/**
* @fileoverview Rule to flag `else` after a `return` in `if`
* @author Ian Christian Myers
*/
"use strict";
//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const astUtils = require("./utils/ast-utils");
const FixTracker = require("./utils/fix-tracker");
//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
module.exports = {
meta: {
type: "suggestion",
docs: {
description: "disallow `else` blocks after `return` statements in `if` statements",
category: "Best Practices",
recommended: false,
url: "https://eslint.org/docs/rules/no-else-return"
},
schema: [{
type: "object",
properties: {
allowElseIf: {
type: "boolean",
default: true
}
},
additionalProperties: false
}],
fixable: "code",
messages: {
unexpected: "Unnecessary 'else' after 'return'."
}
},
create(context) {
//--------------------------------------------------------------------------
// Helpers
//--------------------------------------------------------------------------
/**
* Checks whether the given names can be safely used to declare block-scoped variables
* in the given scope. Name collisions can produce redeclaration syntax errors,
* or silently change references and modify behavior of the original code.
*
* This is not a generic function. In particular, it is assumed that the scope is a function scope or
* a function's inner scope, and that the names can be valid identifiers in the given scope.
*
* @param {string[]} names Array of variable names.
* @param {eslint-scope.Scope} scope Function scope or a function's inner scope.
* @returns {boolean} True if all names can be safely declared, false otherwise.
*/
function isSafeToDeclare(names, scope) {
if (names.length === 0) {
return true;
}
const functionScope = scope.variableScope;
/*
* If this is a function scope, scope.variables will contain parameters, implicit variables such as "arguments",
* all function-scoped variables ('var'), and block-scoped variables defined in the scope.
* If this is an inner scope, scope.variables will contain block-scoped variables defined in the scope.
*
* Redeclaring any of these would cause a syntax error, except for the implicit variables.
*/
const declaredVariables = scope.variables.filter(({ defs }) => defs.length > 0);
if (declaredVariables.some(({ name }) => names.includes(name))) {
return false;
}
// Redeclaring a catch variable would also cause a syntax error.
if (scope !== functionScope && scope.upper.type === "catch") {
if (scope.upper.variables.some(({ name }) => names.includes(name))) {
return false;
}
}
/*
* Redeclaring an implicit variable, such as "arguments", would not cause a syntax error.
* However, if the variable was used, declaring a new one with the same name would change references
* and modify behavior.
*/
const usedImplicitVariables = scope.variables.filter(({ defs, references }) =>
defs.length === 0 && references.length > 0);
if (usedImplicitVariables.some(({ name }) => names.includes(name))) {
return false;
}
/*
* Declaring a variable with a name that was already used to reference a variable from an upper scope
* would change references and modify behavior.
*/
if (scope.through.some(t => names.includes(t.identifier.name))) {
return false;
}
/*
* If the scope is an inner scope (not the function scope), an uninitialized `var` variable declared inside
* the scope node (directly or in one of its descendants) is neither declared nor 'through' in the scope.
*
* For example, this would be a syntax error "Identifier 'a' has already been declared":
* function foo() { if (bar) { let a; if (baz) { var a; } } }
*/
if (scope !== functionScope) {
const scopeNodeRange = scope.block.range;
const variablesToCheck = functionScope.variables.filter(({ name }) => names.includes(name));
if (variablesToCheck.some(v => v.defs.some(({ node: { range } }) =>
scopeNodeRange[0] <= range[0] && range[1] <= scopeNodeRange[1]))) {
return false;
}
}
return true;
}
/**
* Checks whether the removal of `else` and its braces is safe from variable name collisions.
*
* @param {Node} node The 'else' node.
* @param {eslint-scope.Scope} scope The scope in which the node and the whole 'if' statement is.
* @returns {boolean} True if it is safe, false otherwise.
*/
function isSafeFromNameCollisions(node, scope) {
if (node.type === "FunctionDeclaration") {
// Conditional function declaration. Scope and hoisting are unpredictable, different engines work differently.
return false;
}
if (node.type !== "BlockStatement") {
return true;
}
const elseBlockScope = scope.childScopes.find(({ block }) => block === node);
if (!elseBlockScope) {
// ecmaVersion < 6, `else` block statement cannot have its own scope, no possible collisions.
return true;
}
/*
* elseBlockScope is supposed to merge into its upper scope. elseBlockScope.variables array contains
* only block-scoped variables (such as let and const variables or class and function declarations)
* defined directly in the elseBlockScope. These are exactly the only names that could cause collisions.
