Custom Classes

Define custom JavaScript classes backed by C data structures

Custom JavaScript Classes

QuickJS allows you to attach C opaque data to JavaScript objects using the class system.

Overview

Creating a custom class involves:

1. Allocating a class ID
2. Defining the class with a JSClassDef
3. Creating instances with JS_NewObjectClass()
4. Attaching C data with JS_SetOpaque()
5. Retrieving C data with JS_GetOpaque()

Class ID Management

JS_NewClassID

Allocate a new class ID.

JSClassID JS_NewClassID(JSRuntime *rt, JSClassID *pclass_id);

Parameters

• rt - The JavaScript runtime
• pclass_id - Pointer to a class ID variable (will be set to the new ID)

Returns

Returns the new class ID.

Example

static JSClassID js_point_class_id;

// During initialization
JS_NewClassID(rt, &js_point_class_id);

Class Definition

JSClassDef Structure

typedef struct JSClassDef {
    const char *class_name;
    JSClassFinalizer *finalizer;
    JSClassGCMark *gc_mark;
    JSClassCall *call;
    JSClassExoticMethods *exotic;
} JSClassDef;

JSClassFinalizer

Called when an object is garbage collected.

typedef void JSClassFinalizer(JSRuntime *rt, JSValueConst val);

Important: The finalizer should only release C resources. It is invalid to execute JavaScript code from it.

JSClassGCMark

Called during garbage collection to mark referenced objects.

typedef void JSClassGCMark(JSRuntime *rt, JSValueConst val,
                          JS_MarkFunc *mark_func);

JS_NewClass

Register a new class.

int JS_NewClass(JSRuntime *rt, JSClassID class_id, const JSClassDef *class_def);

Example from point.c

typedef struct {
    int x;
    int y;
} JSPointData;

static void js_point_finalizer(JSRuntime *rt, JSValue val)
{
    JSPointData *s = JS_GetOpaque(val, js_point_class_id);
    /* Note: 's' can be NULL in case JS_SetOpaque() was not called */
    js_free_rt(rt, s);
}

static JSClassDef js_point_class = {
    "Point",
    .finalizer = js_point_finalizer,
};

static int js_point_init(JSContext *ctx, JSModuleDef *m)
{
    JSRuntime *rt = JS_GetRuntime(ctx);
    
    /* create the Point class */
    JS_NewClassID(rt, &js_point_class_id);
    JS_NewClass(rt, js_point_class_id, &js_point_class);
    
    // ... create prototype and constructor
    
    return 0;
}

Setting Up Class Prototype

JS_SetClassProto

Set the prototype for a class.

void JS_SetClassProto(JSContext *ctx, JSClassID class_id, JSValue obj);

Example from point.c

JSValue point_proto = JS_NewObject(ctx);
JS_SetPropertyFunctionList(ctx, point_proto, js_point_proto_funcs,
                          countof(js_point_proto_funcs));

JSValue point_class = JS_NewCFunction2(ctx, js_point_ctor, "Point", 2,
                                       JS_CFUNC_constructor, 0);
JS_SetConstructor(ctx, point_class, point_proto);
JS_SetClassProto(ctx, js_point_class_id, point_proto);

JS_GetClassProto

Get the prototype for a class.

JSValue JS_GetClassProto(JSContext *ctx, JSClassID class_id);

Creating Class Instances

Use JS_NewObjectClass() or JS_NewObjectProtoClass() as shown in the Object Creation page.

Opaque Data Management

JS_SetOpaque

Attach C data to an object.

int JS_SetOpaque(JSValueConst obj, void *opaque);

Parameters

• obj - The object (must be a custom class instance)
• opaque - Pointer to C data

Returns

Returns 0 on success, -1 on error.

Note: Only supported for custom classes.

JS_GetOpaque

Retrieve C data from an object.

void *JS_GetOpaque(JSValueConst obj, JSClassID class_id);

Parameters

• obj - The object
• class_id - Expected class ID

Returns

Returns the opaque pointer, or NULL if the object is not of the specified class or if no opaque data was set.

