Develop Reference

Why slice(-x,0) doesn't produce expected string

If I have this:
console.log('12345'.slice(-3)); // I get '345'
but when I do:
console.log('12345'.slice(-3, 0)); // I get '' (empty string).
what is the reasoning behind this? It makes no sense to me. Is there a JS API that can wrap around strings like I expect here? (I was expecting '345' again.)

The second parameter is the index where the substring to be extracted ends (exclusive). If that index is before (or equal to) the start index (the first parameter), the result is the empty string.
As the docs say about the second parameter:
If end is positioned before or at start after normalization, nothing is extracted.
After normalization, .slice(-3) is equivalent to .slice(2), and the second parameter defaults to the length of the string, so
.slice(-3)
// equivalent to
.slice(2)
is like
.slice(2, 5) // take the substring from index 2 to index 5
and not
.slice(2, 0)

Why does `join()` on an array of undefined elements return 1 less element in javascript?

I was initially looking up what happens on Array.prototype.toString(). From the spec i realised this calls .join(). But I don't understand the output on an array with undefined elements.
[,,,].toString() returns ",,". But I expected ",,,". There appears to be 1 element missing in the serialization. What's happening here?

Because in JavaScript, the last , (the trailing comma) is ignored if it is followed by nothing.
console.log([1,].length);
// -> 1
console.log([1,,].length);
// -> 2
Therefore, when you are doing [,,,], the last , is ignored. Thus the length is only 3.
console.log([,,,].length);
// -> 3
When you join an array with n items, you get n-1 commas. Hence the result that you are getting.

The last comma in an array is optional (for convenience) and therefore ignored in join()

I need an explanation on slice()

I was completing javascript exercises and got stuck on creating a function which checks if a string is a palindrome. For your reference, this was the given solution.
I understand what slice() does as a method, and I followed the function here until the point where it checks to see if the first letter matches the last.
if (cstr[x] != cstr.slice(-1-x)[0])
What is going on with the parameter values of the slice method? I was expecting the parameters to slice the last letter and I presume it does but I would appreciate some help in pointing out how!

Let's pop open the console and take a look!
> var s = 'abcde'
"abcde"
> s[0]
"a"
> s.slice(-1)
"e"
> s.slice(-1)[0]
"e"
> s[1]
"b"
> s.slice(-1 - 1)
"de"
> s.slice(-1 - 1)[0]
"d"
As I hope you can see by now, slicing with a negative number -n will return the last n characters of the string, with n = 1 being the last character. So to have the palindrome test work correctly, you need to slice at -1 - x (the "x-th from last" character).

arr.slice([begin[, end]])
Parameters
begin
Zero-based index at which to begin extraction. As a negative index, begin indicates an offset from the end of the sequence. slice(-2) extracts the last two elements in the sequence. If begin is omitted, slice begins from index 0.
end
Zero-based index at which to end extraction. slice extracts up to but not including end. slice(1,4) extracts the second element up to the fourth element (elements indexed 1, 2, and 3). As a negative index, end indicates an offset from the end of the sequence. slice(2,-1) extracts the third element through the second-to-last element in the sequence. If end is omitted, slice extracts to the end of the sequence (arr.length).
Description
slice returns a shallow copy of elements from the original array. Elements of the original array are copied into the returned array as follows:
For object references (and not the actual object), slice copies object references into the new array. Both the original and new array refer to the same object. If a referenced object changes, the changes are visible to both the new and original arrays.
For strings and numbers (not String and Number objects), slice copies strings and numbers into the new array. Changes to the string or number in one array does not affect the other array.
If a new element is added to either array, the other array is not affected.

The indexOf method in Javascript

I'm trying to understand a lot of the basic components of Javascript and one of the things I came across is a line of code saying
if (varX.indexOf(String(varY),0) < 0)
varX being an array of Strings and varY being obviously one of the strings within that array. Take away the ",0" and I understand that the code is just looking for varY withing array varX. But I don't know what the ,0 does and what means for the if statement. I did what I could to look this up and didn't really come across anything.

