JerryScript debugger interface

JerryScript provides a remote debugger which allows debugging JavaScript programs. The debugger has two main components: a server which is part of the JerryScript binary and a separate client application. Currently two debugger clients are available in the /jerry-debugger subdirectory: an HTML and a Python application. These simple applications demonstrate the communication protocol between the client and server and can be reused by integrated development environments.

Setting up the debugger server

The following arguments must be passed to tools/

--jerry-debugger=on --jerry-libc=off

At the moment only a Websocket-based implementation is provided by JerryScript which transmits messages over TCP/IP networks. This implementation requires a socket API which is not yet supported by jerry-libc so the standard libc is used instead. In the future any reliable stream or datagram based protocol can be used for transmitting debugger messages.

Debugging JavaScript applications

The debugger client must be connected to the server before the JavaScript application runs. On-the-fly attachment is not supported because the debugging information (e.g. line index of each possible breakpoint location) is not preserved by JerryScript. The client is expected to be run on a system with much more resources and it should be capable of storing this information. JerryScript frees all debug information after it is transmitted to the client to save memory.

The following argument makes JerryScript wait for a client connection:


It is also recommended to increase the log level to see the Waiting for client connection message:

--log-level 2

The HTML client can connect to the IP address of the server with the connect command. The IP address can be localhost if the server and the client are running on the same machine.

After the connection is established the execution can be controlled by the debugger. The debugger always stops at the first possible breakpoint location. The effect is the same as using the stop command. This allows inserting breakpoints right before the meaningful part of the execution starts.

All available commands of the client can be queried by the help command.

Integrating debugger support into applications using JerryScript

The debugger can be enabled by passing the JERRY_INIT_DEBUGGER flag to the jerry_init () function which then initializes the debugger and blocks until a client connects.

When the debugger is enabled it is recommended to use jerry_parse_named_resource () instead of jerry_parse () because the resource name (usually a file name) is also passed to this function. This resource name is used by the client to identify the corresponding resource. In general it is always recommended to use jerry_parse_named_resource () when the resource name is available because it silently ignores the resource name if the debugger is disabled.