Is there a similar concept to "Symbol.iterator" for the object spread syntax { ...obj } in JavaScript?

Question

Is there a similar concept to "Symbol.iterator" for the object spread syntax { ...obj } in JavaScript?

One symbol that is widely recognized is Symbol.iterator. When this symbol is defined as a generator function property on an object, it enables the object to be utilized in a [...object] syntax. For example, you can do something like this:

const newArray = [ ...foo, ...object, ...bar ];

However, I have not been able to find a similar functionality that allows for this:

const newObject = { ...foo, ...object, ...etc };

or using Object.assign with non-own properties. An example of this is seen in instances of ES6 classes with accessors like get prop() / set prop() - they are defined on the .prototype property of the class constructor:

const C = class {
  #num = 42;
  #str = 'aaa';
  
  get num() { return this.#num; }
  set num(val) { /* validate new value */ this.#num = val; return true; }

  get str() { return this.#num; }
  set str(val) { /* validate new value */ this.#num = val; return true; }
}

const obj = new C();

Now, obj has validation for obj.num = ... and obj.str = .... However, it cannot be used in {...foo, ...obj}, or in Object.assign(foo, obj), due to the accessors being on the prototype. While Proxies can be spread and property access trapped for validation, my goal is to ensure that accessing obj.prop closely matches the performance of accessing plain object properties. Consider the following:

const p = new Proxy({ num: 0 }, {});

suite.add('instance with class-accessors', () => obj.num++);
suite.add('proxy', () => p.num++);
suite.run();

instance with class-accessors x ***235***,971,841 ops/sec ±0.20% (86 runs sampled)
proxy x ***1***,014,238 ops/sec ±1.91% (84 runs sampled)

This shows a significant difference in speed! So, is there a way to allow { ...obj } / Object.assign for instances with prototype-based getters?

I've experimented by defining *[Symbol.iterator]() {...} in the class, yielding pairs in the style of Object.entries, but it did not work. I checked MDN's "Well known symbols" and didn't find any useful solutions. I expected to find something like Symbol.entries that could control the spread/assign behavior and make working with instances containing getters more transparent.

javascript class prototype getter spread-syntax

Answer 1

Answer №1

Unfortunately, no, there is not an easy way to achieve this with object literal spread syntax. It simply grabs all enumerable own properties and that's the extent of its functionality.

One common workaround involves creating a getter method within a class to serialize the object in a way that can be spread as desired. This approach is often used for JSON conversion as well:

class MyClass {
  #num = 42;
  #str = 'example';
  get num() { return this.#num; }
  get str() { return this.#str; }
  
  toJSON() {
    return {num: this.#num, str: this.#str};
  }
}

const originalObject = new MyClass();

const modifiedObject = { key: 'value', ...originalObject.toJSON(), anotherKey: 'anotherValue' };
console.log(modifiedObject);
console.log(JSON.stringify(originalObject));

Answer 2

Unfortunately, no, there is not an easy way to achieve this with object literal spread syntax. It simply grabs all enumerable own properties and that's the extent of its functionality.

One common workaround involves creating a getter method within a class to serialize the object in a way that can be spread as desired. This approach is often used for JSON conversion as well:

class MyClass {
  #num = 42;
  #str = 'example';
  get num() { return this.#num; }
  get str() { return this.#str; }
  
  toJSON() {
    return {num: this.#num, str: this.#str};
  }
}

const originalObject = new MyClass();

const modifiedObject = { key: 'value', ...originalObject.toJSON(), anotherKey: 'anotherValue' };
console.log(modifiedObject);
console.log(JSON.stringify(originalObject));

Answer 3

Answer №2

Another approach would involve defining the getter and setter from the class prototype to the object using Object.defineProperty(). This way, the spread syntax can be utilized. Here is an example implementation:

function makeProtoGSetterEnumerable(target=this,classF=this?.constructor??target.constructor, bindGet=true){

    let descriptors= Object.getOwnPropertyDescriptors(classF.prototype)

    const log=false;
    if(log)console.log("before (Only proto holds the g|setter)\ntarget Desc:",Object.getOwnPropertyDescriptors(target),"\nproto Desc",descriptors);

