Function to extend existing object by mixing properties and methods from other objects

omixin.js

function omixin(target, ...sources) {
    
    //This function works only when the target is an object
    if(typeof target === 'object') {
        
        sources.forEach(source => {
            
            //When merging from another object
            if(typeof source === 'object') {
                
                //Add all the instance properties and methods from the source
                Object.getOwnPropertyNames(source).forEach(prop => {
                    
                    let descriptor = Object.getOwnPropertyDescriptor(source, prop);
                
                    //To allow overriding the target properties and methods from the source remove the if wrapping
                    if(target[prop] === undefined) {
                        Object.defineProperty(target, prop, descriptor);
                    }
                    
                });
                
                //Add all the properties with symbol as keys which are enumerable
                Object.getOwnPropertySymbols(source).forEach(symbol => {
                    
                    let descriptor = Object.getOwnPropertyDescriptor(source, symbol);
                    
                    if(descriptor.enumerable) {
                        
                        //To allow overriding the target properties and methods from the source remove the if wrapping
                        if(target[symbol] === undefined) {
                            Object.defineProperty(target, symbol, descriptor);
                        }
                        
                    }
                });
            }
        });
    }
}

This is a slightly modified version of the addMixin function by @calebporzio in his post .

By default, it doesn't allow overriding of methods and property values. However, one can remove the if wrappings as mentioned in the comments in the function to enable overriding.