JS_GetOpaque2

Retrieve C data with automatic exception throwing.

void *JS_GetOpaque2(JSContext *ctx, JSValueConst obj, JSClassID class_id);

Returns the opaque pointer, or NULL and throws a TypeError if the object is not of the specified class.

Example from point.c

static JSValue js_point_get_xy(JSContext *ctx, JSValue this_val, int magic)
{
    JSPointData *s = JS_GetOpaque2(ctx, this_val, js_point_class_id);
    if (!s)
        return JS_EXCEPTION;
    if (magic == 0)
        return JS_NewInt32(ctx, s->x);
    else
        return JS_NewInt32(ctx, s->y);
}

static JSValue js_point_set_xy(JSContext *ctx, JSValue this_val, 
                               JSValue val, int magic)
{
    JSPointData *s = JS_GetOpaque2(ctx, this_val, js_point_class_id);
    int v;
    if (!s)
        return JS_EXCEPTION;
    if (JS_ToInt32(ctx, &v, val))
        return JS_EXCEPTION;
    if (magic == 0)
        s->x = v;
    else
        s->y = v;
    return JS_UNDEFINED;
}

Complete Example: Point Class

From examples/point.c:

typedef struct {
    int x;
    int y;
} JSPointData;

static JSClassID js_point_class_id;

static void js_point_finalizer(JSRuntime *rt, JSValue val)
{
    JSPointData *s = JS_GetOpaque(val, js_point_class_id);
    js_free_rt(rt, s);
}

static JSValue js_point_norm(JSContext *ctx, JSValue this_val,
                             int argc, JSValue *argv)
{
    JSPointData *s = JS_GetOpaque2(ctx, this_val, js_point_class_id);
    if (!s)
        return JS_EXCEPTION;
    return JS_NewFloat64(ctx, sqrt((double)s->x * s->x + (double)s->y * s->y));
}

static JSClassDef js_point_class = {
    "Point",
    .finalizer = js_point_finalizer,
};

static const JSCFunctionListEntry js_point_proto_funcs[] = {
    JS_CGETSET_MAGIC_DEF("x", js_point_get_xy, js_point_set_xy, 0),
    JS_CGETSET_MAGIC_DEF("y", js_point_get_xy, js_point_set_xy, 1),
    JS_CFUNC_DEF("norm", 0, js_point_norm),
};

static int js_point_init(JSContext *ctx, JSModuleDef *m)
{
    JSRuntime *rt = JS_GetRuntime(ctx);
    
    JS_NewClassID(rt, &js_point_class_id);
    JS_NewClass(rt, js_point_class_id, &js_point_class);
    
    JSValue point_proto = JS_NewObject(ctx);
    JS_SetPropertyFunctionList(ctx, point_proto, js_point_proto_funcs,
                              countof(js_point_proto_funcs));
    
    JSValue point_class = JS_NewCFunction2(ctx, js_point_ctor, "Point", 2,
                                           JS_CFUNC_constructor, 0);
    JS_SetConstructor(ctx, point_class, point_proto);
    JS_SetClassProto(ctx, js_point_class_id, point_proto);
    
    JS_SetModuleExport(ctx, m, "Point", point_class);
    return 0;
}

Class Utilities

JS_GetClassID

Get the class ID of an object.

JSClassID JS_GetClassID(JSValueConst v);

Returns the class ID if v is an object, otherwise returns JS_INVALID_CLASS_ID.

JS_IsRegisteredClass

Check if a class ID is registered.

bool JS_IsRegisteredClass(JSRuntime *rt, JSClassID class_id);

JS_GetClassName

Get the name of a registered class.

JSAtom JS_GetClassName(JSRuntime *rt, JSClassID class_id);

Returns the class name atom or JS_ATOM_NULL if not registered. Must be freed with JS_FreeAtom().

Notes

• Class IDs are allocated per-runtime
• JSClassDef structures are allocated per-runtime
• Prototypes are set per-context with JS_SetClassProto()
• Always use JS_GetOpaque2() in methods that need type checking
• Finalizers run during garbage collection - don’t execute JavaScript code in them
• Use gc_mark to tell the GC about objects referenced by your C data