/
prefer-nullish-coalescing.ts
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/
prefer-nullish-coalescing.ts
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import {
AST_NODE_TYPES,
AST_TOKEN_TYPES,
TSESLint,
TSESTree,
} from '@typescript-eslint/experimental-utils';
import * as util from '../util';
export type Options = [
{
ignoreConditionalTests?: boolean;
ignoreMixedLogicalExpressions?: boolean;
forceSuggestionFixer?: boolean;
},
];
export type MessageIds = 'preferNullish';
export default util.createRule<Options, MessageIds>({
name: 'prefer-nullish-coalescing',
meta: {
type: 'suggestion',
docs: {
description:
'Enforce the usage of the nullish coalescing operator instead of logical chaining',
category: 'Best Practices',
recommended: false,
suggestion: true,
requiresTypeChecking: true,
},
fixable: 'code',
messages: {
preferNullish:
'Prefer using nullish coalescing operator (`??`) instead of a logical or (`||`), as it is a safer operator.',
},
schema: [
{
type: 'object',
properties: {
ignoreConditionalTests: {
type: 'boolean',
},
ignoreMixedLogicalExpressions: {
type: 'boolean',
},
forceSuggestionFixer: {
type: 'boolean',
},
},
additionalProperties: false,
},
],
},
defaultOptions: [
{
ignoreConditionalTests: true,
ignoreMixedLogicalExpressions: true,
forceSuggestionFixer: false,
},
],
create(
context,
[
{
ignoreConditionalTests,
ignoreMixedLogicalExpressions,
forceSuggestionFixer,
},
],
) {
const parserServices = util.getParserServices(context);
const sourceCode = context.getSourceCode();
const checker = parserServices.program.getTypeChecker();
return {
'LogicalExpression[operator = "||"]'(
node: TSESTree.LogicalExpression,
): void {
const tsNode = parserServices.esTreeNodeToTSNodeMap.get(node);
const type = checker.getTypeAtLocation(tsNode.left);
const isNullish = util.isNullableType(type, { allowUndefined: true });
if (!isNullish) {
return;
}
if (ignoreConditionalTests === true && isConditionalTest(node)) {
return;
}
const isMixedLogical = isMixedLogicalExpression(node);
if (ignoreMixedLogicalExpressions === true && isMixedLogical) {
return;
}
const barBarOperator = util.nullThrows(
sourceCode.getTokenAfter(
node.left,
token =>
token.type === AST_TOKEN_TYPES.Punctuator &&
token.value === node.operator,
),
util.NullThrowsReasons.MissingToken('operator', node.type),
);
function* fix(
fixer: TSESLint.RuleFixer,
): IterableIterator<TSESLint.RuleFix> {
if (node.parent && util.isLogicalOrOperator(node.parent)) {
// '&&' and '??' operations cannot be mixed without parentheses (e.g. a && b ?? c)
if (
node.left.type === AST_NODE_TYPES.LogicalExpression &&
!util.isLogicalOrOperator(node.left.left)
) {
yield fixer.insertTextBefore(node.left.right, '(');
} else {
yield fixer.insertTextBefore(node.left, '(');
}
yield fixer.insertTextAfter(node.right, ')');
}
yield fixer.replaceText(barBarOperator, '??');
}
const fixer =
isMixedLogical || forceSuggestionFixer
? // suggestion instead for cases where we aren't sure if the fixer is completely safe
({
suggest: [
{
messageId: 'preferNullish',
fix,
},
],
} as const)
: { fix };
context.report({
node: barBarOperator,
messageId: 'preferNullish',
...fixer,
});
},
};
},
});
function isConditionalTest(node: TSESTree.Node): boolean {
const parents = new Set<TSESTree.Node | null>([node]);
let current = node.parent;
while (current) {
parents.add(current);
if (
(current.type === AST_NODE_TYPES.ConditionalExpression ||
current.type === AST_NODE_TYPES.DoWhileStatement ||
current.type === AST_NODE_TYPES.IfStatement ||
current.type === AST_NODE_TYPES.ForStatement ||
current.type === AST_NODE_TYPES.WhileStatement) &&
parents.has(current.test)
) {
return true;
}
if (
[
AST_NODE_TYPES.ArrowFunctionExpression,
AST_NODE_TYPES.FunctionExpression,
].includes(current.type)
) {
/**
* This is a weird situation like:
* `if (() => a || b) {}`
* `if (function () { return a || b }) {}`
*/
return false;
}
current = current.parent;
}
return false;
}
function isMixedLogicalExpression(node: TSESTree.LogicalExpression): boolean {
const seen = new Set<TSESTree.Node | undefined>();
const queue = [node.parent, node.left, node.right];
for (const current of queue) {
if (seen.has(current)) {
continue;
}
seen.add(current);
if (current && current.type === AST_NODE_TYPES.LogicalExpression) {
if (current.operator === '&&') {
return true;
} else if (current.operator === '||') {
// check the pieces of the node to catch cases like `a || b || c && d`
queue.push(current.parent, current.left, current.right);
}
}
}
return false;
}