According to the MDN docs:
fromindex
The index to start the search at. If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array. Note: if the provided index is negative, the array is still searched from front to back. If the calculated index is less than 0, then the whole array will be searched. Default: 0 (Entire array is searched).
So, passing in "0" is pretty much pointless, as it starts off the search at 0 anyway.

From mdn:
arr.indexOf(searchElement[, fromIndex = 0])
fromIndex
The index to start the search at. If the index is greater than or
equal to the array's length, -1 is returned, which means the array
will not be searched. If the provided index value is a negative
number, it is taken as the offset from the end of the array. Note: if
the provided index is negative, the array is still searched from front
to back. If the calculated index is less than 0, then the whole array
will be searched. Default: 0 (Entire array is searched).

0 is index from where you start the search. It is 0 by default, so you don't have to pass this parameter.

What is the difference between String.slice and String.substring?

Does anyone know what the difference is between these two methods?
String.prototype.slice
String.prototype.substring

slice() works like substring() with a few different behaviors.
Syntax: string.slice(start, stop);
Syntax: string.substring(start, stop);
What they have in common:
If start equals stop: returns an empty string
If stop is omitted: extracts characters to the end of the string
If either argument is greater than the string's length, the string's length will be used instead.
Distinctions of substring():
If start > stop, then substring will swap those 2 arguments.
If either argument is negative or is NaN, it is treated as if it were 0.
Distinctions of slice():
If start > stop, slice() will return the empty string. ("")
If start is negative: sets char from the end of string, exactly like substr() in Firefox. This behavior is observed in both Firefox and IE.
If stop is negative: sets stop to: string.length – Math.abs(stop) (original value), except bounded at 0 (thus, Math.max(0, string.length + stop)) as covered in the ECMA specification.
Source: Rudimentary Art of Programming & Development: Javascript: substr() v.s. substring()

TL;DR:
If you know the index (the position) on which you'll stop (but NOT include), use slice().
If you know the length of characters to be extracted, you could use substr(), but that is discouraged as it is deprecated.
Otherwise, read on for a full comparison
Syntax
string.slice(start,end)
string.substr(start,length)
string.substring(start,end)
Note #1: slice()==substring()
What it does?
slice() extracts parts of a string and returns the extracted parts in a new string.
substr() extracts parts of a string, beginning at the character at the specified position, and returns the specified number of characters.
substring() extracts parts of a string and returns the extracted parts in a new string.
Note #2: slice()==substring()
Changes the Original String?
slice() doesn't
substr() doesn't
substring() doesn't
Note #3: slice()==substr()==substring()
Using Negative Numbers as an Argument
slice() selects characters starting from the end of the string
substr() selects characters starting from the end of the string
substring() doesn't perform
Note #4: slice()==substr()
If the First Argument is Greater than the Second
slice() doesn't perform
substr() since the Second Argument is NOT a position, but length value, it will perform as usual, with no problems
substring() will swap the two arguments, and perform as usual
The First Argument
slice() required; starting Index
substr() required; starting Index
substring() required; starting Index
Note #5: slice()==substr()==substring()
The Second Argument
slice() optional; the position (up to, but not including) where to end the extraction
substr() optional; the number of characters to extract
substring() optional; the position (up to, but not including) where to end the extraction
Note #6: slice()==substring()
What if the Second Argument is Omitted?
slice() selects all characters from the start-position to the end of the string
substr() selects all characters from the start-position to the end of the string
substring() selects all characters from the start-position to the end of the string
Note #7: slice()==substr()==substring()
So, you can say that there's a difference between slice() and substr(), while substring() is basically a copy of slice().
If you want substr's functionality:
"foobarbaz".substr(index, length);
without using a deprecated feature, you can just do:
"foobarbaz".substring(index, length + index);
And get the exact same results bar all of the edge-cases, like negative index/length.

