logical-assignment-operators.js

/**
 * @fileoverview Rule to replace assignment expressions with logical operator assignment
 * @author Daniel Martens
 */
"use strict";

//------------------------------------------------------------------------------
// Requirements
//------------------------------------------------------------------------------
const astUtils = require("./utils/ast-utils.js");

//------------------------------------------------------------------------------
// Helpers
//------------------------------------------------------------------------------

const baseTypes = new Set(["Identifier", "Super", "ThisExpression"]);

/**
 * Returns true iff either "undefined" or a void expression (eg. "void 0")
 * @param {ASTNode} expression Expression to check
 * @param {import('eslint-scope').Scope} scope Scope of the expression
 * @returns {boolean} True iff "undefined" or "void ..."
 */
function isUndefined(expression, scope) {
    if (expression.type === "Identifier" && expression.name === "undefined") {
        return astUtils.isReferenceToGlobalVariable(scope, expression);
    }

    return expression.type === "UnaryExpression" &&
           expression.operator === "void" &&
           expression.argument.type === "Literal" &&
           expression.argument.value === 0;
}

/**
 * Returns true iff the reference is either an identifier or member expression
 * @param {ASTNode} expression Expression to check
 * @returns {boolean} True for identifiers and member expressions
 */
function isReference(expression) {
    return (expression.type === "Identifier" && expression.name !== "undefined") ||
           expression.type === "MemberExpression";
}

/**
 * Returns true iff the expression checks for nullish with loose equals.
 * Examples: value == null, value == void 0
 * @param {ASTNode} expression Test condition
 * @param {import('eslint-scope').Scope} scope Scope of the expression
 * @returns {boolean} True iff implicit nullish comparison
 */
function isImplicitNullishComparison(expression, scope) {
    if (expression.type !== "BinaryExpression" || expression.operator !== "==") {
        return false;
    }

    const reference = isReference(expression.left) ? "left" : "right";
    const nullish = reference === "left" ? "right" : "left";

    return isReference(expression[reference]) &&
           (astUtils.isNullLiteral(expression[nullish]) || isUndefined(expression[nullish], scope));
}

/**
 * Condition with two equal comparisons.
 * @param {ASTNode} expression Condition
 * @returns {boolean} True iff matches ? === ? || ? === ?
 */
function isDoubleComparison(expression) {
    return expression.type === "LogicalExpression" &&
           expression.operator === "||" &&
           expression.left.type === "BinaryExpression" &&
           expression.left.operator === "===" &&
           expression.right.type === "BinaryExpression" &&
           expression.right.operator === "===";
}

/**
 * Returns true iff the expression checks for undefined and null.
 * Example: value === null || value === undefined
 * @param {ASTNode} expression Test condition
 * @param {import('eslint-scope').Scope} scope Scope of the expression
 * @returns {boolean} True iff explicit nullish comparison
 */
function isExplicitNullishComparison(expression, scope) {
    if (!isDoubleComparison(expression)) {
        return false;
    }
    const leftReference = isReference(expression.left.left) ? "left" : "right";
    const leftNullish = leftReference === "left" ? "right" : "left";
    const rightReference = isReference(expression.right.left) ? "left" : "right";
    const rightNullish = rightReference === "left" ? "right" : "left";

    return astUtils.isSameReference(expression.left[leftReference], expression.right[rightReference]) &&
           ((astUtils.isNullLiteral(expression.left[leftNullish]) && isUndefined(expression.right[rightNullish], scope)) ||
           (isUndefined(expression.left[leftNullish], scope) && astUtils.isNullLiteral(expression.right[rightNullish])));
}

/**
 * Returns true for Boolean(arg) calls
 * @param {ASTNode} expression Test condition
 * @param {import('eslint-scope').Scope} scope Scope of the expression
 * @returns {boolean} Whether the expression is a boolean cast
 */
function isBooleanCast(expression, scope) {
    return expression.type === "CallExpression" &&
           expression.callee.name === "Boolean" &&
           expression.arguments.length === 1 &&
           astUtils.isReferenceToGlobalVariable(scope, expression.callee);
}

