Debugger.Script

A Debugger.Script instance may refer to a sequence of bytecode in the debuggee or to a block of WebAssembly code. For the former, it is the Debugger API’s presentation of a JSAPI JSScript object. The two cases are distinguished by their format property being "js" or "wasm".

Debugger.Script for JSScripts

For Debugger.Script instances referring to a JSScript, they are distinguished by their format property being "js".

Each of the following is represented by a single JSScript object:

  • The body of a function—that is, all the code in the function that is not contained within some nested function.

  • The code passed to a single call to eval, excluding the bodies of any functions that code defines.

  • The contents of a <script> element.

  • A DOM event handler, whether embedded in HTML or attached to the element by other JavaScript code.

  • Code appearing in a javascript: URL.

The Debugger interface constructs Debugger.Script objects as scripts of debuggee code are uncovered by the debugger: via the onNewScript handler method; via Debugger.Frame’s script properties; via the functionScript method of Debugger.Object instances; and so on. For a given Debugger instance, SpiderMonkey constructs exactly one Debugger.Script instance for each underlying script object; debugger code can add its own properties to a script object and expect to find them later, use == to decide whether two expressions refer to the same script, and so on.

(If more than one Debugger instance is debugging the same code, each Debugger gets a separate Debugger.Script instance for a given script. This allows the code using each Debugger instance to place whatever properties it likes on its Debugger.Script instances, without worrying about interfering with other debuggers.)

A Debugger.Script instance is a strong reference to a JSScript object; it protects the script it refers to from being garbage collected.

Note that SpiderMonkey may use the same Debugger.Script instances for equivalent functions or evaluated code—that is, scripts representing the same source code, at the same position in the same source file, evaluated in the same lexical environment.

Debugger.Script for WebAssembly

For Debugger.Script instances referring to a block of WebAssembly code, they are distinguished by their format property being "wasm".

Currently only entire modules evaluated via new WebAssembly.Module are represented.

Debugger.Script objects for WebAssembly are uncovered via onNewScript when a new WebAssembly module is instantiated and via the findScripts method on Debugger instances. SpiderMonkey constructs exactly one Debugger.Script for each underlying WebAssembly module per Debugger instance.

A Debugger.Script instance is a strong reference to the underlying WebAssembly module; it protects the module it refers to from being garbage collected.

Please note at the time of this writing, support for WebAssembly is very preliminary. Many properties and methods below throw.

Convention

For descriptions of properties and methods below, if the behavior of the property or method differs between the instance referring to a JSScript or to a block of WebAssembly code, the text will be split into two sections, headed by “if the instance refers to a JSScript” and “if the instance refers to WebAssembly code”, respectively. If the behavior does not differ, no such emphasized headings will appear.

Accessor Properties of the Debugger.Script Prototype Object

A Debugger.Script instance inherits the following accessor properties from its prototype:

isGeneratorFunction

True if this instance refers to a JSScript for a function defined with a function* expression or statement. False otherwise.

isAsyncFunction

True if this instance refers to a JSScript for an async function, defined with an async function expression or statement. False otherwise.

isFunction

True if this instance refers to a JSScript for a function. False otherwise.

isModule

True if this instance refers to a JSScript that was parsed and loaded as an ECMAScript module. False otherwise.

displayName

If the instance refers to a JSScript, this is the script’s display name, if it has one. If the script has no display name — for example, if it is a top-level eval script — this is undefined.

If the script’s function has a given name, its display name is the same as its function’s given name.

If the script’s function has no name, SpiderMonkey attempts to infer an appropriate name for it given its context. For example:

function f() {}          // display name: f (the given name)
var g = function () {};  // display name: g
o.p = function () {};    // display name: o.p
var q = {
  r: function () {}      // display name: q.r
};

Note that the display name may not be a proper JavaScript identifier, or even a proper expression: we attempt to find helpful names even when the function is not immediately assigned as the value of some variable or property. Thus, we use a/b to refer to the b defined within a, and a< to refer to a function that occurs somewhere within an expression that is assigned to a. For example:

function h() {
  var i = function() {};    // display name: h/i
  f(function () {});        // display name: h/<
}
var s = f(function () {});  // display name: s<

If the instance refers to WebAssembly code, throw a TypeError.

parameterNames

If the instance refers to a JSScript, the names of its parameters, as an array of strings. If the script is not a function script this is undefined.

If the function uses destructuring parameters, the corresponding array elements are undefined. For example, if the referent is a function script declared in this way:

function f(a, [b, c], {d, e:f}) { ... }

then this Debugger.Script instance’s parameterNames property would have the value:

["a", undefined, undefined]

If the instance refers to WebAssembly code, throw a TypeError.

url

If the instance refers to a JSScript, the filename or URL from which this script’s code was loaded. For scripts created by eval or the Function constructor, this may be a synthesized filename, starting with a valid URL and followed by information tracking how the code was introduced into the system; the entire string is not a valid URL. For Function.prototype’s script, this is null. If this Debugger.Script’s source property is non-null, then this is equal to source.url.

