Discuss
  • Add an easier method for slicing and stepping strings and arrays

Add an easier method for slicing and stepping strings and arrays

# Alexander Craggs (7 years ago)

I always found the Python methodology of selecting characters from strings (or items from arrays) to be the most natural I've found among a number of language solutions.  Was considering whether adding the syntax would be beneficial for JavaScript.  For those who do not know, the syntax is as so:

const str = 'example'
const arr = ['e', 'x', 'a', 'm', 'p', 'l', 'e']
// Single elements are selected as in JS:
str[1] // 'x'
arr[2] // 'a'
// Multiple elements are selected by providing two numbers, delimited by a colon
// The first number is inclusive, the last number is exclusive
str[1:3] // 'xa'
arr[3:6] // ['m', 'p', 'l']
// These numbers can also be negative, resulting in indexing from the end of the string/array
str[-1] // 'e'
str[-3:-1] // 'pl'
arr[-5:-3] // ['a', 'm']
arr[0:-5] // ['e', 'x']
// There is also a third number, which steps through the selection chosen by the first two
str[0:5:2] // 'eap'
// This number can be negative, which results in stepping backwards through the string/array
str[::-1] // 'elpmaxe'
// An empty item represents one of the following based upon it's position: [0:length:1]

The most official documentation I could find

# Darien Valentine (7 years ago)

Here are some related threads from the past:

I think I’ve seen it mentioned a few other times too.

The arr[-1] syntax is a non-starter I’m afraid, cause it’s just property access. An array can already have a property with the key "-1".

Personally I’d find it weird for slicing to be singled out for special syntax. It’s just a method. However I’d agree that it’s quite awkward to do things like arr[arr.length - 1] or arr.slice().pop() etc to access from the right. I think in one of those threads someone proposed a method like Array.prototype.nth() which would accept negative indices, though this creates an odd asymmetry since (presumably) we would not want to use -0 for the last index.

In general I think the functionality you’re describing could be useful but that it could be served better by adding methods rather than syntax.

# Dante Federici (7 years ago)

This feels like an attempt to get toward python's style of slice stackoverflow.com/questions/509211/explain-slice-notation. Which

I'm not opposed to, and would like to hear how it's better than just the method.

# Tab Atkins Jr. (7 years ago)

On Sun, Jul 23, 2017 at 12:36 AM, Darien Valentine <valentinium at gmail.com> wrote:

Here are some related threads from the past:

esdiscuss.org/topic/negative-indices-for-arrays, esdiscuss.org/topic/array-slice-syntax, esdiscuss.org/topic/javascript-language-feature-idea

I think I’ve seen it mentioned a few other times too.

The arr[-1] syntax is a non-starter I’m afraid, cause it’s just property access. An array can already have a property with the key "-1".

Personally I’d find it weird for slicing to be singled out for special syntax. It’s just a method. However I’d agree that it’s quite awkward to do things like arr[arr.length - 1] or arr.slice().pop() etc to access from the right. I think in one of those threads someone proposed a method like Array.prototype.nth() which would accept negative indices, though this creates an odd asymmetry since (presumably) we would not want to use -0 for the last index.

Yeah, -1 is the last index; it just does one round of "underflow" basically.

In general I think the functionality you’re describing could be useful but that it could be served better by adding methods rather than syntax.

Agreed. Slicing via Array#slice() can likely be extended to allow negative indexes, and even a third argument for steps to match Python (yay for the -1 step value, to get a slice in reverse!). Adding an nth() that just takes a single positive or negative index would be nice. That we still have to do arr[arr.length -1] is barbaric. ^_^

(Only downside of negative indexes is that, when using it on iterators, you have to consume the entire iterator before you can return the desired value.)

# Jordan Harband (7 years ago)

Array slice already allows negative indexes. arr.slice(-1)[0] works fine for a quick and dirty "last item".

# Alexander Craggs (7 years ago)

In which case,  perhaps change this proposal to .slice(start, end, step)

# Isiah Meadows (7 years ago)

That, I could support.

# Isiah Meadows (7 years ago)

To clarify, I'm just a normal, non-committee person. So that's a "I like the idea and it looks workable", but not "I'll champion this for you" (when I can't).