Constructor Functions

Call JavaScript constructors from C code

Constructor Functions

QuickJS provides functions to invoke JavaScript constructors from C code.

JS_CallConstructor

Call a JavaScript constructor to create a new object.

JSValue JS_CallConstructor(JSContext *ctx, JSValueConst func_obj,
                          int argc, JSValueConst *argv);

Parameters

• ctx - The JavaScript context
• func_obj - The constructor function to call
• argc - Number of arguments to pass to the constructor
• argv - Array of argument values

Returns

Returns the newly created object, or JS_EXCEPTION on error.

Example

// Call: new Date(2024, 0, 1)
JSValue date_ctor = JS_GetPropertyStr(ctx, global, "Date");

JSValue args[3];
args[0] = JS_NewInt32(ctx, 2024);
args[1] = JS_NewInt32(ctx, 0);
args[2] = JS_NewInt32(ctx, 1);

JSValue date_obj = JS_CallConstructor(ctx, date_ctor, 3, args);
if (JS_IsException(date_obj)) {
    js_std_dump_error(ctx);
}

JS_FreeValue(ctx, date_obj);
JS_FreeValue(ctx, args[0]);
JS_FreeValue(ctx, args[1]);
JS_FreeValue(ctx, args[2]);
JS_FreeValue(ctx, date_ctor);

JS_CallConstructor2

Call a JavaScript constructor with explicit new.target support.

JSValue JS_CallConstructor2(JSContext *ctx, JSValueConst func_obj,
                           JSValueConst new_target,
                           int argc, JSValueConst *argv);

Parameters

• ctx - The JavaScript context
• func_obj - The constructor function to call
• new_target - The new.target value (used for class inheritance)
• argc - Number of arguments
• argv - Array of argument values

Returns

Returns the newly created object, or JS_EXCEPTION on error.

Example from point.c

static JSValue js_point_ctor(JSContext *ctx,
                             JSValue new_target,
                             int argc, JSValue *argv)
{
    JSPointData *s;
    JSValue obj = JS_UNDEFINED;
    JSValue proto;

    s = js_mallocz(ctx, sizeof(*s));
    if (!s)
        return JS_EXCEPTION;
    if (JS_ToInt32(ctx, &s->x, argv[0]))
        goto fail;
    if (JS_ToInt32(ctx, &s->y, argv[1]))
        goto fail;
    
    /* using new_target to get the prototype is necessary when the
       class is extended. */
    proto = JS_GetPropertyStr(ctx, new_target, "prototype");
    if (JS_IsException(proto))
        goto fail;
    obj = JS_NewObjectProtoClass(ctx, proto, js_point_class_id);
    JS_FreeValue(ctx, proto);
    if (JS_IsException(obj))
        goto fail;
    JS_SetOpaque(obj, s);
    return obj;
 fail:
    js_free(ctx, s);
    JS_FreeValue(ctx, obj);
    return JS_EXCEPTION;
}

Creating Constructor Functions

JS_SetConstructorBit

Mark a function as a constructor.

bool JS_SetConstructorBit(JSContext *ctx, JSValueConst func_obj, bool val);

JS_SetConstructor

Set up the bidirectional link between a constructor and its prototype.

void JS_SetConstructor(JSContext *ctx, JSValueConst func_obj,
                      JSValueConst proto);

Example

From examples/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);
/* set proto.constructor and ctor.prototype */
JS_SetConstructor(ctx, point_class, point_proto);
JS_SetClassProto(ctx, js_point_class_id, point_proto);

Checking Constructor Functions

JS_IsConstructor

Check if a value is a constructor function.

bool JS_IsConstructor(JSContext* ctx, JSValueConst val);

Example

JSValue func = JS_GetPropertyStr(ctx, global, "MyClass");
if (JS_IsConstructor(ctx, func)) {
    printf("MyClass is a constructor\n");
}
JS_FreeValue(ctx, func);

Notes

• new.target is essential for proper class inheritance in ES6 classes
• When creating custom constructors, always use new_target to get the prototype
• Constructor functions should return the newly created object
• The JS_CALL_FLAG_CONSTRUCTOR flag is set when a function is called as a constructor