Is it possible to destructure onto an existing object? (Javascript ES6)

For example if I have two objects:

var foo = {
  x: "bar",
  y: "baz"
}

and

var oof = {}

and I wanted to transfer the x and y values from foo to oof. Is there a way to do that using the es6 destructuring syntax?

perhaps something like:

oof{x,y} = foo

Answers:

Answer

While ugly and a bit repetitive, you can do

({x: oof.x, y: oof.y} = foo);

which will read the two values of the foo object, and write them to their respective locations on the oof object.

Personally I'd still rather read

oof.x = foo.x;
oof.y = foo.y;

or

['x', 'y'].forEach(prop => oof[prop] = foo[prop]);

though.

Answer

No, destructuring does not support member expressions in shorthands but only plain propertynames at the current time. There have been talks about such on esdiscuss, but no proposals will make it into ES6.

You might be able to use Object.assign however - if you don't need all own properties, you still can do

var foo = …,
    oof = {};
{
    let {x, y} = foo;
    Object.assign(oof, {x, y})
}
Answer

IMO this is the easiest way to accomplish what you're looking for:

let { prop1, prop2, prop3 } = someObject;
let data = { prop1, prop2, prop3 };

  // data === { prop1: someObject.prop1, ... }

Basically, destructure into variables and then use the initializer shorthand to make a new object. No need for Object.assign

I think this is the most readable way, anyways. You can hereby select the exact props out of someObject that you want. If you have an existing object you just want to merge the props into, do something like this:

let { prop1, prop2, prop3 } = someObject;
let data = Object.assign(otherObject, { prop1, prop2, prop3 });
    // Makes a new copy, or...
Object.assign(otherObject, { prop1, prop2, prop3 });
    // Merges into otherObject

Another, arguably cleaner, way to write it is:

let { prop1, prop2, prop3 } = someObject;
let newObject = { prop1, prop2, prop3 };

// Merges your selected props into otherObject
Object.assign(otherObject, newObject);

I use this for POST requests a lot where I only need a few pieces of discrete data. But, I agree there should be a one liner for doing this.

Answer

Other than Object.assign there is the object spread syntax which is a Stage 2 proposal for ECMAScript.

var foo = {
  x: "bar",
  y: "baz"
}

var oof = { z: "z" }

oof =  {...oof, ...foo }

console.log(oof)

/* result 
{
  "x": "bar",
  "y": "baz",
  "z": "z"
}
*/

But to use this feature you need to use stage-2 or transform-object-rest-spread plugin for babel. Here is a demo on babel with stage-2

Answer

BabelJS plugin

If you are using BabelJS you can now activate my plugin babel-plugin-transform-object-from-destructuring (see npm package for installation and usage).

I had the same issue described in this thread and for me it was very exhausting when you create an object from a destructuring expression, especially when you have to rename, add or remove a property. With this plugin maintaining such scenarios gets much more easier for you.

Object example

let myObject = {
  test1: "stringTest1",
  test2: "stringTest2",
  test3: "stringTest3"
};
let { test1, test3 } = myObject,
  myTest = { test1, test3 };

can be written as:

let myTest = { test1, test3 } = myObject;

Array example

let myArray = ["stringTest1", "stringTest2", "stringTest3"];
let [ test1, , test3 ] = myArray,
  myTest = [ test1, test3 ];

can be written as:

let myTest = [ test1, , test3 ] = myArray;
Answer

It's totally possible. Just not in one statement.

var foo = {
    x: "bar",
    y: "baz"
};
var oof = {};
({x: oof.x, y: oof.y} = foo); // {x: "bar", y: "baz"}

(Do note the parenthesis around the statement.) But keep in mind legibility is more important than code-golfing :).

Source: http://exploringjs.com/es6/ch_destructuring.html#sec_assignment-targets

Answer

You can return the destructured object in an arrow function, and use Object.assign() to assign it to a variable.

const foo = {
  x: "bar",
  y: "baz"
}

const oof = Object.assign({}, () => ({ x, y } = foo));
Answer

You can just use restructuring for that like this:

const foo = {x:"a", y:"b"};
const {...oof} = foo; // {x:"a", y:"b"} 

Or merge both objects if oof has values:

const foo = {x:"a", y:"b"};
let oof = {z:"c"}
oof = Object.assign({}, oof, foo)
Answer

