/**
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* @fileoverview Rule to flag no-unneeded-ternary
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* @author Gyandeep Singh
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*/
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"use strict";
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const astUtils = require("./utils/ast-utils");
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// Operators that always result in a boolean value
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const BOOLEAN_OPERATORS = new Set(["==", "===", "!=", "!==", ">", ">=", "<", "<=", "in", "instanceof"]);
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const OPERATOR_INVERSES = {
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"==": "!=",
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"!=": "==",
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"===": "!==",
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"!==": "==="
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// Operators like < and >= are not true inverses, since both will return false with NaN.
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};
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const OR_PRECEDENCE = astUtils.getPrecedence({ type: "LogicalExpression", operator: "||" });
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//------------------------------------------------------------------------------
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// Rule Definition
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//------------------------------------------------------------------------------
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module.exports = {
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meta: {
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type: "suggestion",
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docs: {
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description: "disallow ternary operators when simpler alternatives exist",
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category: "Stylistic Issues",
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recommended: false,
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url: "https://eslint.org/docs/rules/no-unneeded-ternary"
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},
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schema: [
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{
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type: "object",
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properties: {
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defaultAssignment: {
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type: "boolean",
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default: true
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}
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},
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additionalProperties: false
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}
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],
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fixable: "code"
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},
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create(context) {
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const options = context.options[0] || {};
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const defaultAssignment = options.defaultAssignment !== false;
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const sourceCode = context.getSourceCode();
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/**
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* Test if the node is a boolean literal
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* @param {ASTNode} node - The node to report.
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* @returns {boolean} True if the its a boolean literal
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* @private
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*/
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function isBooleanLiteral(node) {
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return node.type === "Literal" && typeof node.value === "boolean";
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}
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/**
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* Creates an expression that represents the boolean inverse of the expression represented by the original node
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* @param {ASTNode} node A node representing an expression
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* @returns {string} A string representing an inverted expression
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*/
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function invertExpression(node) {
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if (node.type === "BinaryExpression" && Object.prototype.hasOwnProperty.call(OPERATOR_INVERSES, node.operator)) {
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const operatorToken = sourceCode.getFirstTokenBetween(
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node.left,
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node.right,
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token => token.value === node.operator
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);
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const text = sourceCode.getText();
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return text.slice(node.range[0],
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operatorToken.range[0]) + OPERATOR_INVERSES[node.operator] + text.slice(operatorToken.range[1], node.range[1]);
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}
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if (astUtils.getPrecedence(node) < astUtils.getPrecedence({ type: "UnaryExpression" })) {
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return `!(${astUtils.getParenthesisedText(sourceCode, node)})`;
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}
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return `!${astUtils.getParenthesisedText(sourceCode, node)}`;
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}
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/**
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* Tests if a given node always evaluates to a boolean value
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* @param {ASTNode} node - An expression node
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* @returns {boolean} True if it is determined that the node will always evaluate to a boolean value
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*/
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function isBooleanExpression(node) {
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return node.type === "BinaryExpression" && BOOLEAN_OPERATORS.has(node.operator) ||
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node.type === "UnaryExpression" && node.operator === "!";
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}
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/**
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* Test if the node matches the pattern id ? id : expression
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* @param {ASTNode} node - The ConditionalExpression to check.
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* @returns {boolean} True if the pattern is matched, and false otherwise
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* @private
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*/
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function matchesDefaultAssignment(node) {
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return node.test.type === "Identifier" &&
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node.consequent.type === "Identifier" &&
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node.test.name === node.consequent.name;
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}
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return {
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ConditionalExpression(node) {
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if (isBooleanLiteral(node.alternate) && isBooleanLiteral(node.consequent)) {
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context.report({
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node,
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loc: node.consequent.loc.start,
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message: "Unnecessary use of boolean literals in conditional expression.",
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fix(fixer) {
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if (node.consequent.value === node.alternate.value) {
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// Replace `foo ? true : true` with just `true`, but don't replace `foo() ? true : true`
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return node.test.type === "Identifier" ? fixer.replaceText(node, node.consequent.value.toString()) : null;
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}
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if (node.alternate.value) {
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// Replace `foo() ? false : true` with `!(foo())`
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return fixer.replaceText(node, invertExpression(node.test));
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}
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// Replace `foo ? true : false` with `foo` if `foo` is guaranteed to be a boolean, or `!!foo` otherwise.
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return fixer.replaceText(node, isBooleanExpression(node.test) ? astUtils.getParenthesisedText(sourceCode, node.test) : `!${invertExpression(node.test)}`);
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}
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});
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} else if (!defaultAssignment && matchesDefaultAssignment(node)) {
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context.report({
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node,
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loc: node.consequent.loc.start,
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message: "Unnecessary use of conditional expression for default assignment.",
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fix: fixer => {
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const shouldParenthesizeAlternate = (
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astUtils.getPrecedence(node.alternate) < OR_PRECEDENCE &&
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!astUtils.isParenthesised(sourceCode, node.alternate)
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);
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const alternateText = shouldParenthesizeAlternate
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? `(${sourceCode.getText(node.alternate)})`
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: astUtils.getParenthesisedText(sourceCode, node.alternate);
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const testText = astUtils.getParenthesisedText(sourceCode, node.test);
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return fixer.replaceText(node, `${testText} || ${alternateText}`);
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}
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});
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}
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}
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};
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}
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};
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