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 1 | # ECMAScript As-Patterns for Matching and Destructuring
 2 | 
 3 | ## [Status](https://tc39.github.io/process-document/)
 4 | 
 5 | **Stage**: 0
 6 | 
 7 | **Author**: Kat Marchán (npm, [@maybekatz](https://twitter.com/maybekatz))
 8 | 
 9 | **Champions**: Kat Marchán (npm, [@maybekatz](https://twitter.com/maybekatz))
10 | 
11 | ## Introduction
12 | 
13 | When matching non-Identifier values, it's often the case that users might want
14 | to also bind that value to an Identifier while doing the matching. For this
15 | reason, it's proposed that destructuring be extended with the ability to do
16 | this sort of binding. Furthermore, the separate [pattern matching
17 | proposal](https://github.com/tc39/proposal-pattern-matching) will benefit from
18 | this change by allowing matching operations against values that are also put
19 | into identifiers, since identifiers are irrefutable patterns.
20 | 
21 | The syntax uses an `as` keyword, and looks as follows:
22 | 
23 | ```js
24 | const {x: {y} as x} = {x: {y: 1}}
25 | // x is {y: 1}
26 | // y is 1
27 | ```
28 | 
29 | Or:
30 | ```js
31 | function foo ([{y} as x, [z] as zed = [1]]) {
32 |   // x is {y: ...}
33 |   // y is x.y
34 |   // z is runs an initializer if arguments[0][1] is undefined
35 | }
36 | ```
37 | 
38 | This applies similarly to `match`:
39 | ```js
40 | match (x) {
41 |   when {x: {y: 1} as x} ~> console.log(x.y === 1)
42 | }
43 | ```
44 | 
45 | Note: This syntax is [used by
46 | F#](https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/pattern-matching).
47 | It's also reminiscent of `as` syntax in `import` statements, so there's some
48 | precedent in the language for this sort of binding (`import * as bar from
49 | './x.js'`)
50 | 
51 | ## The Big Picture
52 | 
53 | ### Related Active Proposals
54 | 
55 | * [Pattern matching](https://github.com/tc39/proposal-pattern-matching)
56 | * [Collection literals](https://github.com/zkat/proposal-collection-literals)
57 | 
58 | ## As-Patterns
59 | 
60 | ### Syntax
61 | 
62 | **12.15.5 Destructuring Assignment Changes**:
63 | ```
64 | AssignmentRebinding :
65 |   `as` IdentifierReference
66 | 
67 | AssignmentElement :
68 |   DestructuringAssignmentTarget
69 |   DestructuringAssignmentTarget AssignmentRebinding
70 |   DestructuringAssignmentTarget Initializer
71 |   DestructuringAssignmentTarget AssignmentRebinding Initializer
72 | ```
73 | 
74 | **13.3.3 Destructuring Binding Patterns Changes**:
75 | ```
76 | BindingRebinding :
77 |   `as` IdentifierReference
78 | 
79 | BindingElement :
80 |   SingleNameBinding
81 |   BindingPattern
82 |   BindingPattern BindingRebinding
83 |   BindingPattern Initializer
84 |   BindingPattern BindingRebinding Initializer
85 | ```
86 | 
87 | **[Match Operator](https://github.com/tc39/proposal-pattern-matching) Syntax Changes**:
88 | ```
89 | MatchRebinding :
90 |   `as` IdentifierReference
91 | 
92 | MatchElement :
93 |   SingleNameBinding
94 |   MatchPattern
95 |   MatchPattern MatchRebinding
96 |   MatchPattern Initializer
97 |   MatchPattern MatchRebinding Initializer
98 | ```
99 | 


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