DRY

var a = {a1:1, a2: 2, a3: 3};
var b = {b1:1, b2: 2, b3: 3};

const newVar = (() => ({a1, a2, b1, b2})).bind({...a, ...b});
const val = newVar();
console.log({...val});
// print: Object { a1: 1, a2: 2, b1: 1, b2: 2 }

or

console.log({...(() => ({a1, a2, b1, b2})).bind({...a, ...b})()});
Answer

This works in chrome 53.0.2785.89

let foo = {
  x: "bar",
  y: "baz"
};

let oof = {x, y} = foo;

console.log(`oof: ${JSON.stringify(oof)});

//prints
oof: {
  "x": "bar",
  "y": "baz"
}
Answer

I came up with this method:

exports.pick = function pick(src, props, dest={}) {
    return Object.keys(props).reduce((d,p) => {
        if(typeof props[p] === 'string') {
            d[props[p]] = src[p];
        } else if(props[p]) {
            d[p] = src[p];
        }
        return d;
    },dest);
};

Which you can use like this:

let cbEvents = util.pick(this.props.events, {onFocus:1,onBlur:1,onCheck:'onChange'});
let wrapEvents = util.pick(this.props.events, {onMouseEnter:1,onMouseLeave:1});

i.e., you can pick which properties you want out and put them into a new object. Unlike _.pick you can also rename them at the same time.

If you want to copy the props onto an existing object, just set the dest arg.

Answer

This is kind of cheating, but you can do something like this...

const originalObject = {
  hello: 'nurse',
  meaningOfLife: 42,
  your: 'mom',
};

const partialObject = (({ hello, your }) => {
  return { hello, your };
})(originalObject);

console.log(partialObject); // ?????{ hello: 'nurse', your: 'mom' }?????

In practice, I think you'd rarely want to use that though. The following is MUCH more clear... but not nearly as fun.

const partialObject = {
  hello: originalObject.hello,
  your: originalObject.your,
};

Another completely different route, which includes mucking with the prototype (careful now...):

if (!Object.prototype.pluck) {
  Object.prototype.pluck = function(...props) {
    return props.reduce((destObj, prop) => {
      destObj[prop] = this[prop];

      return destObj;
    }, {});
  }
}

const originalObject = {
  hello: 'nurse',
  meaningOfLife: 42,
  your: 'mom',
};

const partialObject2 = originalObject.pluck('hello', 'your');

console.log(partialObject2); // { hello: 'nurse', your: 'mom' }
Answer

This is the most readable and shortest solution I could come up with:

let props = { 
  isValidDate: 'yes',
  badProp: 'no!',
};

let { isValidDate } = props;
let newProps = { isValidDate };

console.log(newProps);

It will output { isValidDate: 'yes' }

It would be nice to some day be able to say something like let newProps = ({ isValidDate } = props) but unfortunately it is not something ES6 supports.

Answer

It's not a beautiful way, nor I recommend it, but it's possible this way, just for knowledge.

const myObject = {
  name: 'foo',
  surname: 'bar',
  year: 2018
};

const newObject = ['name', 'surname'].reduce(
  (prev, curr) => (prev[curr] = myObject[curr], prev),
  {},
);

console.log(JSON.stringify(newObject)); // {"name":"foo","surname":"bar"}
Answer

You can destruct an object assigning directly to another object attribute.

Working example:

let user = {};
[user.name, user.username] = "Stack Overflow".split(' ');
document.write(`
1st attr: ${user.name} <br /> 
2nd attr: ${user.username}`);

You can work with destructing using variables with the same name of object attribute you want to catch, this way you don't need to do:

let user = { name: 'Mike' }
let { name: name } = user;

Use this way:

let user = { name: 'Mike' }
let { name } = user;

The same way you can set new values to object structures if they have the same attribute name.

Look this working example:

// The object to be destructed
let options = {
  title: "Menu",
  width: 100,
  height: 200
};

// Destructing
let {width: w, height: h, title} = options;

// Feedback
document.write(title + "<br />");  // Menu
document.write(w + "<br />");      // 100
document.write(h);                 // 200

Answer

You can use JSON class methods to achieve it as follows

const foo = {
   x: "bar",
   y: "baz"
};

const oof = JSON.parse(JSON.stringify(foo, ['x','y']));
// output -> {x: "bar", y: "baz"}

Pass properties that need to be added to the resulting object as second argument to stringify function in an array format.

MDN Doc for JSON.stringify

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