eslint/lib/rules/operator-assignment.js

   1 /**
   2  * @fileoverview Rule to replace assignment expressions with operator assignment
   3  * @author Brandon Mills
   4  */
   5 "use strict";
   6
   7 //------------------------------------------------------------------------------
   8 // Requirements
   9 //------------------------------------------------------------------------------
  10
  11 const astUtils = require("./utils/ast-utils");
  12
  13 //------------------------------------------------------------------------------
  14 // Helpers
  15 //------------------------------------------------------------------------------
  16
  17 /**
  18  * Checks whether an operator is commutative and has an operator assignment
  19  * shorthand form.
  20  * @param {string} operator Operator to check.
  21  * @returns {boolean} True if the operator is commutative and has a
  22  *     shorthand form.
  23  */
  24 function isCommutativeOperatorWithShorthand(operator) {
  25     return ["*", "&", "^", "|"].includes(operator);
  26 }
  27
  28 /**
  29  * Checks whether an operator is not commutative and has an operator assignment
  30  * shorthand form.
  31  * @param {string} operator Operator to check.
  32  * @returns {boolean} True if the operator is not commutative and has
  33  *     a shorthand form.
  34  */
  35 function isNonCommutativeOperatorWithShorthand(operator) {
  36     return ["+", "-", "/", "%", "<<", ">>", ">>>", "**"].includes(operator);
  37 }
  38
  39 //------------------------------------------------------------------------------
  40 // Rule Definition
  41 //------------------------------------------------------------------------------
  42
  43 /**
  44  * Determines if the left side of a node can be safely fixed (i.e. if it activates the same getters/setters and)
  45  * toString calls regardless of whether assignment shorthand is used)
  46  * @param {ASTNode} node The node on the left side of the expression
  47  * @returns {boolean} `true` if the node can be fixed
  48  */
  49 function canBeFixed(node) {
  50     return (
  51         node.type === "Identifier" ||
  52         (
  53             node.type === "MemberExpression" &&
  54             (node.object.type === "Identifier" || node.object.type === "ThisExpression") &&
  55             (!node.computed || node.property.type === "Literal")
  56         )
  57     );
  58 }
  59
  60 /** @type {import('../shared/types').Rule} */
  61 module.exports = {
  62     meta: {
  63         type: "suggestion",
  64
  65         docs: {
  66             description: "Require or disallow assignment operator shorthand where possible",
  67             recommended: false,
  68             url: "https://eslint.org/docs/rules/operator-assignment"
  69         },
  70
  71         schema: [
  72             {
  73                 enum: ["always", "never"]
  74             }
  75         ],
  76
  77         fixable: "code",
  78         messages: {
  79             replaced: "Assignment (=) can be replaced with operator assignment ({{operator}}).",
  80             unexpected: "Unexpected operator assignment ({{operator}}) shorthand."
  81         }
  82     },
  83
  84     create(context) {
  85
  86         const sourceCode = context.getSourceCode();
  87
  88         /**
  89          * Returns the operator token of an AssignmentExpression or BinaryExpression
  90          * @param {ASTNode} node An AssignmentExpression or BinaryExpression node
  91          * @returns {Token} The operator token in the node
  92          */
  93         function getOperatorToken(node) {
  94             return sourceCode.getFirstTokenBetween(node.left, node.right, token => token.value === node.operator);
  95         }
  96
  97         /**
  98          * Ensures that an assignment uses the shorthand form where possible.
  99          * @param {ASTNode} node An AssignmentExpression node.
 100          * @returns {void}
 101          */
 102         function verify(node) {
 103             if (node.operator !== "=" || node.right.type !== "BinaryExpression") {
 104                 return;
 105             }
 106
 107             const left = node.left;
 108             const expr = node.right;
 109             const operator = expr.operator;
 110
 111             if (isCommutativeOperatorWithShorthand(operator) || isNonCommutativeOperatorWithShorthand(operator)) {
 112                 const replacementOperator = `${operator}=`;
 113
 114                 if (astUtils.isSameReference(left, expr.left, true)) {
 115                     context.report({
 116                         node,
 117                         messageId: "replaced",
 118                         data: { operator: replacementOperator },
 119                         fix(fixer) {
 120                             if (canBeFixed(left) && canBeFixed(expr.left)) {
 121                                 const equalsToken = getOperatorToken(node);
 122                                 const operatorToken = getOperatorToken(expr);
 123                                 const leftText = sourceCode.getText().slice(node.range[0], equalsToken.range[0]);
 124                                 const rightText = sourceCode.getText().slice(operatorToken.range[1], node.right.range[1]);
 125
 126                                 // Check for comments that would be removed.
