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https://github.com/tc39/proposal-logical-assignment
A proposal to combine Logical Operators and Assignment Expressions
https://github.com/tc39/proposal-logical-assignment
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A proposal to combine Logical Operators and Assignment Expressions
- Host: GitHub
- URL: https://github.com/tc39/proposal-logical-assignment
- Owner: tc39
- License: mit
- Archived: true
- Created: 2018-02-16T04:00:35.000Z (almost 7 years ago)
- Default Branch: master
- Last Pushed: 2020-07-21T20:36:57.000Z (over 4 years ago)
- Last Synced: 2024-08-03T09:07:12.090Z (4 months ago)
- Language: HTML
- Homepage: https://tc39.es/proposal-logical-assignment/
- Size: 212 KB
- Stars: 300
- Watchers: 60
- Forks: 13
- Open Issues: 4
-
Metadata Files:
- Readme: README.md
- License: LICENSE
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README
# proposal-logical-assignment
A proposal to combine Logical Operators and Assignment Expressions:
```js
// "Or Or Equals" (or, the Mallet operator :wink:)
a ||= b;
a || (a = b);
// "And And Equals"
a &&= b;
a && (a = b);
// "QQ Equals"
a ??= b;
a ?? (a = b);
```
## Status
Current [Stage](https://tc39.es/process-document/): 4
## Champions
- Justin Ridgewell ([@jridgewell](https://github.com/jridgewell/))
- Hemanth HM ([@hemanth](https://github.com/hemanth/))
## Motivation
Convenience operators, inspired by
[Ruby's](https://docs.ruby-lang.org/en/2.5.0/syntax/assignment_rdoc.html#label-Abbreviated+Assignment).
We already have a dozen [mathematical assignment
operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Assignment_Operators),
but we don't have ones for the often used logical operators.
```js
function example(a) {
// Default `a` to "foo"
if (!a) {
a = 'foo';
}
}
```
If statements work, but terseness would be nice. Especially when dealing
with deep property assignment:
```js
function example(opts) {
// Ok, but could trigger setter.
opts.foo = opts.foo ?? 'bar'
// No setter, but 'feels wrong' to write.
opts.baz ?? (opts.baz = 'qux');
}
example({ foo: 'foo' })
```
With this proposal, we get terseness and we don't have to suffer from
setter calls:
```js
function example(opts) {
// Setters are not needlessly called.
opts.foo ??= 'bar'
// No repetition of `opts.baz`.
opts.baz ??= 'qux';
}
example({ foo: 'foo' })
```
## Semantics
The logical assignment operators function a bit differently than their
mathematical assignment friends. While math assignment operators
_always_ trigger a set operation, logical assignment embraces their
short-circuiting semantics to avoid it when possible.
```js
let x = 0;
const obj = {
get x() {
return x;
},
set x(value) {
console.log('setter called');
x = value;
}
};
// This always logs "setter called"
obj.x += 1;
assert.equal(obj.x, 1);
// Logical operators do not call setters unnecessarily
// This will not log.
obj.x ||= 2;
assert.equal(obj.x, 1);
// But setters are called if the operator does not short circuit
// "setter called"
obj.x &&= 3;
assert.equal(obj.x, 3);
```
In most cases, the fact that the set operation is short-circuited has no
observable impact beyond performance. But when it has side effects, it
is often desirable to avoid it when appropriate. In the following
example, if the `.innerHTML` setter was triggered uselessly, it could
result in the loss of state (such as focus) that is not serialized in
HTML:
```js
document.getElementById('previewZone').innerHTML ||= 'Nothing to preview';
```
See discussion of short-circuit semantics in [#3](https://github.com/tc39/proposal-logical-assignment/issues/3). It also highlights differences already present in mathematical assignment operators in code like `obj.deep[key++] += 1` vs `obj.deep[key] = obj.deep[key++] + 1`.
## Related
- [Ruby's logical operators](https://docs.ruby-lang.org/en/2.5.0/syntax/assignment_rdoc.html#label-Abbreviated+Assignment)
- [Explainer on no-set semantics](http://www.rubyinside.com/what-rubys-double-pipe-or-equals-really-does-5488.html)
- [CoffeeScript](http://coffeescript.org/#try:a%20%3D%201%0Ab%20%3D%202%0A%0A%0A%23%20%22Or%20Or%20Equals%22%20(or%2C%20the%20Mallet%20operator%20%3Awink%3A)%0Aa%20%7C%7C%3D%20b%3B%0Aa%20%7C%7C%20(a%20%3D%20b)%3B%0A%0A%23%20%22And%20And%20Equals%22%0Aa%20%26%26%3D%20b%3B%0Aa%20%26%26%20(a%20%3D%20b)%3B%0A%0A%23%20Eventually....%0A%23%20%22QQ%20Equals%22%0A%23a%20%3F%3F%3D%20b%3B%0A%23a%20%3F%3F%20(a%20%3D%20b)%3B%0A)
- [C#'s null coalescing assignment](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/null-coalescing-assignment#detailed-design)
- My very first [Babel PR](https://github.com/babel/babel/pull/516) (back when it was still [6to5](https://github.com/babel/babel/tree/ecd85f53b4764ada862537aa767699814f1f1fe2)). 😄