7.6 KiB
Boa Debug Object
The $boa object contains useful utilities that can be used to debug JavaScript in JavaScript.
It's injected into the context as global variable with the --debug-object command-line flag,
the object is separated into modules.
Module $boa.gc
This module contains functions that are related the garbage collector. It currently has the .collect() method.
$boa.gc.collect()
This force triggers the GC to scan the heap and collect garbage.
Module $boa.function
In this module are untility functions related to execution and debugging function.
Function $boa.function.bytecode(func)
This function returns the compiled bytecode of a function as a string,
>> function add(x, y) {
return x + y
}
>> $boa.function.bytecode(add)
"
------------------------Compiled Output: 'add'------------------------
Location Count Handler Opcode Operands
000000 0000 none PutLexicalValue 0000: 'x'
000005 0001 none PutLexicalValue 0001: 'y'
000010 0002 none RestParameterPop
000011 0003 none GetName 0000: 'x'
000016 0004 none GetName 0001: 'y'
000021 0005 none Add
000022 0006 none SetReturnValue
000023 0007 none Return
000024 0008 none Return
Literals:
<empty>
Bindings:
0000: x
0001: y
Functions:
<empty>
Handlers:
<empty>
"
>>
Function $boa.function.trace(func, this, ...args)
It only traces the specified function. If the specified function calls other functions, their instructions aren't traced.
>> const add = (a, b) => a + b
>> $boa.function.trace(add, undefined, 1, 2)
5μs DefInitArg 0000: 'a' 2
4μs DefInitArg 0001: 'b' <empty>
0μs RestParameterPop <empty>
3μs GetName 0000: 'a' 1
1μs GetName 0001: 'b' 2
2μs Add 3
1μs Return 3
3
>>
The this value can be changed as well as the arguments that are passed to the function.
Function $boa.function.traceable(func, mode)
Marks a single function as traceable on all future executions of the function. Both useful to mark several functions as traceable and to trace functions that suspend their execution (async functions, generators, async generators).
Input
function* g() {
yield 1;
yield 2;
yield 3;
}
$boa.function.traceable(g, true);
var iter = g();
iter.next();
iter.next();
iter.next();
Output
1μs RestParameterPop <empty>
1μs PushUndefined undefined
2μs Yield undefined
4μs GetName 0000: 'a' 1
0μs Yield 1
1μs GeneratorNext undefined
1μs Pop <empty>
15μs GetName 0001: 'b' 2
1μs Yield 2
1μs GeneratorNext undefined
1μs Pop <empty>
4μs GetName 0002: 'c' 3
1μs Yield 3
Function $boa.function.flowgraph(func, options)
It can be used to get the instruction flowgraph, like the command-line flag. This works on the function level, allows getting the flow graph without quiting the boa shell and adding the specified flags.
Besides the function it also takes an argument that, can be a string or an object. If it is a string it represets the flowgraph format, otherwire if it's an object:
// These are the defaults, if not specified.
{
format: 'mermaid'
direction: 'LeftRight' // or 'LR' shorthand.
}
Example:
$boa.function.flowgraph(func, 'graphviz')
$boa.function.flowgraph(func, { format: 'mermaid', direction: 'TopBottom' })
Module $boa.object
Contains utility functions for getting internal information about an object.
Function $boa.object.id(object)
This function returns memory address of the given object, as a string.
Example:
let o = { x: 10, y: 20 }
$boa.object.id(o) // '0x7F5B3251B718'
// Geting the address of the $boa object in memory
$boa.object.id($boa) // '0x7F5B3251B5D8'
Module $boa.optimizer
This modules contains getters and setters for enabling and disabling optimizations.
Getter & Setter $boa.optimizer.constantFolding
This is and accessor property on the module, its getter returns true if enabled or false otherwise.
Its setter can be used to enable/disable the constant folding optimization.
$boa.optimizer.constantFolding = true
$boa.optimizer.constantFolding // true
Getter & Setter $boa.optimizer.statistics
This is an accessor property on the module, its getter returns true if enabled or false otherwise.
Its setter can be used to enable/disable optimization statistics, which are printed to stdout.
>> $boa.optimizer.constantFolding = true
>> $boa.optimizer.statistics = true
>> 1 + 1
Optimizer {
constant folding: 1 run(s), 2 pass(es) (1 mutating, 1 checking)
}
2
>>
Module $boa.realm
This module contains realm utilities to test cross-realm behaviour.
$boa.realm.create
Creates a new realm with a new set of builtins and returns its global object.
let global = $boa.realm.create();
Object != global.Object; // true
Module $boa.shape
This module contains helpful functions for getting information about a shape of an object.
Function $boa.shape.id(object)
Returns the pointer of the object's shape in memory as a string encoded in hexadecimal format.
$boa.shape.id(Number) // '0x7FC35A073868'
$boa.shape.id({}) // '0x7FC35A046258'
Function $boa.shape.type(object)
Returns the object's shape type.
$boa.shape.type({x: 3}) // 'shared'
$boa.shape.type(Number) // 'unique'
Function $boa.shape.same(o1, o2)
Returns true if both objects have the same shape.
// The values of the properties are not important!
let o1 = { x: 10 }
let o2 = {}
$boa.shape.same(o1, o2) // false
o2.x = 20
$boa.shape.same(o1, o2) // true
o2.y = 200
$boa.shape.same(o1, o2) // false
Module $boa.limits
This module contains utilities for changing runtime limits.
Getter & Setter $boa.limits.loop
This is an accessor property on the module, its getter returns the loop iteration limit before an error is thrown. Its setter can be used to set the loop iteration limit.
$boa.limits.loop = 10;
while (true) {} // RuntimeLimit: Maximum loop iteration limit 10 exceeded
Getter & Setter $boa.limits.recursion
This is an accessor property on the module, its getter returns the recursion limit before an error is thrown. Its setter can be used to set the recursion limit.
$boa.limits.recursion = 100;
function x() {
return x();
}
x(); // RuntimeLimit: Maximum recursion limit 100 exceeded