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strict-boolean-expressions.ts
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strict-boolean-expressions.ts
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import {
AST_NODE_TYPES,
ParserServices,
TSESTree,
} from '@typescript-eslint/utils';
import * as tsutils from 'tsutils';
import * as ts from 'typescript';
import * as util from '../util';
export type Options = [
{
allowString?: boolean;
allowNumber?: boolean;
allowNullableObject?: boolean;
allowNullableBoolean?: boolean;
allowNullableString?: boolean;
allowNullableNumber?: boolean;
allowAny?: boolean;
allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
},
];
export type MessageId =
| 'conditionErrorOther'
| 'conditionErrorAny'
| 'conditionErrorNullish'
| 'conditionErrorNullableBoolean'
| 'conditionErrorString'
| 'conditionErrorNullableString'
| 'conditionErrorNumber'
| 'conditionErrorNullableNumber'
| 'conditionErrorObject'
| 'conditionErrorNullableObject'
| 'noStrictNullCheck'
| 'conditionFixDefaultFalse'
| 'conditionFixDefaultEmptyString'
| 'conditionFixDefaultZero'
| 'conditionFixCompareNullish'
| 'conditionFixCastBoolean'
| 'conditionFixCompareTrue'
| 'conditionFixCompareFalse'
| 'conditionFixCompareStringLength'
| 'conditionFixCompareEmptyString'
| 'conditionFixCompareZero'
| 'conditionFixCompareNaN';
export default util.createRule<Options, MessageId>({
name: 'strict-boolean-expressions',
meta: {
type: 'suggestion',
fixable: 'code',
hasSuggestions: true,
docs: {
description: 'Disallow certain types in boolean expressions',
recommended: false,
requiresTypeChecking: true,
},
schema: [
{
type: 'object',
properties: {
allowString: { type: 'boolean' },
allowNumber: { type: 'boolean' },
allowNullableObject: { type: 'boolean' },
allowNullableBoolean: { type: 'boolean' },
allowNullableString: { type: 'boolean' },
allowNullableNumber: { type: 'boolean' },
allowAny: { type: 'boolean' },
allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: {
type: 'boolean',
},
},
additionalProperties: false,
},
],
messages: {
conditionErrorOther:
'Unexpected value in conditional. ' +
'A boolean expression is required.',
conditionErrorAny:
'Unexpected any value in conditional. ' +
'An explicit comparison or type cast is required.',
conditionErrorNullish:
'Unexpected nullish value in conditional. ' +
'The condition is always false.',
conditionErrorNullableBoolean:
'Unexpected nullable boolean value in conditional. ' +
'Please handle the nullish case explicitly.',
conditionErrorString:
'Unexpected string value in conditional. ' +
'An explicit empty string check is required.',
conditionErrorNullableString:
'Unexpected nullable string value in conditional. ' +
'Please handle the nullish/empty cases explicitly.',
conditionErrorNumber:
'Unexpected number value in conditional. ' +
'An explicit zero/NaN check is required.',
conditionErrorNullableNumber:
'Unexpected nullable number value in conditional. ' +
'Please handle the nullish/zero/NaN cases explicitly.',
conditionErrorObject:
'Unexpected object value in conditional. ' +
'The condition is always true.',
conditionErrorNullableObject:
'Unexpected nullable object value in conditional. ' +
'An explicit null check is required.',
noStrictNullCheck:
'This rule requires the `strictNullChecks` compiler option to be turned on to function correctly.',
conditionFixDefaultFalse:
'Explicitly treat nullish value the same as false (`value ?? false`)',
conditionFixDefaultEmptyString:
'Explicitly treat nullish value the same as an empty string (`value ?? ""`)',
conditionFixDefaultZero:
'Explicitly treat nullish value the same as 0 (`value ?? 0`)',
conditionFixCompareNullish:
'Change condition to check for null/undefined (`value != null`)',
conditionFixCastBoolean:
'Explicitly cast value to a boolean (`Boolean(value)`)',
conditionFixCompareTrue:
