no-floating-promises.ts

import * as tsutils from 'tsutils';
import * as ts from 'typescript';

import * as util from '../util';

export default util.createRule({
  name: 'no-floating-promises',
  meta: {
    docs: {
      description: 'Requires Promise-like values to be handled appropriately.',
      category: 'Best Practices',
      recommended: false,
    },
    messages: {
      floating: 'Promises must be handled appropriately',
    },
    schema: [],
    type: 'problem',
  },
  defaultOptions: [],

  create(context) {
    const parserServices = util.getParserServices(context);
    const checker = parserServices.program.getTypeChecker();

    return {
      ExpressionStatement(node) {
        const { expression } = parserServices.esTreeNodeToTSNodeMap.get(
          node,
        ) as ts.ExpressionStatement;

        if (isUnhandledPromise(checker, expression)) {
          context.report({
            messageId: 'floating',
            node,
          });
        }
      },
    };
  },
});

function isUnhandledPromise(checker: ts.TypeChecker, node: ts.Node): boolean {
  // First, check expressions whose resulting types may not be promise-like
  if (
    ts.isBinaryExpression(node) &&
    node.operatorToken.kind === ts.SyntaxKind.CommaToken
  ) {
    // Any child in a comma expression could return a potentially unhandled
    // promise, so we check them all regardless of whether the final returned
    // value is promise-like.
    return (
      isUnhandledPromise(checker, node.left) ||
      isUnhandledPromise(checker, node.right)
    );
  } else if (ts.isVoidExpression(node)) {
    // Similarly, a `void` expression always returns undefined, so we need to
    // see what's inside it without checking the type of the overall expression.
    return isUnhandledPromise(checker, node.expression);
  }

  // Check the type. At this point it can't be unhandled if it isn't a promise
  if (!isPromiseLike(checker, node)) {
    return false;
  }

  if (ts.isCallExpression(node)) {
    // If the outer expression is a call, it must be either a `.then()` or
    // `.catch()` that handles the promise.
    return (
      !isPromiseCatchCallWithHandler(node) &&
      !isPromiseThenCallWithRejectionHandler(node)
    );
  } else if (ts.isConditionalExpression(node)) {
    // We must be getting the promise-like value from one of the branches of the
    // ternary. Check them directly.
    return (
      isUnhandledPromise(checker, node.whenFalse) ||
      isUnhandledPromise(checker, node.whenTrue)
    );
  } else if (
    ts.isPropertyAccessExpression(node) ||
    ts.isIdentifier(node) ||
    ts.isNewExpression(node)
  ) {
    // If it is just a property access chain or a `new` call (e.g. `foo.bar` or
    // `new Promise()`), the promise is not handled because it doesn't have the
    // necessary then/catch call at the end of the chain.
    return true;
  }

  // We conservatively return false for all other types of expressions because
  // we don't want to accidentally fail if the promise is handled internally but
  // we just can't tell.
  return false;
}

// Modified from tsutils.isThenable() to only consider thenables which can be
// rejected/caught via a second parameter. Original source (MIT licensed):
//
//   https://github.com/ajafff/tsutils/blob/49d0d31050b44b81e918eae4fbaf1dfe7b7286af/util/type.ts#L95-L125
function isPromiseLike(checker: ts.TypeChecker, node: ts.Node): boolean {
  const type = checker.getTypeAtLocation(node);
  for (const ty of tsutils.unionTypeParts(checker.getApparentType(type))) {
    const then = ty.getProperty('then');
    if (then === undefined) {
      continue;
    }

    const thenType = checker.getTypeOfSymbolAtLocation(then, node);
    if (
      hasMatchingSignature(
        thenType,
        signature =>
          signature.parameters.length >= 2 &&
          isFunctionParam(checker, signature.parameters[0], node) &&
          isFunctionParam(checker, signature.parameters[1], node),
      )
    ) {
      return true;
    }
  }
  return false;
}

function hasMatchingSignature(
  type: ts.Type,
  matcher: (signature: ts.Signature) => boolean,
): boolean {
  for (const t of tsutils.unionTypeParts(type)) {
    if (t.getCallSignatures().some(matcher)) {
      return true;
    }
  }

  return false;
}

function isFunctionParam(
  checker: ts.TypeChecker,
  param: ts.Symbol,
  node: ts.Node,
): boolean {
  const type: ts.Type | undefined = checker.getApparentType(
    checker.getTypeOfSymbolAtLocation(param, node),
  );
  for (const t of tsutils.unionTypeParts(type)) {
    if (t.getCallSignatures().length !== 0) {
      return true;
    }
  }
  return false;
}

function isPromiseCatchCallWithHandler(expression: ts.CallExpression): boolean {
  return (
    tsutils.isPropertyAccessExpression(expression.expression) &&
    expression.expression.name.text === 'catch' &&
    expression.arguments.length >= 1
  );
}

function isPromiseThenCallWithRejectionHandler(
  expression: ts.CallExpression,
): boolean {
  return (
    tsutils.isPropertyAccessExpression(expression.expression) &&
    expression.expression.name.text === 'then' &&
    expression.arguments.length >= 2
  );
}