 127                                 if (sourceCode.commentsExistBetween(equalsToken, operatorToken)) {
 128                                     return null;
 129                                 }
 130
 131                                 return fixer.replaceText(node, `${leftText}${replacementOperator}${rightText}`);
 132                             }
 133                             return null;
 134                         }
 135                     });
 136                 } else if (astUtils.isSameReference(left, expr.right, true) && isCommutativeOperatorWithShorthand(operator)) {
 137
 138                     /*
 139                      * This case can't be fixed safely.
 140                      * If `a` and `b` both have custom valueOf() behavior, then fixing `a = b * a` to `a *= b` would
 141                      * change the execution order of the valueOf() functions.
 142                      */
 143                     context.report({
 144                         node,
 145                         messageId: "replaced",
 146                         data: { operator: replacementOperator }
 147                     });
 148                 }
 149             }
 150         }
 151
 152         /**
 153          * Warns if an assignment expression uses operator assignment shorthand.
 154          * @param {ASTNode} node An AssignmentExpression node.
 155          * @returns {void}
 156          */
 157         function prohibit(node) {
 158             if (node.operator !== "=" && !astUtils.isLogicalAssignmentOperator(node.operator)) {
 159                 context.report({
 160                     node,
 161                     messageId: "unexpected",
 162                     data: { operator: node.operator },
 163                     fix(fixer) {
 164                         if (canBeFixed(node.left)) {
 165                             const firstToken = sourceCode.getFirstToken(node);
 166                             const operatorToken = getOperatorToken(node);
 167                             const leftText = sourceCode.getText().slice(node.range[0], operatorToken.range[0]);
 168                             const newOperator = node.operator.slice(0, -1);
 169                             let rightText;
 170
 171                             // Check for comments that would be duplicated.
 172                             if (sourceCode.commentsExistBetween(firstToken, operatorToken)) {
 173                                 return null;
 174                             }
 175
 176                             // If this change would modify precedence (e.g. `foo *= bar + 1` => `foo = foo * (bar + 1)`), parenthesize the right side.
 177                             if (
 178                                 astUtils.getPrecedence(node.right) <= astUtils.getPrecedence({ type: "BinaryExpression", operator: newOperator }) &&
 179                                 !astUtils.isParenthesised(sourceCode, node.right)
 180                             ) {
 181                                 rightText = `${sourceCode.text.slice(operatorToken.range[1], node.right.range[0])}(${sourceCode.getText(node.right)})`;
 182                             } else {
 183                                 const tokenAfterOperator = sourceCode.getTokenAfter(operatorToken, { includeComments: true });
 184                                 let rightTextPrefix = "";
 185
 186                                 if (
 187                                     operatorToken.range[1] === tokenAfterOperator.range[0] &&
 188                                     !astUtils.canTokensBeAdjacent({ type: "Punctuator", value: newOperator }, tokenAfterOperator)
 189                                 ) {
 190                                     rightTextPrefix = " "; // foo+=+bar -> foo= foo+ +bar
 191                                 }
 192
 193                                 rightText = `${rightTextPrefix}${sourceCode.text.slice(operatorToken.range[1], node.range[1])}`;
 194                             }
 195
 196                             return fixer.replaceText(node, `${leftText}= ${leftText}${newOperator}${rightText}`);
 197                         }
 198                         return null;
 199                     }
 200                 });
 201             }
 202         }
 203
 204         return {
 205             AssignmentExpression: context.options[0] !== "never" ? verify : prohibit
 206         };
 207
 208     }
 209 };