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fix: empty destructuring with var should be converted to const #351

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fix: empty destructuring with var should be converted to const #351

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24 changes: 23 additions & 1 deletion src/transform/let.js
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,18 @@ export default function(ast, loggerInstance) {
variableMarker.markModified(decl.id.name);
}
});


// Empty destructuring should be converted to const.
//
// var [] = foo;
// var {} = foo;
//
// Mix of empty destructuring and other variable declarations will be handled
// by other parts of the transform.
if (node.kind === 'var' && node.declarations.every(isEmptyDestructuring)) {
node.kind = 'const';
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Modifying the node.kind field here is definitely the easy way to fix this, but it doesn't align with how the rest of this transform is organized. Until now, there have been two distinct phases:

  • first gather all the data of all the var-declarations to be modified.
  • then go and convert the vars to let/const.

This change breaks this pattern. Now some var-declarations get modified during this first loop of gathering data, which breaks expectations of somebody reading this code in the future.

I'd rather leave these var-s unchanged than do it this way.

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At first, I was trying to do it in the transformation phase, but nothing about empty restructuring will be collected in the "gathering data" stage. It's empty. 😢 That's why I ended up putting it here.

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@nene nene Oct 1, 2023 •
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Yeah, it would need to involve some more reorganization. Currently this conversion loop goes through all variables, and takes the var-declarations of them, but as there are no variables here such an empty declaration will never be reached.

It's definitely not an easy change to make. There's a currently a heavily beaked-in assumption that all variable declarations actually declare at least a single variable.

One possible middle-ground solution would be to separately gather these empty declaration nodes and then separately loop through them in the modification-phase. This way at least the modification and gathering phases will be kept separate, avoiding confusion in that area.

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Good idea. I will try to separate them.

}
}
else if (node.type === 'AssignmentExpression') {
destructuring.extractVariableNames(node.left).forEach(name => {
Expand Down Expand Up @@ -90,6 +102,16 @@ function isAnyWhileStatement(node) {
node.type === 'DoWhileStatement';
}

function isEmptyDestructuring(node) {
if (node.id.type === 'ObjectPattern') {
return node.id.properties.length === 0;
}
if (node.id.type === 'ArrayPattern') {
return node.id.elements.length === 0;
}
return false;
}

// Program node works almost like a function:
// it hoists all variables which can be transformed to block-scoped let/const.
// It just doesn't have name and parameters.
Expand Down Expand Up @@ -190,7 +212,7 @@ function transformVarsToLetOrConst() {

// let and const declarations aren't allowed in all the same places where
// var declarations are allowed. Notably, only var-declaration can occur
// directlt in if-statement (and other similar statements) body:
// directly in if-statement (and other similar statements) body:
//
// if (true) var x = 10;
//
Expand Down
25 changes: 25 additions & 0 deletions test/transform/letTest.js
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Expand Up @@ -161,6 +161,19 @@ describe('Let/const', () => {
'for (let i=0, len=arr.length; i<len; i++) {}'
);
});

it('should split to let & const when mixed with empty destructuring', () => {
expectTransform(
'var [] = [1, 2], {} = {foo: 1, bar: 2}, x = 1, y = 2;\n' +
'y = 3;'
).toReturn(
'const [] = [1, 2];\n' +
'const {} = {foo: 1, bar: 2};\n' +
'const x = 1;\n' +
'let y = 2;\n' +
'y = 3;'
);
});
});

describe('with variable declaration in restrictive parent', () => {
Expand Down Expand Up @@ -278,6 +291,18 @@ describe('Let/const', () => {
'console.log(x);'
);
});

it('should use const when no variables are destructured', () => {
expectTransform(
'var [] = [1, 2];\n' +
'var {} = {foo: 1, bar: 2};\n' +
'var [] = [1, 2], {} = {foo: 1, bar: 2};'
).toReturn(
'const [] = [1, 2];\n' +
'const {} = {foo: 1, bar: 2};\n' +
'const [] = [1, 2], {} = {foo: 1, bar: 2};'
);
});
});

describe('with nested function', () => {
Expand Down