eslint/lib/rules/no-unneeded-ternary.js

   1 /**
   2  * @fileoverview Rule to flag no-unneeded-ternary
   3  * @author Gyandeep Singh
   4  */
   5
   6 "use strict";
   7
   8 const astUtils = require("./utils/ast-utils");
   9
  10 // Operators that always result in a boolean value
  11 const BOOLEAN_OPERATORS = new Set(["==", "===", "!=", "!==", ">", ">=", "<", "<=", "in", "instanceof"]);
  12 const OPERATOR_INVERSES = {
  13     "==": "!=",
  14     "!=": "==",
  15     "===": "!==",
  16     "!==": "==="
  17
  18     // Operators like < and >= are not true inverses, since both will return false with NaN.
  19 };
  20 const OR_PRECEDENCE = astUtils.getPrecedence({ type: "LogicalExpression", operator: "||" });
  21
  22 //------------------------------------------------------------------------------
  23 // Rule Definition
  24 //------------------------------------------------------------------------------
  25
  26 module.exports = {
  27     meta: {
  28         type: "suggestion",
  29
  30         docs: {
  31             description: "disallow ternary operators when simpler alternatives exist",
  32             recommended: false,
  33             url: "https://eslint.org/docs/rules/no-unneeded-ternary"
  34         },
  35
  36         schema: [
  37             {
  38                 type: "object",
  39                 properties: {
  40                     defaultAssignment: {
  41                         type: "boolean",
  42                         default: true
  43                     }
  44                 },
  45                 additionalProperties: false
  46             }
  47         ],
  48
  49         fixable: "code",
  50
  51         messages: {
  52             unnecessaryConditionalExpression: "Unnecessary use of boolean literals in conditional expression.",
  53             unnecessaryConditionalAssignment: "Unnecessary use of conditional expression for default assignment."
  54         }
  55     },
  56
  57     create(context) {
  58         const options = context.options[0] || {};
  59         const defaultAssignment = options.defaultAssignment !== false;
  60         const sourceCode = context.getSourceCode();
  61
  62         /**
  63          * Test if the node is a boolean literal
  64          * @param {ASTNode} node The node to report.
  65          * @returns {boolean} True if the its a boolean literal
  66          * @private
  67          */
  68         function isBooleanLiteral(node) {
  69             return node.type === "Literal" && typeof node.value === "boolean";
  70         }
  71
  72         /**
  73          * Creates an expression that represents the boolean inverse of the expression represented by the original node
  74          * @param {ASTNode} node A node representing an expression
  75          * @returns {string} A string representing an inverted expression
  76          */
  77         function invertExpression(node) {
  78             if (node.type === "BinaryExpression" && Object.prototype.hasOwnProperty.call(OPERATOR_INVERSES, node.operator)) {
  79                 const operatorToken = sourceCode.getFirstTokenBetween(
  80                     node.left,
  81                     node.right,
  82                     token => token.value === node.operator
  83                 );
  84                 const text = sourceCode.getText();
  85
  86                 return text.slice(node.range[0],
  87                     operatorToken.range[0]) + OPERATOR_INVERSES[node.operator] + text.slice(operatorToken.range[1], node.range[1]);
  88             }
  89
  90             if (astUtils.getPrecedence(node) < astUtils.getPrecedence({ type: "UnaryExpression" })) {
  91                 return `!(${astUtils.getParenthesisedText(sourceCode, node)})`;
  92             }
  93             return `!${astUtils.getParenthesisedText(sourceCode, node)}`;
  94         }
  95
  96         /**
  97          * Tests if a given node always evaluates to a boolean value
  98          * @param {ASTNode} node An expression node
  99          * @returns {boolean} True if it is determined that the node will always evaluate to a boolean value
 100          */
 101         function isBooleanExpression(node) {
 102             return node.type === "BinaryExpression" && BOOLEAN_OPERATORS.has(node.operator) ||
 103                 node.type === "UnaryExpression" && node.operator === "!";
 104         }
 105
 106         /**
 107          * Test if the node matches the pattern id ? id : expression
 108          * @param {ASTNode} node The ConditionalExpression to check.
 109          * @returns {boolean} True if the pattern is matched, and false otherwise
 110          * @private
 111          */
 112         function matchesDefaultAssignment(node) {
 113             return node.test.type === "Identifier" &&
 114                    node.consequent.type === "Identifier" &&
 115                    node.test.name === node.consequent.name;
 116         }
 117
 118         return {
 119
 120             ConditionalExpression(node) {
 121                 if (isBooleanLiteral(node.alternate) && isBooleanLiteral(node.consequent)) {
 122                     context.report({
 123                         node,
 124                         messageId: "unnecessaryConditionalExpression",
 125                         fix(fixer) {
 126                             if (node.consequent.value === node.alternate.value) {
 127
 128                                 // Replace `foo ? true : true` with just `true`, but don't replace `foo() ? true : true`
 129                                 return node.test.type === "Identifier" ? fixer.replaceText(node, node.consequent.value.toString()) : null;
 130                             }
 131                             if (node.alternate.value) {
 132
 133                                 // Replace `foo() ? false : true` with `!(foo())`
 134                                 return fixer.replaceText(node, invertExpression(node.test));
 135                             }
 136
 137                             // Replace `foo ? true : false` with `foo` if `foo` is guaranteed to be a boolean, or `!!foo` otherwise.
 138
 139                             return fixer.replaceText(node, isBooleanExpression(node.test) ? astUtils.getParenthesisedText(sourceCode, node.test) : `!${invertExpression(node.test)}`);
 140                         }
 141                     });
 142                 } else if (!defaultAssignment && matchesDefaultAssignment(node)) {
 143                     context.report({
 144                         node,
 145                         messageId: "unnecessaryConditionalAssignment",
 146                         fix: fixer => {
 147                             const shouldParenthesizeAlternate =
 148                                 (
 149                                     astUtils.getPrecedence(node.alternate) < OR_PRECEDENCE ||
 150                                     astUtils.isCoalesceExpression(node.alternate)
 151                                 ) &&
 152                                 !astUtils.isParenthesised(sourceCode, node.alternate);
 153                             const alternateText = shouldParenthesizeAlternate
 154                                 ? `(${sourceCode.getText(node.alternate)})`
 155                                 : astUtils.getParenthesisedText(sourceCode, node.alternate);
 156                             const testText = astUtils.getParenthesisedText(sourceCode, node.test);
 157
 158                             return fixer.replaceText(node, `${testText} || ${alternateText}`);
 159                         }
 160                     });
 161                 }
 162             }
 163         };
 164     }
 165 };