While John's solution doesn't run into conflicts with downward compatibility, it still wouldn't solve the problem of getting the n-th last element of an array. To solve this, it'd probably be a good idea to extend the prototype and specify a parameter, defaulting to 1.
`Array.prototype.last` doesn't show up a terrible lot on search engines, either, so we might actually be lucky here. Other than that, I also found two more threads[1][2] on the EcmaScript discussion archives that propose it. They might be worth a read. [1] https://esdiscuss.org/topic/array-prototype-last [2] https://esdiscuss.org/topic/proposal-array-prototype-last On Samstag, 23. Januar 2016 15:44:19 CET John Gardner wrote: > Using a well-known symbol to access an array's last element is probably > wiser than a typical method or property: > > let a = [0, 1, 2, 3]; > console.log( > a[Symbol.last] === 3 > /* true */ > ); > > There're obviously instances where authors have extended Array prototypes > with "last" methods or properties, but we can't guarantee they'd all work > the same. For instance, assume there are some implementations that skip > undefined values: > > var a = [0, 1, 2, undefined, undefined]; > Array.prototype.last = function(){ > return this[this.length - 1]; > }; > /** One that skips undefined values */ > Array.prototype.last = function(){ > var offset = 1; > while(offset < this.length && undefined === this[this.length - offset]) > ++offset; > return this[this.length - offset]; > } > > These discrepancies are subtle, but have the potential to break backwards > compatibility. > > Using a well-known symbol eliminates the potential for conflict. > Furthermore, it also offers an opportunity to complement any iterable > object's ability to synthesise array-like behaviour. For instance, it > enables iterables to also return the last object in their list of values: > > let pruebas = { > data: ["Probando", "la", "mierda", "esta", undefined], > > [Symbol.iterator](){ > /* Stuff with .data */ > }, > [Symbol.last](){ > /** Stuff to skip undefined values or whatever */ > let offset = 1; > let data = this.data; > while( > offset < data.length && > undefined === data[data.length - offset] > ) > ++offset; > return data[data.length - offset]; > } > } > > On 23 January 2016 at 08:36, <[email protected]> wrote: > > Send es-discuss mailing list submissions to > > > > [email protected] > > > > To subscribe or unsubscribe via the World Wide Web, visit > > > > https://mail.mozilla.org/listinfo/es-discuss > > > > or, via email, send a message with subject or body 'help' to > > > > [email protected] > > > > You can reach the person managing the list at > > > > [email protected] > > > > When replying, please edit your Subject line so it is more specific > > than "Re: Contents of es-discuss digest..." > > > > Today's Topics: > > 1. Re: Javascript Language feature Idea (Bradley Meck) > > 2. Re: Re: Javascript Language feature Idea (Alican ?ubuk?uo?lu) > > 3. Re: Javascript Language feature Idea (kdex) > > 4. Re: Javascript Language feature Idea (kdex) > > 5. Re: Javascript Language feature Idea (Benoit Marchant) > > > > ---------- Forwarded message ---------- > > From: Bradley Meck <[email protected]> > > To: Dmitry Soshnikov <[email protected]> > > Cc: es-discuss <[email protected]> > > Date: Fri, 22 Jan 2016 13:51:54 -0600 > > Subject: Re: Javascript Language feature Idea > > Personally I prefer a well known symbol for marking you are grabbing from > > the end of the list rather than this wrapping behavior like D ( > > https://dlang.org/spec/arrays.html#array-length ). That said I think > > `.last` is more reasonable than changing prototypes. > > > > On Fri, Jan 22, 2016 at 1:17 PM, Dmitry Soshnikov < > > > > [email protected]> wrote: > >> On Fri, Jan 22, 2016 at 9:53 AM, RacerD123 <[email protected]> wrote: > >>> In other programming languages such as python, you can specify the last > >>> element of a list with "list[-1]" but in javascript you have to do > >>> "list[list.length-1]". Is there maybe a way to make this feature in > >>> javascript? > >> > >> This could be a good addition, although `slice` may work as well. This > >> has been discussed previously as well in the last 