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no-vars.js
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no-vars.js
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module.exports = function(file, api) {
const j = api.jscodeshift;
const root = j(file.source);
const TOP_LEVEL_TYPES = [
'Function',
'FunctionDeclaration',
'FunctionExpression',
'ArrowFunctionExpression',
'Program',
];
const FOR_STATEMENTS = [
'ForStatement',
'ForOfStatement',
'ForInStatement',
];
const getScopeNode = blockScopeNode => {
let scopeNode = blockScopeNode;
let isInFor = FOR_STATEMENTS.indexOf(blockScopeNode.value.type) !== -1;
while (TOP_LEVEL_TYPES.indexOf(scopeNode.node.type) === -1) {
scopeNode = scopeNode.parentPath;
isInFor = isInFor || FOR_STATEMENTS.indexOf(scopeNode.value.type) !== -1;
}
return {scopeNode, isInFor};
};
const findFunctionDeclaration = (node, container) => {
while (node.value.type !== 'FunctionDeclaration' && node !== container) {
node = node.parentPath;
}
return node !== container ? node : null;
};
const isForLoopDeclarationWithoutInit = declaration => {
const parentType = declaration.parentPath.value.type;
return parentType === 'ForOfStatement' || parentType === 'ForInStatement';
};
const extractNamesFromIdentifierLike = id => {
if (!id) {
return [];
} else if (id.type === 'ObjectPattern') {
return id.properties.map(
d => (d.type === 'SpreadProperty' ? [d.argument.name] : extractNamesFromIdentifierLike(d.value))
).reduce((acc, val) => acc.concat(val), []);
} else if (id.type === 'ArrayPattern') {
return id.elements.map(extractNamesFromIdentifierLike).reduce((acc, val) => acc.concat(val), []);
} else if (id.type === 'Identifier') {
return [id.name];
} else if (id.type === 'RestElement') {
return [id.argument.name];
} else {
return [];
}
};
const getDeclaratorNames = (declarator) => {
return extractNamesFromIdentifierLike(declarator.id);
};
const isIdInDeclarator = (declarator, name) => {
return getDeclaratorNames(declarator).indexOf(name) !== -1;
};
const getLocalScope = (scope, parentScope) => {
const names = [];
while (scope !== parentScope) {
if (Array.isArray(scope.value.body)) {
scope.value.body.forEach(node => {
if (node.type === 'VariableDeclaration') {
node.declarations.map(getDeclaratorNames).forEach(dNames => {
dNames.forEach(name => {
if (names.indexOf(name) === -1) {
names.push(name);
}
});
});
}
});
}
if (Array.isArray(scope.value.params)) {
scope.value.params.forEach(id => {
extractNamesFromIdentifierLike(id).forEach(name => {
if (names.indexOf(name) === -1) {
names.push(name);
}
});
});
}
scope = scope.parentPath;
}
return names;
};
const hasLocalDeclarationFor = (nodePath, parentScope, name) => {
return (
getLocalScope(nodePath, parentScope).indexOf(name) !== -1
);
};
const isTruelyVar = (node, declarator) => {
const blockScopeNode = node.parentPath;
const {scopeNode, isInFor} = getScopeNode(blockScopeNode);
// if we are in a for loop of some kind, and the variable
// is referenced within a closure, revert to `var`
// It would be safe to do the conversion if you can verify
// that the callback is run synchronously
const isUsedInClosure = (
isInFor &&
j(blockScopeNode).find(j.Function).filter(
functionNode => (
j(functionNode).find(j.Identifier).filter(
id => isIdInDeclarator(declarator, id.value.name)
).size() !== 0
)
).size() !== 0
);
// if two attempts are made to declare the same variable,
// revert to `var`
// TODO: if they are in different block scopes, it may be
// safe to convert them anyway
const isDeclaredTwice = j(scopeNode)
.find(j.VariableDeclarator)
.filter(otherDeclarator => {
return (
otherDeclarator.value !== declarator &&
getScopeNode(otherDeclarator).scopeNode === scopeNode &&
getDeclaratorNames(otherDeclarator.value).some(
name => isIdInDeclarator(declarator, name)
)
);
}).size() !== 0;
return isUsedInClosure || isDeclaredTwice || j(scopeNode)
.find(j.Identifier)
.filter(n => {
if (!isIdInDeclarator(declarator, n.value.name)) {
return false;
}
// If the variable is used in a function declaration that gets
// hoisted, it could get called early
const functionDeclaration = findFunctionDeclaration(n, scopeNode);
const isCalledInHoistedFunction = (
functionDeclaration &&
j(scopeNode)
.find(j.Identifier)
.filter(n => {
return (
n.value.name === functionDeclaration.value.id.name &&
n.value.start < declarator.start
);
}).size() !== 0
);
if (isCalledInHoistedFunction) {
return true;
}
const referenceScope = getScopeNode(n.parent).scopeNode;
if (
referenceScope === scopeNode ||
!hasLocalDeclarationFor(n, scopeNode, n.value.name)
) {
// if the variable is referenced outside the current block
// scope, revert to using `var`
const isOutsideCurrentScope = (
j(blockScopeNode).find(j.Identifier).filter(
innerNode => innerNode.node.start === n.node.start
).size() === 0
);
// if a variable is used before it is declared, revert to
// `var`
// TODO: If `isDeclaredTwice` is improved, and there is
// another declaration for this variable, it may be
// safe to convert this anyway
const isUsedBeforeDeclaration = (
n.value.start < declarator.start
);
return (
isOutsideCurrentScope ||
isUsedBeforeDeclaration
);
}
}).size() > 0;
};
/**
* isMutated utility function to determine whether a VariableDeclaration
* contains mutations. Takes an optional VariableDeclarator node argument to
* return only whether that specific Identifier is mutated
*
* @param {ASTPath} node VariableDeclaration path
* @param {ASTNode} [declarator] VariableDeclarator node
* @return {Boolean}
*/
const isMutated = (node, declarator) => {
const scopeNode = node.parent;
const hasAssignmentMutation = j(scopeNode)
.find(j.AssignmentExpression)
.filter(n => {
return extractNamesFromIdentifierLike(n.value.left).some(name => {
return isIdInDeclarator(declarator, name);
});
}).size() > 0;
const hasUpdateMutation = j(scopeNode)
.find(j.UpdateExpression)
.filter(n => {
return isIdInDeclarator(declarator, n.value.argument.name);
}).size() > 0;
return hasAssignmentMutation || hasUpdateMutation;
};
const updatedAnything = root.find(j.VariableDeclaration).filter(
dec => dec.value.kind === 'var'
).filter(declaration => {
return declaration.value.declarations.every(declarator => {
return !isTruelyVar(declaration, declarator);
});
}).forEach(declaration => {
const forLoopWithoutInit = isForLoopDeclarationWithoutInit(declaration);
if (
declaration.value.declarations.some(declarator => {
return (!declarator.init && !forLoopWithoutInit) || isMutated(declaration, declarator);
})
) {
declaration.value.kind = 'let';
} else {
declaration.value.kind = 'const';
}
}).size() !== 0;
return updatedAnything ? root.toSource() : null;
}