0.23.0 • Public • Published

objc Build Status npm node

NodeJS Objective-C bridge (experimental)


$ npm install --save objc


const objc = require('objc');

const {
} = objc;

let now = NSDate.date()
let localizedDate = NSDateFormatter.localizedStringFromDate_dateStyle_timeStyle_(now, 2, 2);

console.log(localizedDate); // -> "19. Apr 2017, 22:41:13"




Import an Objective-C framework. Foundation is always imported by default

objc.ns(object, [hint = '@'])

Convert a JavaScript object to its objc equivalent. Returns null if the object doesn't have an objc counterpart.
Takes an optional second parameter to specify whether strings should be converted to NSString objects (default), SEL or Class

objc.js(object, [returnInputIfUnableToConvert = false])

Convert an objc object to its JavaScript equivalent.
Takes an optional second parameter to specify whether it should return null or the input if the object doesn't have a JS counterpart

Calling methods

When calling Objective-C methods, all you need to do is replace the colons in the selector with underscores.

For example, this Objective-C code:

#import <AppKit/AppKit.h>

NSPasteboard *pasteboard = [NSPasteboard generalPasteboard];
[pasteboard declareTypes:@[NSPasteboardTypeString] owner:nil];

[pasteboard setString:@"44 > 45" forType:NSPasteboardTypeString];

is equivalent to the following JavaScript code:

const objc = require('objc');

const {NSPasteboard, NSPasteboardTypeString} = objc;

const pasteboard = NSPasteboard.generalPasteboard();
pasteboard.declareTypes_owner_([NSPasteboardTypeString], null);

pasteboard.setString_forType_("44 > 45", NSPasteboardTypeString);


You can create a block with the objc.Block helper class:

const block = new objc.Block(() => {
  console.log('In the block!');
}, 'v', []);

When creating a block, you need to explicitly declare the type encoding of the block's return value and all its parameters.

If a block takes an Objective-C object as its parameter, you'll need to manually wrap that object in an objc.Proxy (via the objc.wrap helper function).

Example: Sort an array by word length, longest to shortest

const {NSArray, Block, wrap} = objc;
const {id, NSInteger} = objc.types;
const array = NSArray.arrayWithArray_(['I', 'Am', 'The', 'Doctor']);

const block = new Block((arg1, arg2) => {
  arg1 = wrap(arg1);
  arg2 = wrap(arg2);
  return arg1.length() > arg2.length() ? -1 : 1;
}, NSInteger, [id, id]);  // Match the NSComparator signature

const sorted = array.sortedArrayUsingComparator_(block);
// => ['Doctor', 'The', 'Am', 'I']


You can load NSString* constants just like you'd access a class:

const {NSFontAttributeName} = objc;
console.log(NSFontAttributeName);   // => 'NSFont'

NSString* constants are returned as native JavaScript String objects.


Use the objc.defineStruct function to define a struct by its name and layout. The returned type can be used to create instances of the struct, and when specifying type encodings in the objc module. It is also compatible with the ffi-napi, ref-napi, ref-struct-di modules.

You can use the StructType.new function to create an instance of the struct. Optionally, you can pass

The objc module already provides a definition for NSRange, accessible via objc.types.

Example 1 Using structs with objc methods

const {NSRange} = objc.types;

const string = objc.ns('Hello World');
const substring = string.substringWithRange_(NSRange.new(0, 5));
// -> 'Hello'
Example 2** Using structs with the ffi module
const ffi = require('ffi-napi');
const CGFloat = objc.types.double;

const CGPoint = objc.defineStruct('CGPoint', {
  x: CGFloat,
  y: CGFloat

const CGSize = objc.defineStruct('CGSize', {
  width: CGFloat,
  height: CGFloat

const CGRect = objc.defineStruct('CGRect', {
  origin: CGPoint,
  size: CGSize

const libFoundation = new ffi.Library(null, {
  NSStringFromRect: ['pointer', [CGRect]]
const rect = CGRect.new(
  CGPoint.new(5, 10),
  CGSize.new(100, 250)
const string = objc.wrap(libFoundation.NSStringFromRect(rect))
// -> '{{5, 10}, {100, 250}}'

Inout parameters

If a method expects an inout parameter (like NSError**), you can use the objc.allocRef function to get a pointer to a nil objc object that can be passed to a method expecting an id*:

const {NSAppleScript} = objc;

const script = NSAppleScript.alloc().initWithSource_('foobar');

const error = objc.allocRef();
script.executeAndReturnError_(error); // `executeAndReturnError:` takes a `NSDictionary**`

console.log(error); // `error` is now a `NSDictionary*`


[objc.InstanceProxy {
    NSAppleScriptErrorBriefMessage = "The variable foobar is not defined.";
    NSAppleScriptErrorMessage = "The variable foobar is not defined.";
    NSAppleScriptErrorNumber = "-2753";
    NSAppleScriptErrorRange = "NSRange: {0, 6}";

If you need more advanced inout functionality (using primitive types, etc), simply use the ref module.

Method swizzling

Method swizzling allows you to replace a method's implementation:

const {NSProcessInfo} = objc;
objc.swizzle(NSProcessInfo, 'processorCount', (self, _cmd) => {
  return 12;

NSProcessInfo.processInfo().processorCount(); // => 12

The method's original implementation is still available, with the xxx__ prefix:

const {NSDate, wrap} = objc;
objc.swizzle(NSDate, 'dateByAddingTimeInterval:', (self, _cmd, timeInterval) => {
  self = wrap(self);
  return self.xxx__dateByAddingTimeInterval_(timeInterval * 2);

const now = NSDate.date();
const a = now.dateByAddingTimeInterval_(2);
const b = now.xxx__dateByAddingTimeInterval_(4);

a.isEqualToDate_(b); // => true


  • Just like with blocks, you have to wrap all non-primitive parameters
  • If you want to swizzle a class method, pass 'class' as the swizzle function's last parameter
  • objc.swizzle returns a function that - if called - restores the original implementation of the swizzled method

Custom Classes

Use the objc.createClass function to register custom classes with the Objective-C runtime:

const objc = require('objc');

const LKGreeter = objc.createClass('LKGreeter', 'NSObject', {
  'greet:': (self, cmd, name) => {
    name = objc.wrap(name);
    return objc.ns(`Hello, ${name}!`);

  _encodings: {
    'greet:': ['@', ['@', ':', '@']]

LKGreeter.new().greet('Lukas'); // => 'Hello, Lukas!'

Note: You might have to specify individual offsets in the type encoding, see this example.


In the future, I'd like to add support for:

  • improved support for inout parameters (id*)
  • c-style arrays, unions as method parameter/return type
  • runtime introspection (accessing an object's properties, ivars, methods, etc)
  • improved class creation api


MIT © Lukas Kollmer

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