This repository contains a proposal for adding slice notation syntax to JavaScript. This is currently at stage 0 of the TC39 process.
The slice notation provides an ergonomic alternative to the various slice methods present on Array.prototype, String.prototype, etc.
const arr = ['a', 'b', 'c', 'd'];
arr[1:3];
// → ['b', 'c']
arr.slice(1, 3);
// → ['b', 'c']
const str = 'hello world';
str[6:];
// → 'world'
str.slice(6);
// → 'world'This notation can be used for slice operations on primitives like
String and any object that provides indexed access using [[Get]]
like Array and TypedArray.
The length used for these operations is the length property of the
object.
const obj = { 0: 'a', 1: 'b', 2: 'c', 3: 'd', length: 4 };
obj[1:3];
// → ['b', 'c']The slice notation extends the slice operations by accepting an optional step argument. The step argument is set to 1 if not provided.
const arr = ['a', 'b', 'c', 'd'];
arr[1:4:2];
// → ['b', 'd']const arr = ['a', 'b', 'c', 'd'];
arr.slice(3);
// → ['a', 'b', 'c'] or ['d'] ?In the above example, it's not immediately clear if the newly created
array is a slice from the range 0 to 3 or from 3 to len(arr).
const arr = ['a', 'b', 'c', 'd'];
arr.slice(1, 3);
// → ['b', 'c'] or ['b', 'c', 'd'] ?Adding a second argument is also ambigous since it's not clear if the second argument specifies an upper bound or the length of the new slice.
Programming language like Ruby and C++ take the length of the new slice as the second argument, but JavaScript's slice methods take the upper bound as the second argument.
const arr = ['a', 'b', 'c', 'd'];
arr[3:];
// → ['d']
arr[1:3];
// → ['b', 'c']With the new slice syntax, it's immediately clear that the lower bound
is 3 and the upper bound is len(arr). It makes the intent
explicit.
The syntax is also much shorter and more ergonomic than a function call.
The step argument is useful for patterns like creating a slice with every other element in an array or for reversing an array.
const arr = ['a', 'b', 'c', 'd'];
arr[1:4:2];
// → ['b', 'd']
arr[::-1];
// → ['d', 'c', 'b', 'a']The step argument also makes it really easy to work with matrices.
const matrix = [ 1, 2, 3,
4, 5, 6,
7, 8, 9 ];
getColumn = col => matrix[col::3];This is used a lot in scientific computing projects in other programming languages. For example:
In the following text, 'length of the object' refers to the length
property of the object.
The lower bound, upper bound and the step argument are all optional.
The default value for the lower bound is 0.
const arr = ['a', 'b', 'c', 'd'];
arr[:3:1];
// → ['a', 'b', 'c']The default value for the upper bound is the length of the object.
const arr = ['a', 'b', 'c', 'd'];
arr[1::1];
// → ['b', 'c', 'd']The default value for the step argument is 1.
const arr = ['a', 'b', 'c', 'd'];
arr[1:];
// → ['b', 'c', 'd']
arr[:3];
// → ['a', 'b', 'c']
arr[1::2];
// → ['b', 'd']
arr[:3:2];
// → ['a', 'c']Omitting all lower bound and upper bound value, produces a new copy of the object.
const arr = ['a', 'b', 'c', 'd'];
arr[:];
// → ['a', 'b', 'c', 'd']
arr[::];
// → ['a', 'b', 'c', 'd']If the lower bound is negative, then the start index is computed as follows:
start = max(lowerBound + len, 0)where len is the length of the object.
const arr = ['a', 'b', 'c', 'd'];
arr[-2:];
// → ['c', 'd']In the above example, start = max((-2 + 4), 0) = max(2, 0) = 2.
const arr = ['a', 'b', 'c', 'd'];
arr[-10:];
// → ['a', 'b', 'c', 'd']In the above example, start = max((-10 + 4), 0) = max(-6, 0) = 0.
Similarly, if the upper bound is negative, the end index is computed as follows:
end = max(upperBound + len, 0)const arr = ['a', 'b', 'c', 'd'];
arr[:-2];
// → ['a', 'b']
arr[:-10];
// → []These semantics exactly match the behavior of existing slice operations.
If the step argument is negative, then the object is traversed in reverse.
const arr = ['a', 'b', 'c', 'd'];
arr[::-1];
// → ['d', 'c', 'b', 'a']Both the lower and upper bounds are capped at the length of the object.
const arr = ['a', 'b', 'c', 'd'];
arr[100:];
// → []
arr[:100];
// → ['a', 'b', 'c', 'd']These semantics exactly match the behavior of existing slice operations.