If the instance refers to WebAssembly code, throw a TypeError.

startLine

If the instance refers to a JSScript, the number of the line at which this script’s code starts, within the file or document named by url.

startColumn

If the instance refers to a JSScript, the zero-indexed number of the column at which this script’s code starts, within the file or document named by url. For functions, this is the start of the function’s arguments:

function f() { ... }
//        ^ start (column 10)
let g = x => x*x;
//      ^ start (column 8)
let h = (x) => x*x;
//      ^ start (column 8)

For default class constructors, it is the start of the class keyword:

let MyClass = class { };
//            ^ start (column 14)

For scripts from other sources, such as eval or the Function constructor, it is typically 0:

let f = new Function("  console.log('hello world');");
//                    ^ start (column 0, from the string's perspective)

lineCount

If the instance refers to a JSScript, the number of lines this script’s code occupies, within the file or document named by url.

source

If the instance refers to a JSScript, the Debugger.Source instance representing the source code from which this script was produced. This is null if the source code was not retained.

If the instance refers to WebAssembly code, the Debugger.Source instance representing the serialized text format of the WebAssembly code.

sourceStart

If the instance refers to a JSScript, the character within the Debugger.Source instance given by source at which this script’s code starts; zero-based. If this is a function’s script, this is the index of the start of the function token in the source code.

If the instance refers to WebAssembly code, throw a TypeError.

sourceLength

If the instance refers to a JSScript, the length, in characters, of this script’s code within the Debugger.Source instance given by source.

If the instance refers to WebAssembly code, throw a TypeError.

mainOffset

If the instance refers to a JSScript, the offset of the main entry point of the script, excluding any prologue.

If the instance refers to WebAssembly code, throw a TypeError.

global

If the instance refers to a JSScript, a Debugger.Object instance referring to the global object in whose scope this script runs. The result refers to the global directly, not via a wrapper or a WindowProxy (“outer window”, in Firefox).

If the instance refers to WebAssembly code, throw a TypeError.

format

If the instance refers to a JSScript, "js".

If the instance refers to WebAssembly code, "wasm".

Function Properties of the Debugger.Script Prototype Object

The functions described below may only be called with a this value referring to a Debugger.Script instance; they may not be used as methods of other kinds of objects.

getChildScripts()

If the instance refers to a JSScript, return a new array whose elements are Debugger.Script objects for each function in this script. Only direct children are included; nested children can be reached by walking the tree.

If the instance refers to WebAssembly code, throw a TypeError.

getPossibleBreakpoints(query)

Query for the recommended breakpoint locations available in SpiderMonkey. Returns a result array of objects with the following properties:

  • offset: number - The offset the breakpoint.

  • lineNumber: number - The line number of the breakpoint.

  • columnNumber: number - The column number of the breakpoint.

  • isStepStart: boolean - True if SpiderMonkey recommends that the breakpoint be treated as a step location when users of debuggers step to the next item. This roughly translates to the start of each statement, though not entirely.

The query argument can be used to filter the set of breakpoints. The query object can contain the following properties:

  • minOffset: number - The inclusive lower bound of offset values to include.

  • maxOffset: number - The exclusive upper bound of offset values to include.

  • line: number - Limit to breakpoints on the given line.

  • minLine: number - The inclusive lower bound of lines to include.

  • minColumn: number - The inclusive lower bound of the line/minLine column to include.

  • maxLine: number - The exclusive upper bound of lines to include.

  • maxColumn: number - The exclusive upper bound of the line/maxLine column to include.

getPossibleBreakpointOffsets(query)

Query for the recommended breakpoint locations available in SpiderMonkey. Identical to getPossibleBreakpoints except this returns an array of offset values instead of offset metadata objects.

getOffsetMetadata(offset)

Get metadata about a given bytecode offset. Returns an object with the following properties:

  • lineNumber: number - The line number of the breakpoint.

  • columnNumber: number - The column number of the breakpoint.

  • isBreakpoint: boolean - True if this offset qualifies as a breakpoint, defined using the same semantics used for getPossibleBreakpoints().

  • isStepStart: boolean - True if SpiderMonkey recommends that the breakpoint be treated as a step location when users of debuggers step to the next item. This roughly translates to the start of each statement, though not entirely.

setBreakpoint(offset, handler)

If the instance refers to a JSScript, set a breakpoint at the bytecode instruction at offset in this script, reporting hits to the hit method of handler. If offset is not a valid offset in this script, throw an error. Also, even if offset is valid offset in this script, some instructions for engine-internal operation (e.g. SetAliasedVar in the generator function initialization) don’t allow setting breakpoints, and in that case, this also throws an error.

When execution reaches the given instruction, SpiderMonkey calls the hit method of handler, passing a Debugger.Frame instance representing the currently executing stack frame. The hit method’s return value should be a resumption value, determining how execution should continue.

Any number of breakpoints may be set at a single location; when control reaches that point, SpiderMonkey calls their handlers in an unspecified order.

Any number of breakpoints may use the same handler object.

Breakpoint handler method calls are cross-compartment, intra-thread calls: the call takes place in the same thread that hit the breakpoint, and in the compartment containing the handler function (typically the debugger’s compartment).

