/**
|
* @fileoverview A rule to suggest using of const declaration for variables that are never reassigned after declared.
|
* @author Toru Nagashima
|
*/
|
|
"use strict";
|
|
const astUtils = require("./utils/ast-utils");
|
|
//------------------------------------------------------------------------------
|
// Helpers
|
//------------------------------------------------------------------------------
|
|
const PATTERN_TYPE = /^(?:.+?Pattern|RestElement|SpreadProperty|ExperimentalRestProperty|Property)$/u;
|
const DECLARATION_HOST_TYPE = /^(?:Program|BlockStatement|SwitchCase)$/u;
|
const DESTRUCTURING_HOST_TYPE = /^(?:VariableDeclarator|AssignmentExpression)$/u;
|
|
/**
|
* Checks whether a given node is located at `ForStatement.init` or not.
|
*
|
* @param {ASTNode} node - A node to check.
|
* @returns {boolean} `true` if the node is located at `ForStatement.init`.
|
*/
|
function isInitOfForStatement(node) {
|
return node.parent.type === "ForStatement" && node.parent.init === node;
|
}
|
|
/**
|
* Checks whether a given Identifier node becomes a VariableDeclaration or not.
|
*
|
* @param {ASTNode} identifier - An Identifier node to check.
|
* @returns {boolean} `true` if the node can become a VariableDeclaration.
|
*/
|
function canBecomeVariableDeclaration(identifier) {
|
let node = identifier.parent;
|
|
while (PATTERN_TYPE.test(node.type)) {
|
node = node.parent;
|
}
|
|
return (
|
node.type === "VariableDeclarator" ||
|
(
|
node.type === "AssignmentExpression" &&
|
node.parent.type === "ExpressionStatement" &&
|
DECLARATION_HOST_TYPE.test(node.parent.parent.type)
|
)
|
);
|
}
|
|
/**
|
* Checks if an property or element is from outer scope or function parameters
|
* in destructing pattern.
|
*
|
* @param {string} name - A variable name to be checked.
|
* @param {eslint-scope.Scope} initScope - A scope to start find.
|
* @returns {boolean} Indicates if the variable is from outer scope or function parameters.
|
*/
|
function isOuterVariableInDestructing(name, initScope) {
|
|
if (initScope.through.find(ref => ref.resolved && ref.resolved.name === name)) {
|
return true;
|
}
|
|
const variable = astUtils.getVariableByName(initScope, name);
|
|
if (variable !== null) {
|
return variable.defs.some(def => def.type === "Parameter");
|
}
|
|
return false;
|
}
|
|
/**
|
* Gets the VariableDeclarator/AssignmentExpression node that a given reference
|
* belongs to.
|
* This is used to detect a mix of reassigned and never reassigned in a
|
* destructuring.
|
*
|
* @param {eslint-scope.Reference} reference - A reference to get.
|
* @returns {ASTNode|null} A VariableDeclarator/AssignmentExpression node or
|
* null.
|
*/
|
function getDestructuringHost(reference) {
|
if (!reference.isWrite()) {
|
return null;
|
}
|
let node = reference.identifier.parent;
|
|
while (PATTERN_TYPE.test(node.type)) {
|
node = node.parent;
|
}
|
|
if (!DESTRUCTURING_HOST_TYPE.test(node.type)) {
|
return null;
|
}
|
return node;
|
}
|
|
/**
|
* Determines if a destructuring assignment node contains
|
* any MemberExpression nodes. This is used to determine if a
|
* variable that is only written once using destructuring can be
|
* safely converted into a const declaration.
|
* @param {ASTNode} node The ObjectPattern or ArrayPattern node to check.
|
* @returns {boolean} True if the destructuring pattern contains
|
* a MemberExpression, false if not.
|
*/
|
function hasMemberExpressionAssignment(node) {
|
switch (node.type) {
|
case "ObjectPattern":
|
return node.properties.some(prop => {
|
if (prop) {
|
|
/*
|
* Spread elements have an argument property while
|
* others have a value property. Because different
|
* parsers use different node types for spread elements,
|
* we just check if there is an argument property.
|
*/
|
return hasMemberExpressionAssignment(prop.argument || prop.value);
|
}
|
|
return false;
|
});
|
|
case "ArrayPattern":
|
return node.elements.some(element => {
|
if (element) {
|
return hasMemberExpressionAssignment(element);
|
}
|
|
return false;
|
});
|
|
case "AssignmentPattern":
|
return hasMemberExpressionAssignment(node.left);
|
|
case "MemberExpression":
|
return true;
|
|
// no default
|
}
|
|
return false;
|
}
|
|
/**
|
* Gets an identifier node of a given variable.
|
*
|
* If the initialization exists or one or more reading references exist before
|
* the first assignment, the identifier node is the node of the declaration.
|
* Otherwise, the identifier node is the node of the first assignment.
|
*
|
* If the variable should not change to const, this function returns null.
|
* - If the variable is reassigned.
|
* - If the variable is never initialized nor assigned.
|
* - If the variable is initialized in a different scope from the declaration.