This proposal is highly inspired by Python. Unsurprisingly, the Python syntax for slice notation is strikingly similar:
slicing ::= primary "[" slice_list "]"
slice_list ::= slice_item ("," slice_item)* [","]
slice_item ::= expression | proper_slice
proper_slice ::= [lower_bound] ":" [upper_bound] [ ":" [stride] ]
lower_bound ::= expression
upper_bound ::= expression
stride ::= expressionExamples:
arr = [1, 2, 3, 4];
arr[1:3];
// → [2, 3]
arr[1:4:2]
// → [2, 4]CoffeeScript provides a Range operator that is inclusive with respect to the upper bound.
arr = [1, 2, 3, 4];
arr[1..3];
// → [2, 3, 4]CoffeeScript also provides another form the Range operator that is exclusive with respect to the upper bound.
arr = [1, 2, 3, 4];
arr[1...3];
// → [2, 3]Go offers slices:
arr := []int{1,2,3,4};
arr[1:3]
// → [2, 3]There is also ability to not provide lower or upper bound:
arr := []int{1,2,3,4};
arr[1:]
// → [2, 3, 4]
arr := []int{1,2,3,4};
arr[:3]
// → [1, 2, 3]Ruby seems to have two different ways to get a slice:
- Using a Range:
arr = [1, 2, 3, 4];
arr[1..3];
// → [2, 3, 4]This is similar to CoffeeScript. The 1..3 produces a Range object
which defines the set of indices to be sliced out.
- Using the comma operator:
arr = [1, 2, 3, 4];
arr[1, 3];
// → [2, 3, 4]The difference here is that the second argument is actually the length of the new slice, not the upper bound index.
This is currently valid ECMAScript syntax which makes this a non starter.
const s = 'foobar'
s[1, 3]
// → 'b'The Python syntax allows us to provide an optional step argument.
Also, the Python syntax which excludes the upper bound index is similar to the existing slice methods in JavaScript.
We could use exclusive Range operator (...) from CoffeeScript, but
that doesn't quite work for all cases because it's ambiguous with the
spread syntax. Example code from
getify:
Object.defineProperty(Number.prototype,Symbol.iterator,{
*value({ start = 0, step = 1 } = {}) {
var inc = this > 0 ? step : -step;
for (let i = start; Math.abs(i) <= Math.abs(this); i += inc) {
yield i;
}
},
enumerable: false,
writable: true,
configurable: true
});
const range = [ ...8 ];
// → [0, 1, 2, 3, 4, 5, 6, 7, 8]The iterator protocol isn't restricted to index lookup making it incompatible with this slice notation which works only on indices.
For example, Map and Sets have iterators but we shouldn't be able to slice them as they don't have indices.
CoffeeScript allows similar syntax to be used on the left hand side of
an AssignmentExpression leading to splice operation.
numbers = [1, 2, 3, 4]
numbers[2..4] = [7, 8]
// → [1, 2, 7, 8]This doesn't work with Strings as they are immutable, but could be
made to work with any object using a [Set]] operation.
This feature is currently omitted to limit the scope of the proposal, but can be incorporated in a follow on proposal.
Unfortunately, yes. The ambiguity arises from this production:
const x = [2];
const arr = [1, 2, 3, 4];
arr[::x[0]];Is the above creating a new array with values [1, 3] or is it
creating a bound method?
Go creates a slice over the underlying array, instead of allocating a new array.
arr := []int{1,2,3,4};
v = arr[1:3];
// → [2, 3]Here, v is just descriptor that holds a reference to the original
array arr. No new array allocation is performed. See this blog
post for more
details.
This doesn't map to any existing contruct in JavaScript and this would
be a step away from how methods work in JavaScript. To make this
syntax work well within the JavaScript model, such a view data
structure is not included in this proposal.
The slice notation maintains the behavior of the existing
String.prototype.slice method.
The String.prototype.slice method doesn't work well with unicode
characters. This blog
post by Mathias
Bynens, explains the problem.
Given that the existing method doesn't work well, banning the slice notation for strings might be a good idea to prevent more footguns.
const arr = [1, 2, 3, 4] + [5, 6];
// → [1, 2, 3, 4, 5, 6]This is not included in order to keep the proposal's scope maximally minimal.
The operator overloading proposal may be a better fit for this.
Languages like Ruby, evaluate their slice (well, range) syntax to create a Range object.
range = 1..4
// → 1..4A similar construct is already possible with the spread operator as shown in an example in an above FAQ.
This is actually doing a property lookup using [[Get]] on the
underlying object. For example,
const arr = [1, 2, 3, 4];
arr[1:3];
// → [1, 2, 3]This is doing a property lookup for the keys 1, 2 and 3.
But, shouldn't it do a lookup for the string '1:3'?
const arr = [1, 2, 3, 4];
arr['1:3'];
// → undefinedNo. The slice notation makes it analogus with how keyed lookup works. The key is first evaluated to a value and then the lookup happens using this value.
const arr = [1, 2, 3, 4];
const x = 0;
arr[x] !== arr['x'];
// → trueThe slice notation works similarly. The notation is first evaluated to a range of values and then each of the values are looked up.
Depending on context a:b, can mean:
LabelledStatementwithaas the label- Property a with value b in an object literal:
{a: b } - ConditionalExpression:
confused ? a : b - Potential type systems (like TypeScript and Flow) that might make it to JavaScript in the future.
Is it a lot of overhead to disambiguate between modes with context? Major mainstream programming languages like Python have all these modes and are being used as a primary tool for teaching programming.
Currently the proposal (arbitrarily) restricts them to be an
IdentifierReference or DecimalDigits.