ECMAScript Proposal, specs, and reference implementation for Object.pick, Object.omit.

Authors: @Aleen && Hemanth HM

This proposal is currently stage 1 of the process.

Let us consider a few scenarios from the real world to understand what we are trying to solve in this proposal.

• On MouseEvent we are interested on 'ctrlKey', 'shiftKey', 'altKey', 'metaKey' events only.
• We have a configObject and we need ['dependencies', 'devDependencies', 'peerDependencies'] from it.
• We have an optionsBagand we would allow on ['shell', 'env', 'extendEnv', 'uid', 'gid'] on it.
• From a req.body we want to extract ['name', 'company', 'email', 'password']
• Checking if a component shouldReload by extracting compareKeys from props and compare it with prevProps.
• Say we have a depsObject and we need to ignore all @internal/packages from it.
• We have props from which we need to remove [‘_csrf’, ‘_method’]
• We need to construct a newModelData by removing action.deleted from ({ ...state.models, ...action.update })
• Filtering configuration objects when the filter list is given by a CLI argument.

Well, you see life is all about picking what we want and omiting what we don't!

Would life be easier if the language provided a convenient method to help us during similar scenarios?

Now, one might argue saying we can implement pick and omit as below:

const pick = (obj, keys) => Object.fromEntries(
    keys.map(k => obj.hasOwnProperty(k) && [k, obj[k]]).filter(x => x)
);

/*
We can also use a Destructuring assignment
const { authKey, ...toLog } = userInfo;
*/

const omit = (obj, keys) => Object.fromEntries(
    keys.map(k => !obj.hasOwnProperty(k) && [k, obj[k]]).filter(x => x)
);

The major challenges we see with the above implementations:

• It is not ergonomic!
• If we opt for the destructuring way it doesn't work at all for pick, or for omit with dynamic values.
• Destructuring cannot clone a new object while Object.pick can
• Destructuring cannot pick up properties from the prototype while Object.pick can
• Destructuring cannot pick properties dynamically, while Object.pick can
• Destructuring cannot omit some properties, and we can only clone and delete without this proposal

We can read more about such use-cases and challenges from es.discourse below:

• Object.{pick,omit}.
• Object restructuring syntax
• Object Array Pick
• js-pick-notation
• slect multiple object values

With that in mind would it not be easier if we had Object.pick and Object.omit static methods?!

Let us now discuss what the API of such a helpful method would be?

Object.pick(obj[, pickedKeys | predictedFunction(currentValue[, key[, object]])[, thisArg])
Object.omit(obj[, omittedKeys | predictedFunction(currentValue[, key[, object]])[, thisArg])

• obj: which object you want to pick or omit.
• pickedKeys (optional): keys of properties you want to pick from the object. The default value is an empty array.
• omittedKeys (optional): keys of properties you want to pick from the object. The default value is an empty array.
• predictedFunction (optional): the function to predicted whether the property should be picked or omitted. The default value is an identity: x => x.
  • currentValue: the current value processed in the object.
  • key: the key of the currentValue in the object.
  • object: the object pick was called upon.
• thisArg (optional): the object used as this inside the predicted function.

• Returns a new object, which has picked or omitted properties from the object.

// default
Object.pick({a : 1}); // => {}
Object.omit({a : 1}); // => {a: 1}
Object.pick({a : 0, b : 1}, v => v); // => {b: 1}
Object.pick({a : 0, b : 1}, v => !v); // => {a: 0}
Object.pick({}, function () { console.log(this) }); // => the object itself
Object.pick({}, function () { console.log(this) }, window); // => Window

Object.pick({a : 1, b : 2}, ['a']); // => {a: 1}
Object.omit({a : 1, b : 2}, ['b']); // => {a: 1}

Object.pick({a : 1, b : 2}, ['c']); // => {}
Object.omit({a : 1, b : 2}, ['c']); // => {a: 1, b: 2}

Object.pick([], [Symbol.iterator]); // => {Symbol(Symbol.iterator): f}
Object.pick([], ['length']); // => {length: 0}

Object.pick({a : 1, b : 2}, v => v === 1); // => {a: 1}
Object.pick({a : 1, b : 2}, v => v !== 2); // => {a: 1}
Object.pick({a : 1, b : 2}, (v, k) => k === 'a'); // => {a: 1}
Object.pick({a : 1, b : 2}, (v, k) => k !== 'b'); // => {a: 1}

1. A syntax sugar in the case of picking:

   To extend the motivation of this proposal, there may be some syntax notations as an alternative of picking properties from objects, like the proposal, proposal-slice-notation:

   There are two ideas around how to wrap picking keys:

   • square brackets:

