└── readme.markdown


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 1 | # Undefined Coalescing Operator
 2 | 
 3 | This is a proposal for introducing a Undefined Coalescing [(Null Coalescing Operator in C#)](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operator) operator in [ECMAScript](https://github.com/tc39/ecma262/).
 4 | 
 5 | ## Motivation
 6 | 
 7 | We often provide default values using the logical or operator `||`, for convenience.
 8 | 
 9 | ```js
10 | var name = user.name || 'Bob'
11 | var bio = user.bio || 'Nothing to see here!'
12 | ```
13 | 
14 | Sometimes, however, the falsy check gets in the way. Particularly for those cases where the value is `''` or `0`, in which case we often want to preserve those values.
15 | 
16 | ```js
17 | var spellWand = {
18 |   remaining: 0
19 | }
20 | console.log(spellWand.remaining || 5) // <- 5
21 | ```
22 | 
23 | When the left-hand side value is `undefined`, the Undefined Coalescing Operator returns the right-hand side value.
24 | 
25 | ```js
26 | var user = {}
27 | console.log(user.name ?? 'Bob') // 'Bob'
28 | ```
29 | 
30 | The Undefined Coalescing Operator returns the left-hand side value in all other cases.
31 | 
32 | ```js
33 | var spellWand = {
34 |   remaining: 0
35 | }
36 | console.log(spellWand.remaining ?? 5) // <- 0
37 | ```
38 | 
39 | ## Implementation
40 | 
41 | `left ?? right` is equivalent to `left === undefined ? right : left`.
42 | 
43 | ## Limitations
44 | 
45 | C# coalesces `null`, they however don't have a concept for `undefined`. In JavaScript `null` is often treated as interchangeable with `undefined`, but sometimes it's not.
46 | 
47 | Default values in JavaScript function parameters or while destructuring don't consider `null` to be a missing value, and thus logic and the principle of least surprise dictate the Undefined Coalescing Operator shouldn't either. 
48 | 
49 | Under the current proposal, `null` is not covered by the Undefined Coalescing Operator, and `left === null ? right : left` should still be used for this case.
50 | 


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