    //You don't want to modify certain props on class e.g. .constructor
    let not=['constructor'];
    const desc_entries=Object.entries(descriptors).filter( ([prop_key])=> false===not.includes(prop_key)  )

    for( const [prop_key,desc] of desc_entries ){
        //setting unconfigurable prop raises an Error
        if(desc.configurable===false || desc.enumerable===true ){continue }
        const new_desc={
            ...desc,
            get: desc?.get,
            set: desc?.set,
            enumerable:true,
            configurable:true,
        }
        if(bindGet){ 
          //Only for better preview in web console, no need to click (...) , but otherwise uneccessary & slightly more inefficent 
          new_desc.get=  new_desc.get.bind(target);
        }
        Object.defineProperty( target , prop_key ,new_desc )            
    }
    if(!log){return}
    const desc_after_proto=Object.getOwnPropertyDescriptors(target.constructor.prototype);
    const desc_after=Object.getOwnPropertyDescriptors(target);
    console.log("after (Both proto hold the g|setter)\ntarget Desc:",desc_after,"\nproto Desc",desc_after_proto)
  }

This method can be applied within the constructor or on a specific object. Consider the following class:

const C = class {
  #num = 42;
  #str = 'aaa';
    
  // constructor(){ makeProtoGSetterEnumerable(this) }
  get num() { return this.#num; }
  set num(val) { /* validate new value */ this.#num = val;}

  get str() { return this.#str; }
  set str(val) { /* validate new value */ this.#str = val;}
}

const obj = new C();
console.log( obj , {...obj} )

// {#num:42,#str:'aaa'} {}  //Spread is empty

makeProtoGSetterEnumerable(obj);
console.log(obj, ({...obj}))
// {num: 42, str: 'aaa', #num: 42, #str: 'aaa'} {num: 42, str: 'aaa'} 
//Now, the spread operation works correctly and only includes non-private getters

//In Chrome, viewing and editing of .#props in the web console is allowed, but they function as intended in modules

Answer 4

Another approach would involve defining the getter and setter from the class prototype to the object using Object.defineProperty(). This way, the spread syntax can be utilized. Here is an example implementation:

function makeProtoGSetterEnumerable(target=this,classF=this?.constructor??target.constructor, bindGet=true){

    let descriptors= Object.getOwnPropertyDescriptors(classF.prototype)

    const log=false;
    if(log)console.log("before (Only proto holds the g|setter)\ntarget Desc:",Object.getOwnPropertyDescriptors(target),"\nproto Desc",descriptors);

    //You don't want to modify certain props on class e.g. .constructor
    let not=['constructor'];
    const desc_entries=Object.entries(descriptors).filter( ([prop_key])=> false===not.includes(prop_key)  )

    for( const [prop_key,desc] of desc_entries ){
        //setting unconfigurable prop raises an Error
        if(desc.configurable===false || desc.enumerable===true ){continue }
        const new_desc={
            ...desc,
            get: desc?.get,
            set: desc?.set,
            enumerable:true,
            configurable:true,
        }
        if(bindGet){ 
          //Only for better preview in web console, no need to click (...) , but otherwise uneccessary & slightly more inefficent 
          new_desc.get=  new_desc.get.bind(target);
        }
        Object.defineProperty( target , prop_key ,new_desc )            
    }
    if(!log){return}
    const desc_after_proto=Object.getOwnPropertyDescriptors(target.constructor.prototype);
    const desc_after=Object.getOwnPropertyDescriptors(target);
    console.log("after (Both proto hold the g|setter)\ntarget Desc:",desc_after,"\nproto Desc",desc_after_proto)
  }

This method can be applied within the constructor or on a specific object. Consider the following class:

const C = class {
  #num = 42;
  #str = 'aaa';
    
  // constructor(){ makeProtoGSetterEnumerable(this) }
  get num() { return this.#num; }
  set num(val) { /* validate new value */ this.#num = val;}

  get str() { return this.#str; }
  set str(val) { /* validate new value */ this.#str = val;}
}

const obj = new C();
console.log( obj , {...obj} )

// {#num:42,#str:'aaa'} {}  //Spread is empty

makeProtoGSetterEnumerable(obj);
console.log(obj, ({...obj}))
// {num: 42, str: 'aaa', #num: 42, #str: 'aaa'} {num: 42, str: 'aaa'} 
//Now, the spread operation works correctly and only includes non-private getters

//In Chrome, viewing and editing of .#props in the web console is allowed, but they function as intended in modules

Is there a similar concept to "Symbol.iterator" for the object spread syntax { ...obj } in JavaScript?

Answer №1

Answer №2

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