|
* - If the unique assignment of the variable cannot change to a declaration.
|
* e.g. `if (a) b = 1` / `return (b = 1)`
|
* - If the variable is declared in the global scope and `eslintUsed` is `true`.
|
* `/*exported foo` directive comment makes such variables. This rule does not
|
* warn such variables because this rule cannot distinguish whether the
|
* exported variables are reassigned or not.
|
*
|
* @param {eslint-scope.Variable} variable - A variable to get.
|
* @param {boolean} ignoreReadBeforeAssign -
|
* The value of `ignoreReadBeforeAssign` option.
|
* @returns {ASTNode|null}
|
* An Identifier node if the variable should change to const.
|
* Otherwise, null.
|
*/
|
function getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign) {
|
if (variable.eslintUsed && variable.scope.type === "global") {
|
return null;
|
}
|
|
// Finds the unique WriteReference.
|
let writer = null;
|
let isReadBeforeInit = false;
|
const references = variable.references;
|
|
for (let i = 0; i < references.length; ++i) {
|
const reference = references[i];
|
|
if (reference.isWrite()) {
|
const isReassigned = (
|
writer !== null &&
|
writer.identifier !== reference.identifier
|
);
|
|
if (isReassigned) {
|
return null;
|
}
|
|
const destructuringHost = getDestructuringHost(reference);
|
|
if (destructuringHost !== null && destructuringHost.left !== void 0) {
|
const leftNode = destructuringHost.left;
|
let hasOuterVariables = false,
|
hasNonIdentifiers = false;
|
|
if (leftNode.type === "ObjectPattern") {
|
const properties = leftNode.properties;
|
|
hasOuterVariables = properties
|
.filter(prop => prop.value)
|
.map(prop => prop.value.name)
|
.some(name => isOuterVariableInDestructing(name, variable.scope));
|
|
hasNonIdentifiers = hasMemberExpressionAssignment(leftNode);
|
|
} else if (leftNode.type === "ArrayPattern") {
|
const elements = leftNode.elements;
|
|
hasOuterVariables = elements
|
.map(element => element && element.name)
|
.some(name => isOuterVariableInDestructing(name, variable.scope));
|
|
hasNonIdentifiers = hasMemberExpressionAssignment(leftNode);
|
}
|
|
if (hasOuterVariables || hasNonIdentifiers) {
|
return null;
|
}
|
|
}
|
|
writer = reference;
|
|
} else if (reference.isRead() && writer === null) {
|
if (ignoreReadBeforeAssign) {
|
return null;
|
}
|
isReadBeforeInit = true;
|
}
|
}
|
|
/*
|
* If the assignment is from a different scope, ignore it.
|
* If the assignment cannot change to a declaration, ignore it.
|
*/
|
const shouldBeConst = (
|
writer !== null &&
|
writer.from === variable.scope &&
|
canBecomeVariableDeclaration(writer.identifier)
|
);
|
|
if (!shouldBeConst) {
|
return null;
|
}
|
|
if (isReadBeforeInit) {
|
return variable.defs[0].name;
|
}
|
|
return writer.identifier;
|
}
|
|
/**
|
* Groups by the VariableDeclarator/AssignmentExpression node that each
|
* reference of given variables belongs to.
|
* This is used to detect a mix of reassigned and never reassigned in a
|
* destructuring.
|
*
|
* @param {eslint-scope.Variable[]} variables - Variables to group by destructuring.
|
* @param {boolean} ignoreReadBeforeAssign -
|
* The value of `ignoreReadBeforeAssign` option.
|
* @returns {Map<ASTNode, ASTNode[]>} Grouped identifier nodes.
|
*/
|
function groupByDestructuring(variables, ignoreReadBeforeAssign) {
|
const identifierMap = new Map();
|
|
for (let i = 0; i < variables.length; ++i) {
|
const variable = variables[i];
|
const references = variable.references;
|
const identifier = getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign);
|
let prevId = null;
|
|
for (let j = 0; j < references.length; ++j) {
|
const reference = references[j];
|
const id = reference.identifier;
|
|
/*
|
* Avoid counting a reference twice or more for default values of
|
* destructuring.
|
*/
|
if (id === prevId) {
|
continue;
|
}
|
prevId = id;
|
|
// Add the identifier node into the destructuring group.
|
const group = getDestructuringHost(reference);
|
|
if (group) {
|
if (identifierMap.has(group)) {
|
identifierMap.get(group).push(identifier);
|
} else {
|
identifierMap.set(group, [identifier]);
|
}
|
}
|
}
|
}
|
|
return identifierMap;
|
}
|
|
/**
|
* Finds the nearest parent of node with a given type.
|
*
|
* @param {ASTNode} node – The node to search from.
|
* @param {string} type – The type field of the parent node.
|
* @param {Function} shouldStop – a predicate that returns true if the traversal should stop, and false otherwise.
|
* @returns {ASTNode} The closest ancestor with the specified type; null if no such ancestor exists.