'Change condition to check if true (`value === true`)',
conditionFixCompareFalse:
'Change condition to check if false (`value === false`)',
conditionFixCompareStringLength:
"Change condition to check string's length (`value.length !== 0`)",
conditionFixCompareEmptyString:
'Change condition to check for empty string (`value !== ""`)',
conditionFixCompareZero:
'Change condition to check for 0 (`value !== 0`)',
conditionFixCompareNaN:
'Change condition to check for NaN (`!Number.isNaN(value)`)',
},
},
defaultOptions: [
{
allowString: true,
allowNumber: true,
allowNullableObject: true,
allowNullableBoolean: false,
allowNullableString: false,
allowNullableNumber: false,
allowAny: false,
allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
},
],
create(context, [options]) {
const parserServices = util.getParserServices(context);
const typeChecker = parserServices.program.getTypeChecker();
const compilerOptions = parserServices.program.getCompilerOptions();
const sourceCode = context.getSourceCode();
const isStrictNullChecks = tsutils.isStrictCompilerOptionEnabled(
compilerOptions,
'strictNullChecks',
);
if (
!isStrictNullChecks &&
options.allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing !== true
) {
context.report({
loc: {
start: { line: 0, column: 0 },
end: { line: 0, column: 0 },
},
messageId: 'noStrictNullCheck',
});
}
const traversedNodes = new Set<TSESTree.Node>();
return {
ConditionalExpression: traverseTestExpression,
DoWhileStatement: traverseTestExpression,
ForStatement: traverseTestExpression,
IfStatement: traverseTestExpression,
WhileStatement: traverseTestExpression,
'LogicalExpression[operator!="??"]': traverseLogicalExpression,
'UnaryExpression[operator="!"]': traverseUnaryLogicalExpression,
};
type TestExpression =
| TSESTree.ConditionalExpression
| TSESTree.DoWhileStatement
| TSESTree.ForStatement
| TSESTree.IfStatement
| TSESTree.WhileStatement;
/**
* Inspects condition of a test expression. (`if`, `while`, `for`, etc.)
*/
function traverseTestExpression(node: TestExpression): void {
if (node.test == null) {
return;
}
traverseNode(node.test, true);
}
/**
* Inspects the argument of a unary logical expression (`!`).
*/
function traverseUnaryLogicalExpression(
node: TSESTree.UnaryExpression,
): void {
traverseNode(node.argument, true);
}
/**
* Inspects the arguments of a logical expression (`&&`, `||`).
*
* If the logical expression is a descendant of a test expression,
* the `isCondition` flag should be set to true.
* Otherwise, if the logical expression is there on it's own,
* it's used for control flow and is not a condition itself.
*/
function traverseLogicalExpression(
node: TSESTree.LogicalExpression,
isCondition = false,
): void {
// left argument is always treated as a condition
traverseNode(node.left, true);
// if the logical expression is used for control flow,
// then it's right argument is used for it's side effects only
traverseNode(node.right, isCondition);
}
/**
* Inspects any node.
*
* If it's a logical expression then it recursively traverses its arguments.
* If it's any other kind of node then it's type is finally checked against the rule,
* unless `isCondition` flag is set to false, in which case
* it's assumed to be used for side effects only and is skipped.
*/
function traverseNode(node: TSESTree.Node, isCondition: boolean): void {
// prevent checking the same node multiple times
if (traversedNodes.has(node)) {
return;
}
traversedNodes.add(node);
// for logical operator, we check its operands
if (
node.type === AST_NODE_TYPES.LogicalExpression &&
node.operator !== '??'
) {
traverseLogicalExpression(node, isCondition);
return;
}
// skip if node is not a condition
if (!isCondition) {
return;
}
checkNode(node);
}
/**
* This function does the actual type check on a node.
* It analyzes the type of a node and checks if it is allowed in a boolean context.