The new breakpoint belongs to the Debugger instance to which this script belongs. Removing a global from the Debugger instance’s set of debuggees clears all the breakpoints belonging to that Debugger instance in that global’s scripts.

getBreakpoints([offset])

If the instance refers to a JSScript, return an array containing the handler objects for all the breakpoints set at offset in this script. If offset is omitted, return the handlers of all breakpoints set anywhere in this script. If offset is present, but not a valid offset in this script, throw an error.

If the instance refers to WebAssembly code, throw a TypeError.

clearBreakpoint(handler, [offset])

If the instance refers to a JSScript, remove all breakpoints set in this Debugger instance that use handler as their handler. If offset is given, remove only those breakpoints set at offset that use handler; if offset is not a valid offset in this script, throw an error.

Note that, if breakpoints using other handler objects are set at the same location(s) as handler, they remain in place.

clearAllBreakpoints([offset])

If the instance refers to a JSScript, remove all breakpoints set in this script. If offset is present, remove all breakpoints set at that offset in this script; if offset is not a valid bytecode offset in this script, throw an error.

getEffectfulOffsets()

If the instance refers to a JSScript, return an array containing the offsets of all bytecodes in the script which can have direct side effects that are visible outside the currently executing frame. This includes, for example, operations that set properties or elements on objects, or that may set names in environments created outside the frame.

This doesn’t include some instructions for engine-internal operation (e.g. SetAliasedVar in the generator function initialization). Those instructions can be effectful in term of engine-internal, but that’s not user-visible and can be treated as not-effectful here.

getOffsetsCoverage():

If the instance refers to a JSScript, return null or an array which contains information about the coverage of all opcodes. The elements of the array are objects, each of which describes a single opcode, and contains the following properties:

  • lineNumber: the line number of the current opcode.

  • columnNumber: the column number of the current opcode.

  • offset: the bytecode instruction offset of the current opcode.

  • count: the number of times the current opcode got executed.

If this script has no coverage, or if it is not instrumented, then this function will return null. To ensure that the debuggee is instrumented, the flag Debugger.collectCoverageInfo should be set to true.

If the instance refers to WebAssembly code, throw a TypeError.

isInCatchScope([offset])

If the instance refers to a JSScript, this is true if this offset falls within the scope of a try block, and false otherwise.

If the instance refers to WebAssembly code, throw a TypeError.

Deprecated Debugger.Script Prototype Functions

The following functions have all been deprecated in favor of getOffsetMetadata, getPossibleBreakpoints, and getPossibleBreakpointOffsets. These functions all have an under-defined concept of what offsets are and are not included in their results.

getAllOffsets()

If the instance refers to a JSScript, return an array L describing the relationship between bytecode instruction offsets and source code positions in this script. L is sparse, and indexed by source line number. If a source line number line has no code, then L has no line property. If there is code for line, then L[line] is an array of offsets of byte code instructions that are entry points to that line.

For example, suppose we have a script for the following source code:

a=[]
for (i=1; i < 10; i++)
    // It's hip to be square.
    a[i] = i*i;

Calling getAllOffsets() on that code might yield an array like this:

[[0], [5, 20], , [10]]

This array indicates that:

  • the first line’s code starts at offset 0 in the script;

  • the for statement head has two entry points at offsets 5 and 20 (for the initialization, which is performed only once, and the loop test, which is performed at the start of each iteration);

  • the third line has no code;

  • and the fourth line begins at offset 10.

If the instance refers to WebAssembly code, throw a TypeError.

getAllColumnOffsets()

If the instance refers to a JSScript, return an array describing the relationship between bytecode instruction offsets and source code positions in this script. Unlike getAllOffsets(), which returns all offsets that are entry points for each line, getAllColumnOffsets() returns all offsets that are entry points for each (line, column) pair.

The elements of the array are objects, each of which describes a single entry point, and contains the following properties:

  • lineNumber: the line number for which offset is an entry point

  • columnNumber: the column number for which offset is an entry point

  • offset: the bytecode instruction offset of the entry point

For example, suppose we have a script for the following source code:

a=[]
for (i=1; i < 10; i++)
    // It's hip to be square.
    a[i] = i*i;

Calling getAllColumnOffsets() on that code might yield an array like this:

[{ lineNumber: 0, columnNumber: 0, offset: 0 },
  { lineNumber: 1, columnNumber: 5, offset: 5 },
  { lineNumber: 1, columnNumber: 10, offset: 20 },
  { lineNumber: 3, columnNumber: 4, offset: 10 }]

If the instance refers to WebAssembly code, throw a TypeError.

getLineOffsets(line)

If the instance refers to a JSScript, return an array of bytecode instruction offsets representing the entry points to source line line. If the script contains no executable code at that line, the array returned is empty.

getOffsetLocation(offset)

If the instance refers to a JSScript, return an object describing the source code location responsible for the bytecode at offset in this script. The object has the following properties:

  • lineNumber: the line number for which offset is an entry point

  • columnNumber: the column number for which offset is an entry point

  • isEntryPoint: true if the offset is a column entry point, as would be reported by getAllColumnOffsets(); otherwise false.