4-5 years. In addition > >> with non-enumerable properties you can just monkey-patch > >> `Array.prototype` > >> with the `peek` or `last` method. I guess approach when some library > >> forces > >> some method to widely used, can be a good candidate for adding it to the > >> spec (this how it was with `Function.prototype.bind`, e.g., although > >> today > >> monkey-patching is a bad practice of course). > >> > >> In addition, you can monkey-patch just `-1`, and other indexes, e.g. a > >> gist from 5 years old, when a similar topic was proposed here: > >> https://gist.github.com/DmitrySoshnikov/984921 > >> > >> Dmitry > >> > >> _______________________________________________ > >> es-discuss mailing list > >> [email protected] > >> https://mail.mozilla.org/listinfo/es-discuss > > > > ---------- Forwarded message ---------- > > From: "Alican Çubukçuoğlu" <[email protected]> > > To: [email protected] > > Cc: > > Date: Fri, 22 Jan 2016 22:32:42 +0200 > > Subject: Re: Re: Javascript Language feature Idea > > More cool stuff: > > ```javascript > > const arr = [1, 2, 3]; > > > > // Easy push > > arr[] = 4; // => arr[arr.length]; > > > > // Easy s(p)lice > > arr[begin, end];, > > arr[begin,]; // => arr[begin, arr.length]; > > arr[begin, end] = [1, 2, 3]; > > ``` > > > > A terrible example (terrible because this should be done with WebGL > > shaders): > > ```javascript > > const image = [ /* Umbagajillion of RGBA pixels */ ]; > > > > function manipulate(rgba) { > > > > rgba[0] += 10; > > rgba[1] += 10; > > rgba[2] += 10; > > > > } > > > > for (let i = 0; i < image.length / 4; i++) { > > > > const begin = i * 4; > > const end = begin + 4; > > > > /* > > > > In case easy s(p)lice doesn't actually Array.p.slice > > and just creates a limited view of the array > > without breaking reference > > (image[begin, end] === image[begin, end]) > > > > */ > > manipulate(image[begin, end]); > > > > /* > > > > In case easy s(p)lice does Array.p.slice > > and creates a new array > > (image[begin, end] !== image[begin, end]) > > > > */ > > const pixel = image[begin, end]; > > manipulate(pixel); > > image[begin, end] = pixel; > > > > } > > ``` > > > > > > ---------- Forwarded message ---------- > > From: kdex <[email protected]> > > To: [email protected] > > Cc: > > Date: Fri, 22 Jan 2016 22:00:01 +0100 > > Subject: Re: Javascript Language feature Idea > > I really adore the slicing syntax. Just as a quick fix, though, > > > > > arr[begin,]; // => arr[begin, arr.length]; > > > > should obviously return `arr[begin, arr.length - 1]`. > > > > Honestly, I'm not too sure if the "easy push" syntax is particularly self- > > explanatory. There might be better alternatives. > > > > On Freitag, 22. Januar 2016 22:32:42 CET Alican Çubukçuoğlu wrote: > > > More cool stuff: > > > ```javascript > > > const arr = [1, 2, 3]; > > > > > > // Easy push > > > arr[] = 4; // => arr[arr.length]; > > > > > > // Easy s(p)lice > > > arr[begin, end];, > > > arr[begin,]; // => arr[begin, arr.length]; > > > arr[begin, end] = [1, 2, 3]; > > > ``` > > > > > > A terrible example (terrible because this should be done with WebGL > > > shaders): > > > ```javascript > > > const image = [ /* Umbagajillion of RGBA pixels */ ]; > > > > > > function manipulate(rgba) { > > > > > > rgba[0] += 10; > > > rgba[1] += 10; > > > rgba[2] += 10; > > > > > > } > > > > > > for (let i = 0; i < image.length / 4; i++) { > > > > > > const begin = i * 4; > > > const end = begin + 4; > > > > > > /* > > > > > > In case easy s(p)lice doesn't actually Array.p.slice > > > and just creates a limited view of the array > > > without breaking reference > > > (image[begin, end] === image[begin, end]) > > > > > > */ > > > manipulate(image[begin, end]); > > > > > > /* > > > > > > In case easy s(p)lice does Array.p.slice > > > and creates a new array > > > (image[begin, end] !