*/
function checkNode(node: TSESTree.Node): void {
const tsNode = parserServices.esTreeNodeToTSNodeMap.get(node);
const type = util.getConstrainedTypeAtLocation(typeChecker, tsNode);
const types = inspectVariantTypes(tsutils.unionTypeParts(type));
const is = (...wantedTypes: readonly VariantType[]): boolean =>
types.size === wantedTypes.length &&
wantedTypes.every(type => types.has(type));
// boolean
if (is('boolean') || is('truthy boolean')) {
// boolean is always okay
return;
}
// never
if (is('never')) {
// never is always okay
return;
}
// nullish
if (is('nullish')) {
// condition is always false
context.report({ node, messageId: 'conditionErrorNullish' });
return;
}
// Known edge case: boolean `true` and nullish values are always valid boolean expressions
if (is('nullish', 'truthy boolean')) {
return;
}
// nullable boolean
if (is('nullish', 'boolean')) {
if (!options.allowNullableBoolean) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!nullableBoolean)
context.report({
node,
messageId: 'conditionErrorNullableBoolean',
suggest: [
{
messageId: 'conditionFixDefaultFalse',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? false`,
}),
},
{
messageId: 'conditionFixCompareFalse',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} === false`,
}),
},
],
});
} else {
// if (nullableBoolean)
context.report({
node,
messageId: 'conditionErrorNullableBoolean',
suggest: [
{
messageId: 'conditionFixDefaultFalse',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? false`,
}),
},
{
messageId: 'conditionFixCompareTrue',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} === true`,
}),
},
],
});
}
}
return;
}
// Known edge case: truthy primitives and nullish values are always valid boolean expressions
if (
(options.allowNumber && is('nullish', 'truthy number')) ||
(options.allowString && is('nullish', 'truthy string'))
) {
return;
}
// string
if (is('string') || is('truthy string')) {
if (!options.allowString) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!string)
context.report({
node,
messageId: 'conditionErrorString',
suggest: [
{
messageId: 'conditionFixCompareStringLength',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code}.length === 0`,
}),
},
{
messageId: 'conditionFixCompareEmptyString',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} === ""`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `!Boolean(${code})`,
}),
},
],
});
} else {
// if (string)
context.report({
node,
messageId: 'conditionErrorString',
suggest: [
{
messageId: 'conditionFixCompareStringLength',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code}.length > 0`,
}),
},
{
messageId: 'conditionFixCompareEmptyString',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} !== ""`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `Boolean(${code})`,
}),
},
],
});
}
}
return;
}
// nullable string
if (is('nullish', 'string')) {
if (!options.allowNullableString) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!nullableString)
context.report({
node,
messageId: 'conditionErrorNullableString',
suggest: [
{
messageId: 'conditionFixCompareNullish',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} == null`,
}),
},
{
messageId: 'conditionFixDefaultEmptyString',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? ""`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `!Boolean(${code})`,
}),
},
],
});
} else {
// if (nullableString)
context.report({
node,
messageId: 'conditionErrorNullableString',
suggest: [
{
messageId: 'conditionFixCompareNullish',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} != null`,
}),
},
{
messageId: 'conditionFixDefaultEmptyString',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? ""`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `Boolean(${code})`,
}),
},
],
});
}
}
return;
}
// number
if (is('number') || is('truthy number')) {
if (!options.allowNumber) {
if (isArrayLengthExpression(node, typeChecker, parserServices)) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!array.length)
context.report({
node,
messageId: 'conditionErrorNumber',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} === 0`,
}),
});
} else {
// if (array.length)
context.report({
node,
messageId: 'conditionErrorNumber',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} > 0`,
}),
});
}
} else if (isLogicalNegationExpression(node.parent!)) {
// if (!number)
context.report({
node,
messageId: 'conditionErrorNumber',
suggest: [
{
messageId: 'conditionFixCompareZero',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
// TODO: we have to compare to 0n if the type is bigint
wrap: code => `${code} === 0`,
}),
},
{
// TODO: don't suggest this for bigint because it can't be NaN
messageId: 'conditionFixCompareNaN',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `Number.isNaN(${code})`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `!Boolean(${code})`,
}),
},
],
});
} else {
// if (number)
context.report({
node,
messageId: 'conditionErrorNumber',
suggest: [
{
messageId: 'conditionFixCompareZero',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} !== 0`,
}),
},
{
