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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef js_Exception_h
#define js_Exception_h
#include "mozilla/Attributes.h"
#include "mozilla/Maybe.h"
#include "jstypes.h"
#include "js/RootingAPI.h" // JS::{Handle,Rooted}
#include "js/TypeDecls.h"
#include "js/Value.h" // JS::Value, JS::Handle<JS::Value>
class JSErrorReport;
namespace JS {
enum class ExceptionStackBehavior : bool {
// Do not capture any stack.
// Capture the current JS stack when setting the exception. It may be
// retrieved by JS::GetPendingExceptionStack.
} // namespace JS
extern JS_PUBLIC_API bool JS_IsExceptionPending(JSContext* cx);
extern JS_PUBLIC_API bool JS_IsThrowingOutOfMemory(JSContext* cx);
extern JS_PUBLIC_API bool JS_GetPendingException(JSContext* cx,
JS::MutableHandleValue vp);
extern JS_PUBLIC_API void JS_SetPendingException(
JSContext* cx, JS::HandleValue v,
JS::ExceptionStackBehavior behavior = JS::ExceptionStackBehavior::Capture);
extern JS_PUBLIC_API void JS_ClearPendingException(JSContext* cx);
* If the given object is an exception object, the exception will have (or be
* able to lazily create) an error report struct, and this function will return
* the address of that struct. Otherwise, it returns nullptr. The lifetime
* of the error report struct that might be returned is the same as the
* lifetime of the exception object.
extern JS_PUBLIC_API JSErrorReport* JS_ErrorFromException(JSContext* cx,
JS::HandleObject obj);
namespace JS {
// When propagating an exception up the call stack, we store the underlying
// reason on the JSContext as one of the following enum values.
// TODO: Track uncatchable exceptions explicitly.
enum class ExceptionStatus {
// No exception status.
// Used by debugger when forcing an early return from a frame. This uses
// exception machinery, but at the right time is turned back into a normal
// non-error completion.
// Throwing a (catchable) exception. Certain well-known exceptions are
// explicitly tracked for convenience.
// Returns true if the status is a catchable exception. Formerly this was
// indicated by the `JSContext::throwing` flag.
static MOZ_ALWAYS_INLINE bool IsCatchableExceptionStatus(
ExceptionStatus status) {
return status >= ExceptionStatus::Throwing;
// This class encapsulates a (pending) exception and the corresponding optional
// SavedFrame stack object captured when the pending exception was set
// on the JSContext. This fuzzily correlates with a `throw` statement in JS,
// although arbitrary JSAPI consumers or VM code may also set pending exceptions
// via `JS_SetPendingException`.
// This is not the same stack as `e.stack` when `e` is an `Error` object.
// (That would be JS::ExceptionStackOrNull).
class MOZ_STACK_CLASS ExceptionStack {
Rooted<Value> exception_;
Rooted<JSObject*> stack_;
friend JS_PUBLIC_API bool GetPendingExceptionStack(
JSContext* cx, JS::ExceptionStack* exceptionStack);
void init(HandleValue exception, HandleObject stack) {
exception_ = exception;
stack_ = stack;
explicit ExceptionStack(JSContext* cx) : exception_(cx), stack_(cx) {}
ExceptionStack(JSContext* cx, HandleValue exception, HandleObject stack)
: exception_(cx, exception), stack_(cx, stack) {}
HandleValue exception() const { return exception_; }
// |stack| can be null.
HandleObject stack() const { return stack_; }
* Save and later restore the current exception state of a given JSContext.
* This is useful for implementing behavior in C++ that's like try/catch
* or try/finally in JS.
* Typical usage:
* bool ok = JS::Evaluate(cx, ...);
* AutoSaveExceptionState savedExc(cx);
* ... cleanup that might re-enter JS ...
* return ok;
class JS_PUBLIC_API AutoSaveExceptionState {
JSContext* context;
ExceptionStatus status;
RootedValue exceptionValue;
RootedObject exceptionStack;
* Take a snapshot of cx's current exception state. Then clear any current
* pending exception in cx.
explicit AutoSaveExceptionState(JSContext* cx);
* If neither drop() nor restore() was called, restore the exception
* state only if no exception is currently pending on cx.
* Discard any stored exception state.
* If this is called, the destructor is a no-op.
void drop();
* Replace cx's exception state with the stored exception state. Then
* discard the stored exception state. If this is called, the
* destructor is a no-op.
void restore();
// Get the current pending exception value and stack.
// This function asserts that there is a pending exception.
// If this function returns false, then retrieving the current pending exception
// failed and might have been overwritten by a new exception.
extern JS_PUBLIC_API bool GetPendingExceptionStack(
JSContext* cx, JS::ExceptionStack* exceptionStack);
// Similar to GetPendingExceptionStack, but also clears the current
// pending exception.
extern JS_PUBLIC_API bool StealPendingExceptionStack(
JSContext* cx, JS::ExceptionStack* exceptionStack);
// Set both the exception value and its associated stack on the context as
// the current pending exception.
extern JS_PUBLIC_API void SetPendingExceptionStack(
JSContext* cx, const JS::ExceptionStack& exceptionStack);
* If the given object is an exception object (or an unwrappable
* cross-compartment wrapper for one), return the stack for that exception, if
* any. Will return null if the given object is not an exception object
* (including if it's null or a security wrapper that can't be unwrapped) or if
* the exception has no stack.
extern JS_PUBLIC_API JSObject* ExceptionStackOrNull(JS::HandleObject obj);
* If the given object is an exception object, return the error cause for that
* exception, if any, or mozilla::Nothing.
extern JS_PUBLIC_API mozilla::Maybe<JS::Value> GetExceptionCause(JSObject* exc);
} // namespace JS
#endif // js_Exception_h