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Updating spec text #73

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Updating spec text #73

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6a04369
[spec update] updating spec text
Oct 30, 2018
9a63dc1
[spec] reverting null/undefined changes
Oct 31, 2018
b98bd8e
[tiny] fixing missed grammar
Oct 31, 2018
51f37b3
[tiny] fixing a few nits
Nov 29, 2018
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68 changes: 14 additions & 54 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ Current Stage:
## Authors
* Claude Pache (@claudepache)
* Gabriel Isenberg (@the_gisenberg)
* Dustin Savery (@dustinsavery)

## Overview and motivation
When looking for a property value that's deep in a tree-like structure, one often has to check whether intermediate nodes exist:
Expand All @@ -29,19 +30,6 @@ var street = user.address?.street
var fooValue = myForm.querySelector('input[name=foo]')?.value
```

The call variant of Optional Chaining is useful for dealing with interfaces that have optional methods:

```js
iterator.return?.() // manually close an iterator
```
or with methods not universally implemented:
```js
if (myForm.checkValidity?.() === false) { // skip the test in older web browsers
// form validation fails
return;
}
```

## Prior Art
The following languages implement the operator with the same general semantics as this proposal (i.e., 1) guarding against a null base value, and 2) short-circuiting application to the whole chain):
* C#: [Null-conditional operator](https://msdn.microsoft.com/en-us/library/dn986595.aspx) — null-conditional member access or index, in read access.
Expand All @@ -58,7 +46,6 @@ The Optional Chaining operator is spelled `?.`. It may appear in three positions
```javascript
obj?.prop // optional static property access
obj?.[expr] // optional dynamic property access
func?.(...args) // optional function or method call
```

### Notes
Expand All @@ -71,27 +58,27 @@ If the operand at the left-hand side of the `?.` operator evaluates to undefined

Here are basic examples, each one followed by its desugaring. (The desugaring is not exact in the sense that the LHS should be evaluated only once.)
```js
a?.b         // undefined if `a` is null/undefined, `a.b` otherwise.
a?.b // undefined if `a` is null/undefined, `a.b` otherwise.
a == null ? undefined : a.b

a?.[x]       // undefined if `a` is null/undefined, `a[x]` otherwise.
a?.[x] // undefined if `a` is null/undefined, `a[x]` otherwise.
a == null ? undefined : a[x]

a?.b()       // undefined if `a` is null/undefined
a?.b() // undefined if `a` is null/undefined
a == null ? undefined : a.b() // throws a TypeError if `a.b` is not a function
                            // otherwise, evaluates to `a.b()`
// otherwise, evaluates to `a.b()`

a?.()       // undefined if `a` is null/undefined
a?.() // undefined if `a` is null/undefined
a == null ? undefined : a() // throws a TypeError if `a` is neither null/undefined, nor a function
            // invokes the function `a` otherwise
// invokes the function `a` otherwise
```

### Short-circuiting

If the expression on the LHS of `?.` evaluates to null/undefined, the RHS is not evaluated. This concept is called *short-circuiting*.

```js
a?.[++x]         // `x` is incremented if and only if `a` is not null/undefined
a?.[++x] // `x` is incremented if and only if `a` is not null/undefined
a == null ? undefined : a[++x]
```

Expand All @@ -100,8 +87,8 @@ a == null ? undefined : a[++x]
In fact, short-circuiting, when triggered, skips not only the current property access, method or function call, but also the whole chain of property accesses, method or function calls directly following the Optional Chaining operator.

```js
a?.b.c(++x).d  // if `a` is null/undefined, evaluates to undefined. Variable `x` is not incremented.
              // otherwise, evaluates to `a.b.c(++x).d`.
a?.b.c(++x).d // if `a` is null/undefined, evaluates to undefined. Variable `x` is not incremented.
// otherwise, evaluates to `a.b.c(++x).d`.
a == null ? undefined : a.b.c(++x).d
```

Expand Down Expand Up @@ -134,37 +121,22 @@ That follows from the design choice of specifying the scope of short-circuiting

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Above desugaring is incorrect.

Note that, whatever the semantics are, there is no practical reason to use parentheses in that position anyway.

### Optional deletion
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👏

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If you want to remove ”optional deletion”, you must add additional static semantics in Section The delete Operator, in order to forbid specifically a LeftHandSideExpression containing an OptionalChain to be used with the delete operator.

As I said in #40, ”Optional deletion has very few practical use cases; but forbidding it has probably no practical utility either.” However, forbidding optional deletion adds positive complexity to the spec.


Because the `delete` operator is very liberal in what it accepts, we have that feature for free:
```js
delete a?.b
// delete (a == null ? undefined : a.b) // that *would* work if `? :` could return a Reference...
a == null ? undefined : delete a.b // this is what we get, really
```

## Not supported

Although they could be included for completeness, the following are not supported due to lack of real-world use cases or other compelling reasons; see [Issue # 22](https://github.com/tc39/proposal-optional-chaining/issues/22) and [Issue #54](https://github.com/tc39/proposal-optional-chaining/issues/54) for discussion:
Although they could be included for completeness, the following are not supported due to complexity, lack of real-world use cases, or other compelling reasons:

* optional function execution: `a?.()`
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👏#59

* optional construction: `new a?.()`
* optional template literal: ``a?.`{b}` ``
* constructor or template literals in/after an Optional Chain: `new a?.b()`, ``a?.b`{c}` ``

The following is not supported, although it has some use cases; see [Issue #18](//github.com/tc39/proposal-optional-chaining/issues/18) for discussion:

* optional property assignment: `a?.b = c`
* optional property assignment: `a?.b = x`

All the above cases will be forbidden by the grammar or by static semantics so that support might be added later.

## FAQ

[TODO: to be completed. In particular, discuss specific criticisms around long short-circuiting.]


<dl>


<dt>obj?.[expr] and func?.(arg) look ugly. Why not use obj?[expr] and func?(arg) as does &lt;language X>?

<dd>
Expand All @@ -174,22 +146,10 @@ We don’t use the `obj?[expr]` and `func?(arg)` syntax, because of the difficul
Alternative syntaxes for those two cases each have their own flaws; and deciding which one looks the least bad is mostly a question of personal taste. Here is how we made our choice:

* pick the best syntax for the `obj?.prop` case, which is expected to occur most often;
* extend the use of the recognisable `?.` sequence of characters to other cases: `obj?.[expr]`, `func?.(arg)`.
* extend the use of the recognisable `?.` sequence of characters to other cases: `obj?.[expr]`.

As for &lt;language X>, it has different syntactical constraints than JavaScript because of &lt;some construct not supported by X or working differently in X>.



<dt>Why does (null)?.b evaluate to undefined rather than null?

<dd>

Neither `a.b` nor `a?.b` is intended to preserve arbitrary information on the base object `a`, but only to give information about the property `"b"` of that object. If a property `"b"` is absent from `a`, this is reflected by `a.b === undefined` and `a?.b === undefined`.

In particular, the value `null` is considered to have no properties; therefore, `(null)?.b` is undefined.



<dt>Why do you want long short-circuiting?</dt>

<dd>
Expand Down
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