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explicit-function-return-type.ts
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explicit-function-return-type.ts
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import {
TSESTree,
AST_NODE_TYPES,
} from '@typescript-eslint/experimental-utils';
import * as util from '../util';
type Options = [
{
allowExpressions?: boolean;
allowTypedFunctionExpressions?: boolean;
allowHigherOrderFunctions?: boolean;
}
];
type MessageIds = 'missingReturnType';
export default util.createRule<Options, MessageIds>({
name: 'explicit-function-return-type',
meta: {
type: 'problem',
docs: {
description:
'Require explicit return types on functions and class methods',
category: 'Stylistic Issues',
recommended: 'warn',
},
messages: {
missingReturnType: 'Missing return type on function.',
},
schema: [
{
type: 'object',
properties: {
allowExpressions: {
type: 'boolean',
},
allowTypedFunctionExpressions: {
type: 'boolean',
},
allowHigherOrderFunctions: {
type: 'boolean',
},
},
additionalProperties: false,
},
],
},
defaultOptions: [
{
allowExpressions: false,
allowTypedFunctionExpressions: false,
allowHigherOrderFunctions: false,
},
],
create(context, [options]) {
/**
* Checks if a node is a constructor.
* @param node The node to check
*/
function isConstructor(node: TSESTree.Node | undefined): boolean {
return (
!!node &&
node.type === AST_NODE_TYPES.MethodDefinition &&
node.kind === 'constructor'
);
}
/**
* Checks if a node is a setter.
*/
function isSetter(node: TSESTree.Node | undefined): boolean {
return (
!!node &&
(node.type === AST_NODE_TYPES.MethodDefinition ||
node.type === AST_NODE_TYPES.Property) &&
node.kind === 'set'
);
}
/**
* Checks if a node is a variable declarator with a type annotation.
* `const x: Foo = ...`
*/
function isVariableDeclaratorWithTypeAnnotation(
node: TSESTree.Node,
): boolean {
return (
node.type === AST_NODE_TYPES.VariableDeclarator &&
!!node.id.typeAnnotation
);
}
/**
* Checks if a node is a class property with a type annotation.
* `public x: Foo = ...`
*/
function isClassPropertyWithTypeAnnotation(node: TSESTree.Node): boolean {
return (
node.type === AST_NODE_TYPES.ClassProperty && !!node.typeAnnotation
);
}
/**
* Checks if a node is a type cast
* `(() => {}) as Foo`
* `<Foo>(() => {})`
*/
function isTypeCast(node: TSESTree.Node): boolean {
return (
node.type === AST_NODE_TYPES.TSAsExpression ||
node.type === AST_NODE_TYPES.TSTypeAssertion
);
}
/**
* Checks if a node belongs to:
* `const x: Foo = { prop: () => {} }`
* `const x = { prop: () => {} } as Foo`
* `const x = <Foo>{ prop: () => {} }`
*/
function isPropertyOfObjectWithType(
parent: TSESTree.Node | undefined,
): boolean {
if (!parent || parent.type !== AST_NODE_TYPES.Property) {
return false;
}
parent = parent.parent; // this shouldn't happen, checking just in case
/* istanbul ignore if */ if (
!parent ||
parent.type !== AST_NODE_TYPES.ObjectExpression
) {
return false;
}
parent = parent.parent; // this shouldn't happen, checking just in case
/* istanbul ignore if */ if (!parent) {
return false;
}
return (
isTypeCast(parent) ||
isClassPropertyWithTypeAnnotation(parent) ||
isVariableDeclaratorWithTypeAnnotation(parent)
);
}
/**
* Checks if a function belongs to:
* `() => () => ...`
* `() => function () { ... }`
* `() => { return () => ... }`
* `() => { return function () { ... } }`
* `function fn() { return () => ... }`
* `function fn() { return function() { ... } }`
*/
function doesImmediatelyReturnFunctionExpression({
body,
}:
| TSESTree.ArrowFunctionExpression
| TSESTree.FunctionDeclaration
| TSESTree.FunctionExpression): boolean {
// Should always have a body; really checking just in case
/* istanbul ignore if */ if (!body) {
return false;
}
// Check if body is a block with a single statement
if (
body.type === AST_NODE_TYPES.BlockStatement &&
body.body.length === 1
) {
const [statement] = body.body;
// Check if that statement is a return statement with an argument
if (
statement.type === AST_NODE_TYPES.ReturnStatement &&
!!statement.argument
) {
// If so, check that returned argument as body
body = statement.argument;
}
}
// Check if the body being returned is a function expression
return (
body.type === AST_NODE_TYPES.ArrowFunctionExpression ||
body.type === AST_NODE_TYPES.FunctionExpression
);
}
/**
* Checks if a node belongs to:
* `foo(() => 1)`
*/
function isFunctionArgument(
parent: TSESTree.Node,
child: TSESTree.Node,
): boolean {
return (
parent.type === AST_NODE_TYPES.CallExpression &&
// make sure this isn't an IIFE
parent.callee !== child
);
}
/**
* Checks if a function declaration/expression has a return type.
*/
function checkFunctionReturnType(
node:
| TSESTree.ArrowFunctionExpression
| TSESTree.FunctionDeclaration
| TSESTree.FunctionExpression,
): void {
if (
options.allowHigherOrderFunctions &&
doesImmediatelyReturnFunctionExpression(node)
) {
return;
}
if (
node.returnType ||
isConstructor(node.parent) ||
isSetter(node.parent)
) {
return;
}
if (util.isTypeScriptFile(context.getFilename())) {
context.report({
node,
messageId: 'missingReturnType',
});
}
}
/**
* Checks if a function declaration/expression has a return type.
*/
function checkFunctionExpressionReturnType(
node: TSESTree.ArrowFunctionExpression | TSESTree.FunctionExpression,
): void {
// Should always have a parent; checking just in case
/* istanbul ignore else */ if (node.parent) {
if (options.allowTypedFunctionExpressions) {
if (
isTypeCast(node.parent) ||
isVariableDeclaratorWithTypeAnnotation(node.parent) ||
isClassPropertyWithTypeAnnotation(node.parent) ||
isPropertyOfObjectWithType(node.parent) ||
isFunctionArgument(node.parent, node)
) {
return;
}
}
if (
options.allowExpressions &&
node.parent.type !== AST_NODE_TYPES.VariableDeclarator &&
node.parent.type !== AST_NODE_TYPES.MethodDefinition
) {
return;
}
}
checkFunctionReturnType(node);
}
return {
ArrowFunctionExpression: checkFunctionExpressionReturnType,
FunctionDeclaration: checkFunctionReturnType,
FunctionExpression: checkFunctionExpressionReturnType,
};
},
});