     ({a : 1, b : 2, c : 3}).['a', 'b']; // => {a : 1, b : 2}
     
     const keys = ['a', 'b'];
     ({a : 1, b : 2, c : 3}).[keys[0], keys[1]]; // => {a : 1, b : 2}
     ({a : 1, b : 2, c : 3}).[...keys]; // => {a : 1, b : 2}

   • curly brackets

     ({a : 1, b : 2, c : 3}).{a, b} // => {a : 1, b : 2}
     
     const keys = ['a', 'b'];
     ({a : 1, b : 2, c : 3}).{[keys[0]], b}; // => {a : 1}
     ({a : 1, b : 2, c : 3}).{[...keys]}; // => {a : 1, b : 2}
     
     // Similar to destructuring
     ({a : 1, b : 2, c : 3}).{a, b : B}; // => {a : 1, B : 2}

     Currently, there is a disagreement on whether properties with default assignment values should be picked.

     // If considering the meaning of picking, the initial value has no meanings
     ({a : 1, b : 2, c : 3}).{a, d = 2}; // => {a : 1}
     
     // If considering as "restructuring", the shortcut has its reason to pick
     ({a : 1, b : 2, c : 3}).{a, d = 2}; // => {a : 1, d : 2}

   Nevertheless, it is just a simple vision, and feel free to discuss.

1. When it comes to the prototype chain of an object, should the method pick or omit it? (The answer may change)

   A: The implementation of _.pick and _.omit by Lodash has taken care about the chain. To keep the rule, we can pick off properties of prototype, but can't omit them:

   Object.pick({a : 1}, ['toString']); // => {toString: f}
   Object.omit({a : 1}, ['toString']).toString; // => ƒ toString() { [native code] }

   The same rule applies to __proto__ event if it has been deprecated, because the proposal should be pure enough to not specify a special logic to eliminate deprecated properties:

   Object.pick({}, ['__proto__']); // => {__proto__: {...}}
   Object.omit({}, ['__proto__']).__proto__; // => {...} 

   In some opinions, picking off or omitting properties from the prototype chain should make the method more extendable:

   const pickOwn = (obj, keys) => Object.pick(obj, keys.filter(key => obj.hasOwnProperty(key)));
   const omitOwn = (obj, keys) => Object.omit(obj, keys.filter(key => obj.hasOwnProperty(key)));

2. What is the type of the returned value?

   A: All these methods should return plain objects:

   Object.pick([]); // => {} 
   Object.omit([]); // => {}
   Object.pick(new Map()); // => {} 
   Object.omit(new Map()); // => {}

3. How to handle Symbol?

   A: Symbol should just be considered as properties within Symbol keys, and they should obey the rules mentioned above:

   Object.pick([], [Symbol.iterator]); // => {Symbol(Symbol.iterator): f}, pick off from the prototype
   Object.omit([], [Symbol.iterator]); // => {}, plain objects
   
   const symbol = Symbol('key');
   Object.omit({a : 1, [symbol]: 2}, [symbol]); // => {a : 1}
   
   Object.prototype[symbol] = 'test'; // override prototype
   Object.pick({}, [symbol]); // => {Symbol(key): "test"}, pick off from the prototype
   Object.omit({}, [symbol])[symbol]; // => "test", cannot omit properties from the prototype

4. If some properties of an object are not accessible like throwing an error, can Object.pick or Object.omit operate such an object?

   A: I suggest throwing the error wrapped by Object.pick or Object.omit, but it is NOT the final choice:

   Object.pick(Object.defineProperty({}, 'key', {
      get() { throw new Error() }
   }), ['key']);

   The error stack will look like this:

   Uncaught Error
       at Object.get (<anonymous>:2:20)
       at Object.pick (<anonymous>:2:10)
       at <anonymous>:1:8 
   

5. In comparison with proposal-shorthand-improvements, when should we use these two methods?

   A: Multiple properties. Assume that we need to ensure an object without any side-effected keys except key1 and key2:

   postData({[key1] : o[key1], [key2] : o[key2]});
   postData(Object.pick(o, [key1, key2])); 

6. Why can't be defined on the Object.prototype directly?

   A: As Object is especially fundamental, and both of them will result in conflicts of properties of any other objects. In shorthand, if defined, any objects inherited from Object with pick or omit defined in its prototype should break.

7. Why not define filtered methods corresponding to two actions: pickBy and omitBy like Lodash?

   A: It is unnecessary to double two methods, because it can be combined into the argument instead:

   Besides, the passing filtered method can be easily reversed with equal meaning, and it means that omitBy can be easily defined as pickBy's inverse.

   Object.pick({a : 1, b : 2}, v => v);
   
   // Equivalent to the following:
   Object.omitBy({a: 1, b : 2}, v => !v);

Notice: If you have any suggestions or ideas about this proposal? Appreciate your discussions via issues.