messageId: 'conditionFixCompareNaN',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `!Number.isNaN(${code})`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `Boolean(${code})`,
}),
},
],
});
}
}
return;
}
// nullable number
if (is('nullish', 'number')) {
if (!options.allowNullableNumber) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!nullableNumber)
context.report({
node,
messageId: 'conditionErrorNullableNumber',
suggest: [
{
messageId: 'conditionFixCompareNullish',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} == null`,
}),
},
{
messageId: 'conditionFixDefaultZero',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? 0`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `!Boolean(${code})`,
}),
},
],
});
} else {
// if (nullableNumber)
context.report({
node,
messageId: 'conditionErrorNullableNumber',
suggest: [
{
messageId: 'conditionFixCompareNullish',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} != null`,
}),
},
{
messageId: 'conditionFixDefaultZero',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} ?? 0`,
}),
},
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `Boolean(${code})`,
}),
},
],
});
}
}
return;
}
// object
if (is('object')) {
// condition is always true
context.report({ node, messageId: 'conditionErrorObject' });
return;
}
// nullable object
if (is('nullish', 'object')) {
if (!options.allowNullableObject) {
if (isLogicalNegationExpression(node.parent!)) {
// if (!nullableObject)
context.report({
node,
messageId: 'conditionErrorNullableObject',
fix: util.getWrappingFixer({
sourceCode,
node: node.parent,
innerNode: node,
wrap: code => `${code} == null`,
}),
});
} else {
// if (nullableObject)
context.report({
node,
messageId: 'conditionErrorNullableObject',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `${code} != null`,
}),
});
}
}
return;
}
// any
if (is('any')) {
if (!options.allowAny) {
context.report({
node,
messageId: 'conditionErrorAny',
suggest: [
{
messageId: 'conditionFixCastBoolean',
fix: util.getWrappingFixer({
sourceCode,
node,
wrap: code => `Boolean(${code})`,
}),
},
],
});
}
return;
}
// other
context.report({ node, messageId: 'conditionErrorOther' });
}
/** The types we care about */
type VariantType =
| 'nullish'
| 'boolean'
| 'truthy boolean'
| 'string'
| 'truthy string'
| 'number'
| 'truthy number'
| 'object'
| 'any'
| 'never';
/**
* Check union variants for the types we care about
*/
function inspectVariantTypes(types: ts.Type[]): Set<VariantType> {
const variantTypes = new Set<VariantType>();
if (
types.some(type =>
tsutils.isTypeFlagSet(
type,
ts.TypeFlags.Null | ts.TypeFlags.Undefined | ts.TypeFlags.VoidLike,
),
)
) {
variantTypes.add('nullish');
}
const booleans = types.filter(type =>
tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike),
);
// If incoming type is either "true" or "false", there will be one type
// object with intrinsicName set accordingly
// If incoming type is boolean, there will be two type objects with
// intrinsicName set "true" and "false" each because of tsutils.unionTypeParts()
if (booleans.length === 1) {
tsutils.isBooleanLiteralType(booleans[0], true)
? variantTypes.add('truthy boolean')
: variantTypes.add('boolean');
} else if (booleans.length === 2) {
variantTypes.add('boolean');
}
const strings = types.filter(type =>
tsutils.isTypeFlagSet(type, ts.TypeFlags.StringLike),
);
if (strings.length) {
if (strings.some(type => type.isStringLiteral() && type.value !== '')) {
variantTypes.add('truthy string');
} else {
variantTypes.add('string');
}
}
const numbers = types.filter(type =>
tsutils.isTypeFlagSet(
type,
ts.TypeFlags.NumberLike | ts.TypeFlags.BigIntLike,
),
);
if (numbers.length) {
if (numbers.some(type => type.isNumberLiteral() && type.value !== 0)) {
variantTypes.add('truthy number');
} else {
variantTypes.add('number');
}
}
if (
types.some(
type =>
!tsutils.isTypeFlagSet(
type,
ts.TypeFlags.Null |
ts.TypeFlags.Undefined |
ts.TypeFlags.VoidLike |
ts.TypeFlags.BooleanLike |
ts.TypeFlags.StringLike |
ts.TypeFlags.NumberLike |
ts.TypeFlags.BigIntLike |
ts.TypeFlags.TypeParameter |
ts.TypeFlags.Any |
ts.TypeFlags.Unknown |
ts.TypeFlags.Never,
),
)
) {
variantTypes.add('object');
}
if (
types.some(type =>
util.isTypeFlagSet(
type,
ts.TypeFlags.TypeParameter |
ts.TypeFlags.Any |
ts.TypeFlags.Unknown,
),
)
) {
variantTypes.add('any');
}
if (types.some(type => tsutils.isTypeFlagSet(type, ts.TypeFlags.Never))) {
variantTypes.add('never');
}
return variantTypes;
}
},
});
function isLogicalNegationExpression(
node: TSESTree.Node,
): node is TSESTree.UnaryExpression {
return node.type === AST_NODE_TYPES.UnaryExpression && node.operator === '!';
}
function isArrayLengthExpression(
node: TSESTree.Node,
typeChecker: ts.TypeChecker,
parserServices: ParserServices,
): node is TSESTree.MemberExpressionNonComputedName {
if (node.type !== AST_NODE_TYPES.MemberExpression) {
return false;
}
if (node.computed) {
return false;
}
if (node.property.name !== 'length') {
return false;
}
const objectTsNode = parserServices.esTreeNodeToTSNodeMap.get(node.object);
const objectType = util.getConstrainedTypeAtLocation(
typeChecker,
objectTsNode,
);
return util.isTypeArrayTypeOrUnionOfArrayTypes(objectType, typeChecker);
}