/**
 * Returns true for:
 * truthiness checks:  value, Boolean(value), !!value
 * falsiness checks:   !value, !Boolean(value)
 * nullish checks:     value == null, value === undefined || value === null
 * @param {ASTNode} expression Test condition
 * @param {import('eslint-scope').Scope} scope Scope of the expression
 * @returns {?{ reference: ASTNode, operator: '??'|'||'|'&&'}} Null if not a known existence
 */
function getExistence(expression, scope) {
    const isNegated = expression.type === "UnaryExpression" && expression.operator === "!";
    const base = isNegated ? expression.argument : expression;

    switch (true) {
        case isReference(base):
            return { reference: base, operator: isNegated ? "||" : "&&" };
        case base.type === "UnaryExpression" && base.operator === "!" && isReference(base.argument):
            return { reference: base.argument, operator: "&&" };
        case isBooleanCast(base, scope) && isReference(base.arguments[0]):
            return { reference: base.arguments[0], operator: isNegated ? "||" : "&&" };
        case isImplicitNullishComparison(expression, scope):
            return { reference: isReference(expression.left) ? expression.left : expression.right, operator: "??" };
        case isExplicitNullishComparison(expression, scope):
            return { reference: isReference(expression.left.left) ? expression.left.left : expression.left.right, operator: "??" };
        default: return null;
    }
}

/**
 * Returns true iff the node is inside a with block
 * @param {ASTNode} node Node to check
 * @returns {boolean} True iff passed node is inside a with block
 */
function isInsideWithBlock(node) {
    if (node.type === "Program") {
        return false;
    }

    return node.parent.type === "WithStatement" && node.parent.body === node ? true : isInsideWithBlock(node.parent);
}

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------
/** @type {import('../shared/types').Rule} */
module.exports = {
    meta: {
        type: "suggestion",

        docs: {
            description: "Require or disallow logical assignment operator shorthand",
            recommended: false,
            url: "https://eslint.org/docs/latest/rules/logical-assignment-operators"
        },

        schema: {
            type: "array",
            oneOf: [{
                items: [
                    { const: "always" },
                    {
                        type: "object",
                        properties: {
                            enforceForIfStatements: {
                                type: "boolean"
                            }
                        },
                        additionalProperties: false
                    }
                ],
                minItems: 0, // 0 for allowing passing no options
                maxItems: 2
            }, {
                items: [{ const: "never" }],
                minItems: 1,
                maxItems: 1
            }]
        },
        fixable: "code",
        hasSuggestions: true,
        messages: {
            assignment: "Assignment (=) can be replaced with operator assignment ({{operator}}).",
            useLogicalOperator: "Convert this assignment to use the operator {{ operator }}.",
            logical: "Logical expression can be replaced with an assignment ({{ operator }}).",
            convertLogical: "Replace this logical expression with an assignment with the operator {{ operator }}.",
            if: "'if' statement can be replaced with a logical operator assignment with operator {{ operator }}.",
            convertIf: "Replace this 'if' statement with a logical assignment with operator {{ operator }}.",
            unexpected: "Unexpected logical operator assignment ({{operator}}) shorthand.",
            separate: "Separate the logical assignment into an assignment with a logical operator."
        }
    },

    create(context) {
        const mode = context.options[0] === "never" ? "never" : "always";
        const checkIf = mode === "always" && context.options.length > 1 && context.options[1].enforceForIfStatements;
        const sourceCode = context.sourceCode;
        const isStrict = sourceCode.getScope(sourceCode.ast).isStrict;

        /**
         * Returns false if the access could be a getter
         * @param {ASTNode} node Assignment expression
         * @returns {boolean} True iff the fix is safe
         */
        function cannotBeGetter(node) {
            return node.type === "Identifier" &&
                   (isStrict || !isInsideWithBlock(node));
        }

        /**
         * Check whether only a single property is accessed
         * @param {ASTNode} node reference
         * @returns {boolean} True iff a single property is accessed
         */
        function accessesSingleProperty(node) {
            if (!isStrict && isInsideWithBlock(node)) {
                return node.type === "Identifier";
            }

            return node.type === "MemberExpression" &&
                   baseTypes.has(node.object.type) &&
                   (!node.computed || (node.property.type !== "MemberExpression" && node.property.type !== "ChainExpression"));
        }

