/
no-unnecessary-boolean-literal-compare.ts
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/
no-unnecessary-boolean-literal-compare.ts
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import {
AST_NODE_TYPES,
TSESTree,
} from '@typescript-eslint/experimental-utils';
import * as tsutils from 'tsutils';
import * as ts from 'typescript';
import * as util from '../util';
type MessageIds =
| 'direct'
| 'negated'
| 'comparingNullableToTrueDirect'
| 'comparingNullableToTrueNegated'
| 'comparingNullableToFalse';
type Options = [
{
allowComparingNullableBooleansToTrue?: boolean;
allowComparingNullableBooleansToFalse?: boolean;
},
];
interface BooleanComparison {
expression: TSESTree.Expression;
literalBooleanInComparison: boolean;
forTruthy: boolean;
negated: boolean;
range: [number, number];
}
interface BooleanComparisonWithTypeInformation extends BooleanComparison {
expressionIsNullableBoolean: boolean;
}
export default util.createRule<Options, MessageIds>({
name: 'no-unnecessary-boolean-literal-compare',
meta: {
docs: {
description:
'Flags unnecessary equality comparisons against boolean literals',
category: 'Stylistic Issues',
recommended: false,
requiresTypeChecking: true,
},
fixable: 'code',
messages: {
direct:
'This expression unnecessarily compares a boolean value to a boolean instead of using it directly.',
negated:
'This expression unnecessarily compares a boolean value to a boolean instead of negating it.',
comparingNullableToTrueDirect:
'This expression unnecessarily compares a nullable boolean value to true instead of using it directly.',
comparingNullableToTrueNegated:
'This expression unnecessarily compares a nullable boolean value to true instead of negating it.',
comparingNullableToFalse:
'This expression unnecessarily compares a nullable boolean value to false instead of using the ?? operator to provide a default.',
},
schema: [
{
type: 'object',
properties: {
allowComparingNullableBooleansToTrue: {
type: 'boolean',
},
allowComparingNullableBooleansToFalse: {
type: 'boolean',
},
},
additionalProperties: false,
},
],
type: 'suggestion',
},
defaultOptions: [
{
allowComparingNullableBooleansToTrue: true,
allowComparingNullableBooleansToFalse: true,
},
],
create(context, [options]) {
const parserServices = util.getParserServices(context);
const checker = parserServices.program.getTypeChecker();
function getBooleanComparison(
node: TSESTree.BinaryExpression,
): BooleanComparisonWithTypeInformation | undefined {
const comparison = deconstructComparison(node);
if (!comparison) {
return undefined;
}
const expressionType = checker.getTypeAtLocation(
parserServices.esTreeNodeToTSNodeMap.get(comparison.expression),
);
if (isBooleanType(expressionType)) {
return {
...comparison,
expressionIsNullableBoolean: false,
};
}
if (isNullableBoolean(expressionType)) {
return {
...comparison,
expressionIsNullableBoolean: true,
};
}
return undefined;
}
function isBooleanType(expressionType: ts.Type): boolean {
return tsutils.isTypeFlagSet(
expressionType,
ts.TypeFlags.Boolean | ts.TypeFlags.BooleanLiteral,
);
}
/**
* checks if the expressionType is a union that
* 1) contains at least one nullish type (null or undefined)
* 2) contains at least once boolean type (true or false or boolean)
* 3) does not contain any types besides nullish and boolean types
*/
function isNullableBoolean(expressionType: ts.Type): boolean {
if (!expressionType.isUnion()) {
return false;
}
const { types } = expressionType;
const nonNullishTypes = types.filter(
type =>
!tsutils.isTypeFlagSet(
type,
ts.TypeFlags.Undefined | ts.TypeFlags.Null,
),
);
const hasNonNullishType = nonNullishTypes.length > 0;
if (!hasNonNullishType) {
return false;
}
const hasNullableType = nonNullishTypes.length < types.length;
if (!hasNullableType) {
return false;
}
const allNonNullishTypesAreBoolean = nonNullishTypes.every(isBooleanType);
if (!allNonNullishTypesAreBoolean) {
return false;
}
return true;
}
function deconstructComparison(
node: TSESTree.BinaryExpression,
): BooleanComparison | undefined {
const comparisonType = util.getEqualsKind(node.operator);
if (!comparisonType) {
return undefined;
}
for (const [against, expression] of [
[node.right, node.left],
[node.left, node.right],
]) {
if (
against.type !== AST_NODE_TYPES.Literal ||
typeof against.value !== 'boolean'
) {
continue;
}
const { value: literalBooleanInComparison } = against;
const negated = !comparisonType.isPositive;
return {
literalBooleanInComparison,
forTruthy: literalBooleanInComparison ? !negated : negated,
expression,
negated,
range:
expression.range[0] < against.range[0]
? [expression.range[1], against.range[1]]
: [against.range[1], expression.range[1]],
};
}
return undefined;
}
function nodeIsUnaryNegation(node: TSESTree.Node): boolean {
return (
node.type === AST_NODE_TYPES.UnaryExpression &&
node.prefix &&
node.operator === '!'
);
}
return {
BinaryExpression(node): void {
const comparison = getBooleanComparison(node);
if (comparison === undefined) {
return;
}
if (comparison.expressionIsNullableBoolean) {
if (
comparison.literalBooleanInComparison &&
options.allowComparingNullableBooleansToTrue
) {
return;
}
if (
!comparison.literalBooleanInComparison &&
options.allowComparingNullableBooleansToFalse
) {
return;
}
}
context.report({
fix: function* (fixer) {
yield fixer.removeRange(comparison.range);
// if the expression `exp` isn't nullable, or we're comparing to `true`,
// we can just replace the entire comparison with `exp` or `!exp`
if (
!comparison.expressionIsNullableBoolean ||
comparison.literalBooleanInComparison
) {
if (!comparison.forTruthy) {
yield fixer.insertTextBefore(node, '!');
}
return;
}
// if we're here, then the expression is a nullable boolean and we're
// comparing to a literal `false`
// if we're doing `== false` or `=== false`, then we need to negate the expression
if (!comparison.negated) {
const { parent } = node;
// if the parent is a negation, we can instead just get rid of the parent's negation.
// i.e. instead of resulting in `!(!(exp))`, we can just result in `exp`
if (parent != null && nodeIsUnaryNegation(parent)) {
// remove from the beginning of the parent to the beginning of this node
yield fixer.removeRange([parent.range[0], node.range[0]]);
// remove from the end of the node to the end of the parent
yield fixer.removeRange([node.range[1], parent.range[1]]);
} else {
yield fixer.insertTextBefore(node, '!');
}
}
// provide the default `true`
yield fixer.insertTextBefore(node, '(');
yield fixer.insertTextAfter(node, ' ?? true)');
},
messageId: comparison.expressionIsNullableBoolean
? comparison.literalBooleanInComparison
? comparison.negated
? 'comparingNullableToTrueNegated'
: 'comparingNullableToTrueDirect'
: 'comparingNullableToFalse'
: comparison.negated
? 'negated'
: 'direct',
node,
});
},
};
},
});