Ruff 1.6.0 Documentation


Errors#

Errors generated by Ruff fall into two categories: JavaScript errors and system errors. All errors inherit from or are instances of JavaScript's Error class and are guaranteed to provide at least the attributes available on that class.

When an operation is not permitted due to language-syntax or language-runtime-level reasons, a JavaScript error is generated and thrown as an exception. If an operation is not allowed due to system-level restrictions, a system error is generated. Client code is then given the opportunity to intercept this error based on how the API propagates it.

The style of API called determines how generated errors are handed back, or propagated, to client code, which in turn informs how the client may intercept the error. Exceptions can be intercepted using the try / catch construct; other propagation strategies are covered below.

JavaScript Errors#

JavaScript errors typically denote that an API is being used incorrectly, or that there is a problem with the program as written.

Class: Error#

A general error object. Unlike other error objects, Error instances do not denote any specific circumstance of why the error occurred. Errors capture a "stack trace" detailing the point in the program at which they were instantiated, and may provide a description of the error.

Note: Ruff will generate this class of error to encapsulate system errors as well as plain JavaScript errors.

new Error(message)#

Instantiates a new Error object and sets its .message property to the provided message. Its .stack will represent the point in the program at which new Error was called. Stack traces are subject to Duktape's stack trace API. Stack traces only extend to the beginning of synchronous code execution, or a number of frames given by Error.stackTraceLimit, whichever is smaller.

error.message#

A string of the value passed to Error() upon instantiation. The message will also appear in the first line of the stack trace of the error. Changing this property may not change the first line of the stack trace.

error.stack#

A property that, when accessed, returns a string representing the point in the program at which this error was instantiated. An example stacktrace follows:

AssertionError: 6 == 9
  fail /Users/zhulizhong/ruff/ruff_modules/assert/src/index.js:93
  equal /Users/zhulizhong/ruff/ruff_modules/assert/src/index.js:119
  anon /Users/zhulizhong/ruff/ruff_modules/timers/test/timers_test.js:32 preventsyield
  anon  native strict preventsyield

The first line is formatted as <error class name>: <error message>, and it is followed by a series of stack frames (each line beginning with method name). Each frame describes a call site in the program that lead to the error being generated. Duktape attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name.

Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify, which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in stacktraces:

var cheetahify = require('./native-binding.node');

function makeFaster() {
  // cheetahify *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster(); // will throw:
// /home/gbusey/file.js:6
//     throw new Error('oh no!');
//           ^
// Error: oh no!
//      makeFaster speedy (/home/gbusey/file.js:6:11)
//      fail /Users/zhulizhong/ruff/ruff_modules/assert/src/index.js:93
//      equal /Users/zhulizhong/ruff/ruff_modules/assert/src/index.js:119
//      anon /Users/zhulizhong/ruff/ruff_modules/timers/test/timers_test.js:32 preventsyield
//      anon  native strict preventsyield

The location information will be one of:

  • native, if the frame represents a call internal to Duktape (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Ruff.
  • /absolute/path/to/file.js:line, if the frame represents a call in a user program, or its dependencies.

It is important to note that the string representing the stacktrace is only generated on access: it is lazily generated.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

System-level errors are generated as augmented Error instances, which are detailed below.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the program at which Error.captureStackTrace was called.

var myObject = {};

Error.captureStackTrace(myObject);

myObject.stack  // similar to `new Error().stack`

The first line of the trace, instead of being prefixed with ErrorType: message, will be the result of targetObject.toString().

constructorOpt optionally accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

This is useful for hiding implementation details of error generation from the end user. A common way of using this parameter is to pass the current Error constructor to it:


function MyError() {
  Error.captureStackTrace(this, MyError);
}

// without passing MyError to captureStackTrace, the MyError
// frame would should up in the .stack property. by passing
// the constructor, we omit that frame and all frames above it.
new MyError().stack

Error.stackTraceLimit#

Property that determines the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The initial value is 10. It may be set to any valid JavaScript number, which will affect any stack trace captured after the value has been changed. If set to a non-number value, stack traces will not capture any frames and will report undefined on access.

Class: RangeError#

A subclass of Error that indicates that a provided argument was not within the set or range of acceptable values for a function; whether that be a numeric range, or outside the set of options for a given function parameter. An example:

require('net').connect(-1);  // throws RangeError, port should be > 0 && < 65536

Ruff will generate and throw RangeError instances immediately -- they are a form of argument validation.

Class: TypeError#

A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.

require('url').parse(function() { }); // throws TypeError, since it expected a string

Ruff will generate and throw TypeError instances immediately -- they are a form of argument validation.

Class: ReferenceError#

A subclass of Error that indicates that an attempt is being made to access a variable that is not defined. Most commonly it indicates a typo, or an otherwise broken program. While client code may generate and propagate these errors, in practice only Duktape will do so.

doesNotExist; // throws ReferenceError, doesNotExist is not a variable in this program.

ReferenceError instances will have an .arguments member that is an array containing one element -- a string representing the variable that was not defined.

try {
  doesNotExist;
} catch(err) {
  err.arguments[0] === 'doesNotExist';
}

Unless the userland program is dynamically generating and running code, ReferenceErrors should always be considered a bug in the program, or its dependencies.

Class: SyntaxError#

A subclass of Error that indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require("vm").runInThisContext("binary ! isNotOk");
} catch(err) {
  // err will be a SyntaxError
}

SyntaxErrors are unrecoverable from the context that created them – they may only be caught by other contexts.

Exceptions vs. Errors#

A JavaScript "exception" is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values inherit from Error, all exceptions thrown by Ruff or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. These exceptions will always bring down the process. These are usually failed assert() checks or abort() calls in the C++ layer.