|
*/
|
function findUp(node, type, shouldStop) {
|
if (!node || shouldStop(node)) {
|
return null;
|
}
|
if (node.type === type) {
|
return node;
|
}
|
return findUp(node.parent, type, shouldStop);
|
}
|
|
//------------------------------------------------------------------------------
|
// Rule Definition
|
//------------------------------------------------------------------------------
|
|
module.exports = {
|
meta: {
|
type: "suggestion",
|
|
docs: {
|
description: "require `const` declarations for variables that are never reassigned after declared",
|
category: "ECMAScript 6",
|
recommended: false,
|
url: "https://eslint.org/docs/rules/prefer-const"
|
},
|
|
fixable: "code",
|
|
schema: [
|
{
|
type: "object",
|
properties: {
|
destructuring: { enum: ["any", "all"], default: "any" },
|
ignoreReadBeforeAssign: { type: "boolean", default: false }
|
},
|
additionalProperties: false
|
}
|
],
|
messages: {
|
useConst: "'{{name}}' is never reassigned. Use 'const' instead."
|
}
|
},
|
|
create(context) {
|
const options = context.options[0] || {};
|
const sourceCode = context.getSourceCode();
|
const shouldMatchAnyDestructuredVariable = options.destructuring !== "all";
|
const ignoreReadBeforeAssign = options.ignoreReadBeforeAssign === true;
|
const variables = [];
|
let reportCount = 0;
|
let name = "";
|
|
/**
|
* Reports given identifier nodes if all of the nodes should be declared
|
* as const.
|
*
|
* The argument 'nodes' is an array of Identifier nodes.
|
* This node is the result of 'getIdentifierIfShouldBeConst()', so it's
|
* nullable. In simple declaration or assignment cases, the length of
|
* the array is 1. In destructuring cases, the length of the array can
|
* be 2 or more.
|
*
|
* @param {(eslint-scope.Reference|null)[]} nodes -
|
* References which are grouped by destructuring to report.
|
* @returns {void}
|
*/
|
function checkGroup(nodes) {
|
const nodesToReport = nodes.filter(Boolean);
|
|
if (nodes.length && (shouldMatchAnyDestructuredVariable || nodesToReport.length === nodes.length)) {
|
const varDeclParent = findUp(nodes[0], "VariableDeclaration", parentNode => parentNode.type.endsWith("Statement"));
|
const isVarDecParentNull = varDeclParent === null;
|
|
if (!isVarDecParentNull && varDeclParent.declarations.length > 0) {
|
const firstDeclaration = varDeclParent.declarations[0];
|
|
if (firstDeclaration.init) {
|
const firstDecParent = firstDeclaration.init.parent;
|
|
/*
|
* First we check the declaration type and then depending on
|
* if the type is a "VariableDeclarator" or its an "ObjectPattern"
|
* we compare the name from the first identifier, if the names are different
|
* we assign the new name and reset the count of reportCount and nodeCount in
|
* order to check each block for the number of reported errors and base our fix
|
* based on comparing nodes.length and nodesToReport.length.
|
*/
|
|
if (firstDecParent.type === "VariableDeclarator") {
|
|
if (firstDecParent.id.name !== name) {
|
name = firstDecParent.id.name;
|
reportCount = 0;
|
}
|
|
if (firstDecParent.id.type === "ObjectPattern") {
|
if (firstDecParent.init.name !== name) {
|
name = firstDecParent.init.name;
|
reportCount = 0;
|
}
|
}
|
}
|
}
|
}
|
|
let shouldFix = varDeclParent &&
|
|
// Don't do a fix unless the variable is initialized (or it's in a for-in or for-of loop)
|
(varDeclParent.parent.type === "ForInStatement" || varDeclParent.parent.type === "ForOfStatement" || varDeclParent.declarations[0].init) &&
|
|
/*
|
* If options.destructuring is "all", then this warning will not occur unless
|
* every assignment in the destructuring should be const. In that case, it's safe
|
* to apply the fix.
|
*/
|
nodesToReport.length === nodes.length;
|
|
if (!isVarDecParentNull && varDeclParent.declarations && varDeclParent.declarations.length !== 1) {
|
|
if (varDeclParent && varDeclParent.declarations && varDeclParent.declarations.length >= 1) {
|
|
/*
|
* Add nodesToReport.length to a count, then comparing the count to the length
|
* of the declarations in the current block.
|
*/
|
|
reportCount += nodesToReport.length;
|
|
shouldFix = shouldFix && (reportCount === varDeclParent.declarations.length);
|
}
|
}
|
|
nodesToReport.forEach(node => {
|
context.report({
|
node,
|
messageId: "useConst",
|
data: node,
|
fix: shouldFix ? fixer => fixer.replaceText(sourceCode.getFirstToken(varDeclParent), "const") : null
|
});
|
});
|
}
|
}
|
|
return {
|
"Program:exit"() {
|
groupByDestructuring(variables, ignoreReadBeforeAssign).forEach(checkGroup);
|
},
|
|
VariableDeclaration(node) {
|
if (node.kind === "let" && !isInitOfForStatement(node)) {
|
variables.push(...context.getDeclaredVariables(node));
|
}
|
}
|
};
|
}
|
};
|
