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Destructuring: Fix array unpacking assignments with holes on RHS #9412

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merged 1 commit into from Mar 27, 2019
Merged

Destructuring: Fix array unpacking assignments with holes on RHS #9412

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6 changes: 5 additions & 1 deletion packages/babel-plugin-transform-destructuring/src/index.js
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Expand Up @@ -83,7 +83,11 @@ export default declare((api, options) => {

if (op) {
node = t.expressionStatement(
t.assignmentExpression(op, id, t.cloneNode(init)),
t.assignmentExpression(
op,
id,
t.cloneNode(init) || this.scope.buildUndefinedNode(),
),
);
} else {
node = t.variableDeclaration(this.kind, [
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4 changes: 4 additions & 0 deletions ...in-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/input.js
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Expand Up @@ -11,3 +11,7 @@ var [a, b] = [...foo, bar];
var [a, b] = [foo(), bar];
var [a, b] = [clazz.foo(), bar];
var [a, b] = [clazz.foo, bar];
var [a, b] = [, 2];
[a, b] = [1, 2];
[a, b] = [, 2];
; // Avoid completion record special case
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Not in this PR, but this optimization is wrong: ; keeps the previous completion value:

eval("0;;;;")

Can you use something like 0; instead?

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Yeah, I've just realised this on another PR. Ultimately it's a false negative in the isCompletionRecord check, which could be quite expensive to correct without memoising somehow.

I'll change this, but FWIW there are several other tests that currently use ; in this way and would need to change similarly.

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This can/should be handled in another PR

7 changes: 7 additions & 0 deletions ...n-transform-destructuring/test/fixtures/destructuring/array-unpack-optimisation/output.js
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Expand Up @@ -34,3 +34,10 @@ var _ref7 = [clazz.foo(), bar],
var _ref8 = [clazz.foo, bar],
a = _ref8[0],
b = _ref8[1];
var a,
b = 2;
a = 1;
b = 2;
a = void 0;
b = 2;
; // Avoid completion record special case