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sapphi-red 814da55f81
feat(minifier): compress x = x || 1 to x ||= 1 (#8368) 
The simplified version of the evaluation of `a = a || b` is:

> AssignmentExpression : LeftHandSideExpression = LogicalORExpression || LogicalANDExpression
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let llRef be ? Evaluation of LogicalORExpression.
> 3. Let llVal be ? GetValue(llRef).
> 4. If ToBoolean(llVal) is true
>   a. Perform ? PutValue(lRef, llVal).
>   b. return llVal.
> 5. Let lrRef be ? Evaluation of LogicalANDExpression.
> 6. Let rRef be ? GetValue(lrRef).
> 7. Let rVal be ? GetValue(rRef). [Note GetValue(rRef) returns rRef itself]
> 8. Perform ? PutValue(lRef, rVal).
> 9. Return rVal.

The simplified version of the evaluation of `a ||= b` is:

> AssignmentExpression : LeftHandSideExpression ||= AssignmentExpression
> 1. Let lRef be ? Evaluation of LeftHandSideExpression.
> 2. Let lVal be ? GetValue(lRef).
> 3. If ToBoolean(lVal) is true, return lVal.
> 4. Let rRef be ? Evaluation of AssignmentExpression.
> 5. Let rVal be ? GetValue(rRef).
> 6. Perform ? PutValue(lRef, rVal).
> 7. Return rVal.

The difference of these is that

- the evaluation of `a` is done twice for `a = a || b`, one with `1. Let lRef be ? Evaluation of LeftHandSideExpression` and one with `2. Let llRef be ? Evaluation of LogicalORExpression.`. This is same with #8366, #8367.
- `PutValue(lRef, llVal)` is performed when `ToBoolean(lVal)` is `true`.

So `x = x || 1` can be compressed to `x ||= 1` when the conditions written in #8366 are met and `PutValue(lRef, llVal)` does not have a side effect. When `a` is a non-global identifier (and not a reference created by a `with` statement), these conditions are met.

**References**
- [Spec of `||`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-binary-logical-operators-runtime-semantics-evaluation)
- [Spec of `=` / `||=`](https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-binary-logical-operators-runtime-semantics-evaluation)
2025-01-09 03:47:12 +00:00
..
src feat(minifier): fold if(x)return;y -> if(!x)y (#8226) 2025-01-03 05:24:53 +00:00
Cargo.toml chore(tasks): print diff for minify idempotency assertion (#8161) 2024-12-29 00:55:45 +08:00
minsize.snap feat(minifier): compress x = x || 1 to x ||= 1 (#8368) 2025-01-09 03:47:12 +00:00
README.md

README.md

Track Minification size

See minification-benchmarks for baseline.