        /**
         * Adds a fixer or suggestion whether on the fix is safe.
         * @param {{ messageId: string, node: ASTNode }} descriptor Report descriptor without fix or suggest
         * @param {{ messageId: string, fix: Function }} suggestion Adds the fix or the whole suggestion as only element in "suggest" to suggestion
         * @param {boolean} shouldBeFixed Fix iff the condition is true
         * @returns {Object} Descriptor with either an added fix or suggestion
         */
        function createConditionalFixer(descriptor, suggestion, shouldBeFixed) {
            if (shouldBeFixed) {
                return {
                    ...descriptor,
                    fix: suggestion.fix
                };
            }

            return {
                ...descriptor,
                suggest: [suggestion]
            };
        }


        /**
         * Returns the operator token for assignments and binary expressions
         * @param {ASTNode} node AssignmentExpression or BinaryExpression
         * @returns {import('eslint').AST.Token} Operator token between the left and right expression
         */
        function getOperatorToken(node) {
            return sourceCode.getFirstTokenBetween(node.left, node.right, token => token.value === node.operator);
        }

        if (mode === "never") {
            return {

                // foo ||= bar
                "AssignmentExpression"(assignment) {
                    if (!astUtils.isLogicalAssignmentOperator(assignment.operator)) {
                        return;
                    }

                    const descriptor = {
                        messageId: "unexpected",
                        node: assignment,
                        data: { operator: assignment.operator }
                    };
                    const suggestion = {
                        messageId: "separate",
                        *fix(ruleFixer) {
                            if (sourceCode.getCommentsInside(assignment).length > 0) {
                                return;
                            }

                            const operatorToken = getOperatorToken(assignment);

                            // -> foo = bar
                            yield ruleFixer.replaceText(operatorToken, "=");

                            const assignmentText = sourceCode.getText(assignment.left);
                            const operator = assignment.operator.slice(0, -1);

                            // -> foo = foo || bar
                            yield ruleFixer.insertTextAfter(operatorToken, ` ${assignmentText} ${operator}`);

                            const precedence = astUtils.getPrecedence(assignment.right) <= astUtils.getPrecedence({ type: "LogicalExpression", operator });

                            // ?? and || / && cannot be mixed but have same precedence
                            const mixed = assignment.operator === "??=" && astUtils.isLogicalExpression(assignment.right);

                            if (!astUtils.isParenthesised(sourceCode, assignment.right) && (precedence || mixed)) {

                                // -> foo = foo || (bar)
                                yield ruleFixer.insertTextBefore(assignment.right, "(");
                                yield ruleFixer.insertTextAfter(assignment.right, ")");
                            }
                        }
                    };

                    context.report(createConditionalFixer(descriptor, suggestion, cannotBeGetter(assignment.left)));
                }
            };
        }

        return {

            // foo = foo || bar
            "AssignmentExpression[operator='='][right.type='LogicalExpression']"(assignment) {
                if (!astUtils.isSameReference(assignment.left, assignment.right.left)) {
                    return;
                }

                const descriptor = {
                    messageId: "assignment",
                    node: assignment,
                    data: { operator: `${assignment.right.operator}=` }
                };
                const suggestion = {
                    messageId: "useLogicalOperator",
                    data: { operator: `${assignment.right.operator}=` },
                    *fix(ruleFixer) {
                        if (sourceCode.getCommentsInside(assignment).length > 0) {
                            return;
                        }

                        // No need for parenthesis around the assignment based on precedence as the precedence stays the same even with changed operator
                        const assignmentOperatorToken = getOperatorToken(assignment);

                        // -> foo ||= foo || bar
                        yield ruleFixer.insertTextBefore(assignmentOperatorToken, assignment.right.operator);

                        // -> foo ||= bar
                        const logicalOperatorToken = getOperatorToken(assignment.right);
                        const firstRightOperandToken = sourceCode.getTokenAfter(logicalOperatorToken);

                        yield ruleFixer.removeRange([assignment.right.range[0], firstRightOperandToken.range[0]]);
                    }
                };

                context.report(createConditionalFixer(descriptor, suggestion, cannotBeGetter(assignment.left)));
            },

            // foo || (foo = bar)
            'LogicalExpression[right.type="AssignmentExpression"][right.operator="="]'(logical) {

