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prefer-ternary.js
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prefer-ternary.js
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'use strict';
const {isParenthesized} = require('eslint-utils');
const {flatten} = require('lodash');
const getDocumentationUrl = require('./utils/get-documentation-url');
const avoidCapture = require('./utils/avoid-capture');
const extendFixRange = require('./utils/extend-fix-range');
const messageId = 'prefer-ternary';
const selector = [
'IfStatement',
':not(IfStatement > IfStatement.alternate)',
'[test.type!="ConditionalExpression"]',
'[consequent]',
'[alternate]'
].join('');
const isTernary = node => node && node.type === 'ConditionalExpression';
function getNodeBody(node) {
/* istanbul ignore next */
if (!node) {
return;
}
if (node.type === 'ExpressionStatement') {
return getNodeBody(node.expression);
}
if (node.type === 'BlockStatement') {
const body = node.body.filter(({type}) => type !== 'EmptyStatement');
if (body.length === 1) {
return getNodeBody(body[0]);
}
}
return node;
}
function isSameAssignmentLeft(node1, node2) {
// [TODO]: Allow more types of left
return node1.type === node2.type && node1.type === 'Identifier' && node1.name === node2.name;
}
const getIndentString = (node, sourceCode) => {
const {line, column} = sourceCode.getLocFromIndex(node.range[0]);
const lines = sourceCode.getLines();
const before = lines[line - 1].slice(0, column);
return before.match(/\s*$/)[0];
};
const getScopes = scope => [
scope,
...flatten(scope.childScopes.map(scope => getScopes(scope)))
];
const create = context => {
const sourceCode = context.getSourceCode();
const scopeToNamesGeneratedByFixer = new WeakMap();
const isSafeName = (name, scopes) => scopes.every(scope => {
const generatedNames = scopeToNamesGeneratedByFixer.get(scope);
return !generatedNames || !generatedNames.has(name);
});
const getParenthesizedText = node => {
const text = sourceCode.getText(node);
return (
isParenthesized(node, sourceCode) ||
node.type === 'AwaitExpression' ||
// Lower precedence, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table
node.type === 'AssignmentExpression' ||
node.type === 'YieldExpression' ||
node.type === 'SequenceExpression'
) ?
`(${text})` : text;
};
function merge(options, mergeOptions) {
const {
before = '',
after = ';',
consequent,
alternate,
node
} = options;
const {
checkThrowStatement,
returnFalseIfNotMergeable
} = {
checkThrowStatement: false,
returnFalseIfNotMergeable: false,
...mergeOptions
};
if (!consequent || !alternate || consequent.type !== alternate.type) {
return returnFalseIfNotMergeable ? false : options;
}
const {type} = consequent;
if (
type === 'ReturnStatement' &&
!isTernary(consequent.argument) &&
!isTernary(alternate.argument)
) {
return merge({
before: `${before}return `,
after,
consequent: consequent.argument === null ? 'undefined' : consequent.argument,
alternate: alternate.argument === null ? 'undefined' : alternate.argument,
node
});
}
if (
type === 'YieldExpression' &&
consequent.delegate === alternate.delegate &&
!isTernary(consequent.argument) &&
!isTernary(alternate.argument)
) {
return merge({
before: `${before}yield${consequent.delegate ? '*' : ''} (`,
after: `)${after}`,
consequent: consequent.argument === null ? 'undefined' : consequent.argument,
alternate: alternate.argument === null ? 'undefined' : alternate.argument,
node
});
}
if (
type === 'AwaitExpression' &&
!isTernary(consequent.argument) &&
!isTernary(alternate.argument)
) {
return merge({
before: `${before}await (`,
after: `)${after}`,
consequent: consequent.argument,
alternate: alternate.argument,
node
});
}
if (
checkThrowStatement &&
type === 'ThrowStatement' &&
!isTernary(consequent.argument) &&
!isTernary(alternate.argument)
) {
// `ThrowStatement` don't check nested
// If `IfStatement` is not a `BlockStatement`, need add `{}`
const {parent} = node;
const needBraces = parent && parent.type !== 'BlockStatement';
return {
type,
before: `${before}${needBraces ? '{\n{{INDENT_STRING}}' : ''}const {{ERROR_NAME}} = `,
after: `;\n{{INDENT_STRING}}throw {{ERROR_NAME}};${needBraces ? '\n}' : ''}`,
consequent: consequent.argument,
alternate: alternate.argument
};
}
if (
type === 'AssignmentExpression' &&
isSameAssignmentLeft(consequent.left, alternate.left) &&
consequent.operator === alternate.operator &&
!isTernary(consequent.left) &&
!isTernary(alternate.left) &&
!isTernary(consequent.right) &&
!isTernary(alternate.right)
) {
return merge({
before: `${before}${sourceCode.getText(consequent.left)} ${consequent.operator} `,
after,
consequent: consequent.right,
alternate: alternate.right,
node
});
}
return returnFalseIfNotMergeable ? false : options;
}
return {
[selector](node) {
const consequent = getNodeBody(node.consequent);
const alternate = getNodeBody(node.alternate);
const result = merge({node, consequent, alternate}, {
checkThrowStatement: true,
returnFalseIfNotMergeable: true
});
if (!result) {
return;
}
const scope = context.getScope();
const sourceCode = context.getSourceCode();
context.report({
node,
messageId,
* fix(fixer) {
const testText = getParenthesizedText(node.test);
const consequentText = typeof result.consequent === 'string' ?
result.consequent :
getParenthesizedText(result.consequent);
const alternateText = typeof result.alternate === 'string' ?
result.alternate :
getParenthesizedText(result.alternate);
let {type, before, after} = result;
let generateNewVariables = false;
if (type === 'ThrowStatement') {
const scopes = getScopes(scope);
const errorName = avoidCapture('error', scopes, context.parserOptions.ecmaVersion, isSafeName);
for (const scope of scopes) {
if (!scopeToNamesGeneratedByFixer.has(scope)) {
scopeToNamesGeneratedByFixer.set(scope, new Set());
}
const generatedNames = scopeToNamesGeneratedByFixer.get(scope);
generatedNames.add(errorName);
}
const indentString = getIndentString(node, sourceCode);
after = after
.replace('{{INDENT_STRING}}', indentString)
.replace('{{ERROR_NAME}}', errorName);
before = before
.replace('{{INDENT_STRING}}', indentString)
.replace('{{ERROR_NAME}}', errorName);
generateNewVariables = true;
}
const fixed = `${before}${testText} ? ${consequentText} : ${alternateText}${after}`;
yield fixer.replaceText(node, fixed);
if (generateNewVariables) {
yield * extendFixRange(fixer, sourceCode.ast.range);
}
}
});
}
};
};
module.exports = {
create,
meta: {
type: 'suggestion',
docs: {
url: getDocumentationUrl(__filename)
},
messages: {
[messageId]: 'This `if` statement can be replaced by a ternary expression.'
},
fixable: 'code'
}
};