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[DCE] Fix var hoisting #169

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merged 1 commit into from
Sep 30, 2016
Merged

[DCE] Fix var hoisting #169

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35 changes: 35 additions & 0 deletions packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1999,4 +1999,39 @@ describe("dce-plugin", () => {
const expected = source;
expect(transform(source)).toBe(expected);
});

it("should preserve variabledeclarations(var) after completion statements", () => {
const source = unpad(`
function foo() {
a = 1;
return a;
var a;
}
`);

const expected = source;

expect(transform(source)).toBe(expected);
});

it("should NOT preserve variabledeclarations(let) after completion statements", () => {
const source = unpad(`
function foo() {
a = 1;
b = 2;
return a + b;
let a, b;
}
`);

const expected = unpad(`
function foo() {
a = 1;
b = 2;
return a + b;
}
`);

expect(transform(source)).toBe(expected);
});
});
10 changes: 8 additions & 2 deletions packages/babel-plugin-minify-dead-code-elimination/src/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -283,7 +283,7 @@ module.exports = ({ types: t, traverse }) => {
continue;
}

if (purge && !p.isFunctionDeclaration()) {
if (purge && !canExistAfterCompletion(p)) {
removeOrVoid(p);
}
}
Expand All @@ -299,7 +299,7 @@ module.exports = ({ types: t, traverse }) => {

// Not last in it's block? (See BlockStatement visitor)
if (path.container.length - 1 !== path.key &&
!path.getSibling(path.key + 1).isFunctionDeclaration() &&
!canExistAfterCompletion(path.getSibling(path.key + 1)) &&
path.parentPath.isBlockStatement()
) {
// This is probably a new oppurtinity by some other transform
Expand Down Expand Up @@ -900,4 +900,10 @@ module.exports = ({ types: t, traverse }) => {
};
}
}

// things that are hoisted
function canExistAfterCompletion(path) {
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So basically anything that doesn't conform to TDZ? What about function?

a = 1;
return a;
function a() { }

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@boopathi boopathi Sep 29, 2016 •
edited

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That test is already covered in name=should be fine with fun decl after return if that's what you meant.

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https://github.com/babel/babili/blob/32c809b39638aba7d2bcb7bec684f91f2f551be3/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js#L490

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Ah, perfect!

return path.isFunctionDeclaration()
|| path.isVariableDeclaration({ kind: "var" });
}
};