{ "source": "doc/api/debugger.markdown", "stability": 3, "stabilityText": "Stable", "miscs": [ { "textRaw": "Debugger", "name": "Debugger", "stability": 3, "stabilityText": "Stable", "type": "misc", "desc": "
V8 comes with an extensive debugger which is accessible out-of-process via a\nsimple TCP protocol.\nNode has a built-in client for this debugger. To use this, start Node with the\ndebug
argument; a prompt will appear:\n\n
% node debug myscript.js\n< debugger listening on port 5858\nconnecting... ok\nbreak in /home/indutny/Code/git/indutny/myscript.js:1\n 1 x = 5;\n 2 setTimeout(function () {\n 3 debugger;\ndebug>
\nNode's debugger client doesn't support the full range of commands, but\nsimple step and inspection is possible. By putting the statement debugger;
\ninto the source code of your script, you will enable a breakpoint.\n\n
For example, suppose myscript.js
looked like this:\n\n
// myscript.js\nx = 5;\nsetTimeout(function () {\n debugger;\n console.log("world");\n}, 1000);\nconsole.log("hello");
\nThen once the debugger is run, it will break on line 4.\n\n
\n% node debug myscript.js\n< debugger listening on port 5858\nconnecting... ok\nbreak in /home/indutny/Code/git/indutny/myscript.js:1\n 1 x = 5;\n 2 setTimeout(function () {\n 3 debugger;\ndebug> cont\n< hello\nbreak in /home/indutny/Code/git/indutny/myscript.js:3\n 1 x = 5;\n 2 setTimeout(function () {\n 3 debugger;\n 4 console.log("world");\n 5 }, 1000);\ndebug> next\nbreak in /home/indutny/Code/git/indutny/myscript.js:4\n 2 setTimeout(function () {\n 3 debugger;\n 4 console.log("world");\n 5 }, 1000);\n 6 console.log("hello");\ndebug> repl\nPress Ctrl + C to leave debug repl\n> x\n5\n> 2+2\n4\ndebug> next\n< world\nbreak in /home/indutny/Code/git/indutny/myscript.js:5\n 3 debugger;\n 4 console.log("world");\n 5 }, 1000);\n 6 console.log("hello");\n 7\ndebug> quit\n%
\nThe repl
command allows you to evaluate code remotely. The next
command\nsteps over to the next line. There are a few other commands available and more\nto come. Type help
to see others.\n\n
You can watch expression and variable values while debugging your code.\nOn every breakpoint each expression from the watchers list will be evaluated\nin the current context and displayed just before the breakpoint's source code\nlisting.\n\n
\nTo start watching an expression, type watch("my_expression")
. watchers
\nprints the active watchers. To remove a watcher, type\nunwatch("my_expression")
.\n\n
The V8 debugger can be enabled and accessed either by starting Node with\nthe --debug
command-line flag or by signaling an existing Node process\nwith SIGUSR1
.\n