*/
const namesToCheck = elseBlockScope.variables.map(({ name }) => name);
return isSafeToDeclare(namesToCheck, scope);
}
/**
* Display the context report if rule is violated
*
* @param {Node} node The 'else' node
* @returns {void}
*/
function displayReport(node) {
const currentScope = context.getScope();
context.report({
node,
messageId: "unexpected",
fix: fixer => {
if (!isSafeFromNameCollisions(node, currentScope)) {
return null;
}
const sourceCode = context.getSourceCode();
const startToken = sourceCode.getFirstToken(node);
const elseToken = sourceCode.getTokenBefore(startToken);
const source = sourceCode.getText(node);
const lastIfToken = sourceCode.getTokenBefore(elseToken);
let fixedSource, firstTokenOfElseBlock;
if (startToken.type === "Punctuator" && startToken.value === "{") {
firstTokenOfElseBlock = sourceCode.getTokenAfter(startToken);
} else {
firstTokenOfElseBlock = startToken;
}
/*
* If the if block does not have curly braces and does not end in a semicolon
* and the else block starts with (, [, /, +, ` or -, then it is not
* safe to remove the else keyword, because ASI will not add a semicolon
* after the if block
*/
const ifBlockMaybeUnsafe = node.parent.consequent.type !== "BlockStatement" && lastIfToken.value !== ";";
const elseBlockUnsafe = /^[([/+`-]/u.test(firstTokenOfElseBlock.value);
if (ifBlockMaybeUnsafe && elseBlockUnsafe) {
return null;
}
const endToken = sourceCode.getLastToken(node);
const lastTokenOfElseBlock = sourceCode.getTokenBefore(endToken);
if (lastTokenOfElseBlock.value !== ";") {
const nextToken = sourceCode.getTokenAfter(endToken);
const nextTokenUnsafe = nextToken && /^[([/+`-]/u.test(nextToken.value);
const nextTokenOnSameLine = nextToken && nextToken.loc.start.line === lastTokenOfElseBlock.loc.start.line;
/*
* If the else block contents does not end in a semicolon,
* and the else block starts with (, [, /, +, ` or -, then it is not
* safe to remove the else block, because ASI will not add a semicolon
* after the remaining else block contents
*/
if (nextTokenUnsafe || (nextTokenOnSameLine && nextToken.value !== "}")) {
return null;
}
}
if (startToken.type === "Punctuator" && startToken.value === "{") {
fixedSource = source.slice(1, -1);
} else {
fixedSource = source;
}
/*
* Extend the replacement range to include the entire
* function to avoid conflicting with no-useless-return.
* https://github.com/eslint/eslint/issues/8026
*
* Also, to avoid name collisions between two else blocks.
*/
return new FixTracker(fixer, sourceCode)
.retainEnclosingFunction(node)
.replaceTextRange([elseToken.range[0], node.range[1]], fixedSource);
}
});
}
/**
* Check to see if the node is a ReturnStatement
*
* @param {Node} node The node being evaluated
* @returns {boolean} True if node is a return
*/
function checkForReturn(node) {
return node.type === "ReturnStatement";
}
/**
* Naive return checking, does not iterate through the whole
* BlockStatement because we make the assumption that the ReturnStatement
* will be the last node in the body of the BlockStatement.
*
* @param {Node} node The consequent/alternate node
* @returns {boolean} True if it has a return
*/
function naiveHasReturn(node) {
if (node.type === "BlockStatement") {
const body = node.body,
lastChildNode = body[body.length - 1];
return lastChildNode && checkForReturn(lastChildNode);
}
return checkForReturn(node);
}
/**
* Check to see if the node is valid for evaluation,
* meaning it has an else.
*
* @param {Node} node The node being evaluated
* @returns {boolean} True if the node is valid
*/
function hasElse(node) {
return node.alternate && node.consequent;
}
/**
* If the consequent is an IfStatement, check to see if it has an else
* and both its consequent and alternate path return, meaning this is
* a nested case of rule violation. If-Else not considered currently.
*
* @param {Node} node The consequent node
* @returns {boolean} True if this is a nested rule violation
*/
function checkForIf(node) {
return node.type === "IfStatement" && hasElse(node) &&
naiveHasReturn(node.alternate) && naiveHasReturn(node.consequent);
}
/**
* Check the consequent/body node to make sure it is not
* a ReturnStatement or an IfStatement that returns on both
* code paths.
*
* @param {Node} node The consequent or body node
* @returns {boolean} `true` if it is a Return/If node that always returns.
*/
function checkForReturnOrIf(node) {
return checkForReturn(node) || checkForIf(node);
}
/**
* Check whether a node returns in every codepath.
* @param {Node} node The node to be checked
* @returns {boolean} `true` if it returns on every codepath.
*/
function alwaysReturns(node) {
if (node.type === "BlockStatement") {
// If we have a BlockStatement, check each consequent body node.
return node.body.some(checkForReturnOrIf);
}
/*
* If not a block statement, make sure the consequent isn't a
* ReturnStatement or an IfStatement with returns on both paths.
*/
return checkForReturnOrIf(node);
}
/**
* Check the if statement, but don't catch else-if blocks.
* @returns {void}
* @param {Node} node The node for the if statement to check
* @private
*/
function checkIfWithoutElse(node) {
const parent = node.parent;
/*
* Fixing this would require splitting one statement into two, so no error should
* be reported if this node is in a position where only one statement is allowed.
*/
if (!astUtils.STATEMENT_LIST_PARENTS.has(parent.type)) {
return;
}
const consequents = [];
let alternate;
for (let currentNode = node; currentNode.type === "IfStatement"; currentNode = currentNode.alternate) {
if (!currentNode.alternate) {
return;
}
consequents.push(currentNode.consequent);
alternate = currentNode.alternate;
}
if (consequents.every(alwaysReturns)) {
displayReport(alternate);
}
}
/**
* Check the if statement
* @returns {void}
* @param {Node} node The node for the if statement to check
* @private
*/
function checkIfWithElse(node) {
const parent = node.parent;
/*
* Fixing this would require splitting one statement into two, so no error should
* be reported if this node is in a position where only one statement is allowed.
*/
if (!astUtils.STATEMENT_LIST_PARENTS.has(parent.type)) {
return;
}
const alternate = node.alternate;
if (alternate && alwaysReturns(node.consequent)) {
displayReport(alternate);
}
}
const allowElseIf = !(context.options[0] && context.options[0].allowElseIf === false);
//--------------------------------------------------------------------------
// Public API
//--------------------------------------------------------------------------
return {
"IfStatement:exit": allowElseIf ? checkIfWithoutElse : checkIfWithElse
};
}
};
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