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