                // Right side has to be parenthesized, otherwise would be parsed as (foo || foo) = bar which is illegal
                if (isReference(logical.left) && astUtils.isSameReference(logical.left, logical.right.left)) {
                    const descriptor = {
                        messageId: "logical",
                        node: logical,
                        data: { operator: `${logical.operator}=` }
                    };
                    const suggestion = {
                        messageId: "convertLogical",
                        data: { operator: `${logical.operator}=` },
                        *fix(ruleFixer) {
                            if (sourceCode.getCommentsInside(logical).length > 0) {
                                return;
                            }

                            const requiresOuterParenthesis = logical.parent.type !== "ExpressionStatement" &&
                                                             (astUtils.getPrecedence({ type: "AssignmentExpression" }) < astUtils.getPrecedence(logical.parent));

                            if (!astUtils.isParenthesised(sourceCode, logical) && requiresOuterParenthesis) {
                                yield ruleFixer.insertTextBefore(logical, "(");
                                yield ruleFixer.insertTextAfter(logical, ")");
                            }

                            // Also removes all opening parenthesis
                            yield ruleFixer.removeRange([logical.range[0], logical.right.range[0]]); // -> foo = bar)

                            // Also removes all ending parenthesis
                            yield ruleFixer.removeRange([logical.right.range[1], logical.range[1]]); // -> foo = bar

                            const operatorToken = getOperatorToken(logical.right);

                            yield ruleFixer.insertTextBefore(operatorToken, logical.operator); // -> foo ||= bar
                        }
                    };
                    const fix = cannotBeGetter(logical.left) || accessesSingleProperty(logical.left);

                    context.report(createConditionalFixer(descriptor, suggestion, fix));
                }
            },

            // if (foo) foo = bar
            "IfStatement[alternate=null]"(ifNode) {
                if (!checkIf) {
                    return;
                }

                const hasBody = ifNode.consequent.type === "BlockStatement";

                if (hasBody && ifNode.consequent.body.length !== 1) {
                    return;
                }

                const body = hasBody ? ifNode.consequent.body[0] : ifNode.consequent;
                const scope = sourceCode.getScope(ifNode);
                const existence = getExistence(ifNode.test, scope);

                if (
                    body.type === "ExpressionStatement" &&
                    body.expression.type === "AssignmentExpression" &&
                    body.expression.operator === "=" &&
                    existence !== null &&
                    astUtils.isSameReference(existence.reference, body.expression.left)
                ) {
                    const descriptor = {
                        messageId: "if",
                        node: ifNode,
                        data: { operator: `${existence.operator}=` }
                    };
                    const suggestion = {
                        messageId: "convertIf",
                        data: { operator: `${existence.operator}=` },
                        *fix(ruleFixer) {
                            if (sourceCode.getCommentsInside(ifNode).length > 0) {
                                return;
                            }

                            const firstBodyToken = sourceCode.getFirstToken(body);
                            const prevToken = sourceCode.getTokenBefore(ifNode);

                            if (
                                prevToken !== null &&
                                prevToken.value !== ";" &&
                                prevToken.value !== "{" &&
                                firstBodyToken.type !== "Identifier" &&
                                firstBodyToken.type !== "Keyword"
                            ) {

                                // Do not fix if the fixed statement could be part of the previous statement (eg. fn() if (a == null) (a) = b --> fn()(a) ??= b)
                                return;
                            }


                            const operatorToken = getOperatorToken(body.expression);

                            yield ruleFixer.insertTextBefore(operatorToken, existence.operator); // -> if (foo) foo ||= bar

                            yield ruleFixer.removeRange([ifNode.range[0], body.range[0]]); // -> foo ||= bar

                            yield ruleFixer.removeRange([body.range[1], ifNode.range[1]]); // -> foo ||= bar, only present if "if" had a body

                            const nextToken = sourceCode.getTokenAfter(body.expression);

                            if (hasBody && (nextToken !== null && nextToken.value !== ";")) {
                                yield ruleFixer.insertTextAfter(ifNode, ";");
                            }
                        }
                    };
                    const shouldBeFixed = cannotBeGetter(existence.reference) ||
                                          (ifNode.test.type !== "LogicalExpression" && accessesSingleProperty(existence.reference));

                    context.report(createConditionalFixer(descriptor, suggestion, shouldBeFixed));
                }
            }
        };
    }
};