Ben Nadel has written a good article about this, he points out the difference in the parameters to these functions:
String.slice( begin [, end ] )
String.substring( from [, to ] )
String.substr( start [, length ] )
He also points out that if the parameters to slice are negative, they reference the string from the end. Substring and substr doesn't.
Here is his article about this.

The one answer is fine but requires a little reading into. Especially with the new terminology "stop".
My Go -- organized by differences to make it useful in addition to the first answer by Daniel above:
1) negative indexes. Substring requires positive indexes and will set a negative index to 0. Slice's negative index means the position from the end of the string.
"1234".substring(-2, -1) == "1234".substring(0,0) == ""
"1234".slice(-2, -1) == "1234".slice(2, 3) == "3"
2) Swapping of indexes. Substring will reorder the indexes to make the first index less than or equal to the second index.
"1234".substring(3,2) == "1234".substring(2,3) == "3"
"1234".slice(3,2) == ""
--------------------------
General comment -- I find it weird that the second index is the position after the last character of the slice or substring. I would expect "1234".slice(2,2) to return "3". This makes Andy's confusion above justified -- I would expect "1234".slice(2, -1) to return "34". Yes, this means I'm new to Javascript. This means also this behavior:
"1234".slice(-2, -2) == "", "1234".slice(-2, -1) == "3", "1234".slice(-2, -0) == "" <-- you have to use length or omit the argument to get the 4.
"1234".slice(3, -2) == "", "1234".slice(3, -1) == "", "1234".slice(3, -0) == "" <-- same issue, but seems weirder.
My 2c.

The difference between substring and slice - is how they work with negative and overlooking lines abroad arguments:
substring(start, end)
Negative arguments are interpreted as zero. Too large values ​​are truncated to the length of the string:
alert("testme".substring(-2)); // "testme", -2 becomes 0
Furthermore, if start > end, the arguments are interchanged, i.e. plot line returns between the start and end:
alert("testme".substring(4, -1)); // "test"
// -1 Becomes 0 -> got substring (4, 0)
// 4> 0, so that the arguments are swapped -> substring (0, 4) = "test"
slice
Negative values ​​are measured from the end of the line:
alert("testme".slice(-2)); // "me", from the end position 2
alert("testme".slice(1, -1)); // "estm", from the first position to the one at the end.
It is much more convenient than the strange logic substring.
A negative value of the first parameter to substr supported in all browsers except IE8-.
If the choice of one of these three methods, for use in most situations - it will be slice: negative arguments and it maintains and operates most obvious.

substr: It's providing us to fetch part of the string based on specified index.
syntax of substr-
string.substr(start,end)
start - start index tells where the fetching start.
end - end index tells upto where string fetches. It's optional.
slice: It's providing to fetch part of the string based on the specified index. It's allows us to specify positive and index.
syntax of slice - string.slice(start,end)
start - start index tells where the fetching start.It's
end - end index tells upto where string fetches. It's optional.
In 'splice' both start and end index helps to take positive and negative index.
sample code for 'slice' in string
var str="Javascript";
console.log(str.slice(-5,-1));
output: crip
sample code for 'substring' in string
var str="Javascript";
console.log(str.substring(1,5));
output: avas
[*Note: negative indexing starts at the end of the string.]

The only difference between slice and substring method is of arguments
Both take two arguments e.g. start/from and end/to.
You cannot pass a negative value as first argument for substring method but for slice method to traverse it from end.
Slice method argument details:
Arguments
start_index
Index from where slice should begin. If value is provided in negative it means start from last. e.g. -1 for last character.
end_index
Index after end of slice. If not provided slice will be taken from start_index to end of string. In case of negative value index will be measured from end of string.
Substring method argument details:
Arguments
from
It should be a non negative integer to specify index from where sub-string should start.
to
An optional non negative integer to provide index before which sub-string should be finished.

For slice(start, stop), if stop is negative, stop will be set to:
string.length – Math.abs(stop)
rather than:
string.length – 1 – Math.abs(stop)