined, undefined, 0].
Now later in your code when you do the summation and add the arrays entries up (using eval having prepared a string) these ...undefined+0+undefined+8... are producing a Not A Number error.
Instead, one way to prevent this is to push entries onto the fields array like:
if (userInputs[f].value)
{
fields.push(userInputs[f].value);
}
else
{
fields.push(0);
}
push will add one entry to the end of the array. [] will become [0] if you push(0).
I have an array like:
errors = [ {...}, {...}, {...} ]
It's an instanceof array, yet it only returns 1 for .length?
Relevant code:
if(data.error){
errors.push({'element':ele,error:data.error});
}
//Above is looped a few times and there are N number of errors now inside
console.log(errors) //Returns 2+ objects like {...}, {...}
console.log(errors.length) //Returns 1
For Uzi and Muirbot, here's the errors array:
[
Object
element: b.fn.b.init[1]
error: "You must enter "example" into the field to pass"
__proto__: Object
,
Object
element: b.fn.b.init[1]
error: "Crap!"
__proto__: Object
It is correct, this code:
var errors = new Array();
errors.push({'element':'ele', error:'data.error'});
...adds ONE object to the array. The object has two properties.
It's possible your code is executing in an order other than what you're expecting. ie, when you log both errors and errors.length, errors does contain only 1 object. But after that you are adding to the errors array, and only after that are you looking at the console. At that point you could see a larger array in errors for two reasons - first, your actual code isn't logging errors but some object that contains errors. In that case the console display is live, and will show you not what was in errors at the time, but what is in it now. Alternatively, the console could just be taking some time to log errors.
Without more code I can't be sure if this is the case. But you could verify it by replacing console.log(errors); with console.log(errors[1]);. If errors is really only 1 long at the time, it will log undefined.
The problem was that Chrome's Web Inspector's console.log is an async event. So, the length was a property lookup so it gave that back instantly, but the object with two items inside was held off until the rest of the events had fired.
In the future I, and others with this issue, should use debugger; instead.
is it an Array object or something that resembles it?
arrays do work:
> a = [{a:1}, {b:2}]
[Object, Object]
> a.length
2
you'll have to provide more code.
and now that you've provided the relevant code, the correct answer is what Steve Wellens said (which was downvoted, by the way).
Array.push adds a single element, objects may have more than one key but they're still a single object so your real case was different from your original example, which of course works.
another possibility:
> a = []
[]
> a.length = 2
2
> a
[]
> a.length
2
> a instanceof Array
true