== image[begin, end]) > > > > > > */ > > > const pixel = image[begin, end]; > > > manipulate(pixel); > > > image[begin, end] = pixel; > > > > > > } > > > ``` > > > > ---------- Forwarded message ---------- > > From: kdex <[email protected]> > > To: [email protected] > > Cc: > > Date: Fri, 22 Jan 2016 22:14:20 +0100 > > Subject: Re: Javascript Language feature Idea > > Standardizing unused array keys breaks compatibility about as much as > > extending the prototype does, really. Users can already mangle with both. > > > > The problems are a little more subtle, and yes, it would severely break > > backwards compatibility. Consider this example (accessing an element based > > on > > a computed index): > > > > ```js > > let a = [1, 2, 3]; > > let occurrence = a.indexOf(4); > > a[occurrence]; > > ``` > > > > Currently, this should result in `undefined`. After your proposal, you > > could > > introduce a potential bug into every program that accesses arrays using an > > index computed with `Array.prototype.indexOf`, since it would now return > > `3`. > > > > On Freitag, 22. Januar 2016 18:59:06 CET Sander Deryckere wrote: > > > Since Array inherits from Object, it can have any key, including the key > > > "-1". So something like list[-1] would break compatibility as users can > > > > now > > > > > already assign values to the index -1. > > > > > > If you want a short way to access the last element, it should probably > > > > be a > > > > > function in the Array prototype. Something like list.last(). > > > > > > Regards, > > > Sander > > > > > > 2016-01-22 18:53 GMT+01:00 RacerD123 <[email protected]>: > > > > In other programming languages such as python, you can specify the > > > > last > > > > element of a list with "list[-1]" but in javascript you have to do > > > > "list[list.length-1]". Is there maybe a way to make this feature in > > > > javascript? > > > > > > > > _______________________________________________ > > > > es-discuss mailing list > > > > [email protected] > > > > https://mail.mozilla.org/listinfo/es-discuss > > > > ---------- Forwarded message ---------- > > From: Benoit Marchant <[email protected]> > > To: kdex <[email protected]> > > Cc: [email protected] > > Date: Fri, 22 Jan 2016 13:35:55 -0800 > > Subject: Re: Javascript Language feature Idea > > Interesting! More pedestrian, it would be nice to finally have a version > > of splice that doesn't create a new other Array as a side effect. > > > > > On Jan 22, 2016, at 13:00, kdex <[email protected]> wrote: > > > > > > I really adore the slicing syntax. Just as a quick fix, though, > > > > > >> arr[begin,]; // => arr[begin, arr.length]; > > > > > > should obviously return `arr[begin, arr.length - 1]`. > > > > > > Honestly, I'm not too sure if the "easy push" syntax is particularly > > > > self- > > > > > explanatory. There might be better alternatives. > > > > > >> On Freitag, 22. Januar 2016 22:32:42 CET Alican Çubukçuoğlu wrote: > > >> More cool stuff: > > >> ```javascript > > >> const arr = [1, 2, 3]; > > >> > > >> // Easy push > > >> arr[] = 4; // => arr[arr.length]; > > >> > > >> // Easy s(p)lice > > >> arr[begin, end];, > > >> arr[begin,]; // => arr[begin, arr.length]; > > >> arr[begin, end] = [1, 2, 3]; > > >> ``` > > >> > > >> A terrible example (terrible because this should be done with WebGL > > >> shaders): > > >> ```javascript > > >> const image = [ /* Umbagajillion of RGBA pixels */ ]; > > >> > > >> function manipulate(rgba) { > > >> > > >> rgba[0] += 10; > > >> rgba[1] += 10; > > >> rgba[2] += 10; > > >> > > >> } > > >> > > >> for (let i = 0; i < image.length / 4; i++) { > > >> > > >> const begin = i * 4; > > >> const end = begin + 4; > > >> > > >> /* > > >> > > >> In case easy s(p)lice doesn't actually Array.p.slice > > >> and just creates a limited view of the array > > >> without breaking reference > > >> (image[begin, end] === image[begin, end]) > > >> > > >> */ > > >> manipulate(image[begin, end]); > > >> > > >> /* > > >> > > >> In case easy s(p)lice does Array.p.slice > > >> and creates a new array > > >> (image[begin, end] !== image[begin, end]) > > >> > > >> */ > > >> const pixel = image[begin, end]; > > >> manipulate(pixel); > > >> image[begin, end] = pixel; > > >> > > >> } > > >> ``` > > > > > > _______________________________________________ > > > es-discuss mailing list > > > [email protected] > > > https://mail.mozilla.org/listinfo/es-discuss > > > > _______________________________________________ > > es-discuss mailing list > > [email protected] > > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
