Merge Object Properties with the Spread Operator

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Last week I wrote 6 Great Uses of the Spread Operator, a post detailing how awesome the spread operator (...) is for working with arrays and other iterable objects.  As always my readers chimed in with a few other great uses and which you should check out in the comments.  And of course as soon as I publish the post I find another great use of the spread operator while I tinker with Babel and React:  merging multiple objects' properties into one object!

The JavaScript

Wrap any objects you'd like merged into one with braces ({}):

const person = { name: 'David Walsh', gender: 'Male' };
const tools = { computer: 'Mac', editor: 'Atom' };

const summary = {...person, ...tools};
/*
Object {
  "computer": "Mac",
  "editor": "Atom",
  "gender": "Male",
  "name": "David Walsh",
}
*/

A new object is created containing all properties and values from the objects provided with the rest operator.  Also note that you can provide any number of objects to merge:

const person = { name: 'David Walsh', gender: 'Male' };
const tools = { computer: 'Mac', editor: 'Atom' };
const attributes = { handsomeness: 'Extreme', hair: 'Brown', eyes: 'Blue' };

const summary = {...person, ...tools, ...attributes};
/*
Object {
  "computer": "Mac",
  "editor": "Atom",
  "eyes": "Blue",
  "gender": "Male",
  "hair": "Brown",
  "handsomeness": "Extreme",
  "name": "David Walsh",
}
*/

In the case of a key collision, the right-most (last) object's value wins out:

const person1 = { name: 'David Walsh', age: 33 };
const person2 = { name: 'David Walsh Jr.', role: 'kid' };

const merged = {...person1, ...person2}
/*
Object {
  "name": "David Walsh Jr.",
  "age": 33,
  "role": "kid",
}
*/

I love how easy merging objects is using the spread operator.  You can use Object.assign to accomplish the same feat but the spread operator makes things a bit shorter if you don't mind a slightly less descriptive syntax!

Note:  This syntax is not yet support by all browsers but you can use Babel with the transform-object-rest-spread plugin to enable object merging with the spread operator.

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Discussion

  1. Fred

    Excellent tip! Thank you.

  2. MaxArt

    To be precise, only Chrome 58 at the moment supports the object spread operator, behind a flag too. But I’ve been using it with great satisfaction with Babel behind.

    It’s very expressive, and also helps adopting an immutable pattern in your code, as opposed to Object.assign it creates a new object every time.

  3. Rob David

    Sweet! Nice tip :)

  4. gocho

    Object’s key now has order, so

    ({...{b: 1, a: 2}, ...{d: 3, c: 4}})

    will result in

    {b: 1, a: 2, d: 3, c: 4}
  5. var __assign = (this && this.__assign) || Object.assign || function(t) {
        for (var s, i = 1, n = arguments.length; i < n; i++) {
            s = arguments[i];
            for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p))
                t[p] = s[p];
        }
        return t;
    };
    var person = { name: 'David Walsh', gender: 'Male' };
    var tools = { computer: 'Mac', editor: 'Atom' };
    var summary = __assign({}, person, tools);
    
  6. This code above is an TypeScript translation to ES5

  7. Stefano

    And what about common properties? What does the sprea operator manage the merge?

  8. Fred

    Thanks Bigismall, I didn’t know about Object.assign. Here is the MDN doc with the polyfill and the browser support:

    https://developer.mozilla.org/pl/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

  9. A word of warning, spread operator object merging should be used only on one level deep objects, meaning objects won’t be deeply merged.

    • Joao Pedro Furtado Lourenco

      Yeah. Is there any nice way to have deep merge in es6 or typescript.

  10. Con

    What about deep merge? Is there a builtin method via ECMAScript?

  11. Phillip Hall

    Be aware that merging objects with undefined properties may result in unexpected objects as in the following example;

    const defaults = {prop1: 'prop1', prop2: 'prop2', prop3: 'prop3'};
    const params = {prop1: 'overridden', prop2: undefined}
    const result = {...defaults, ...params};
    // Might expect result = {prop1: 'overridden', prop2: 'prop2', prop3: 'prop3'}
    // But result = {prop1: 'overridden', prop2: undefined, prop3: 'prop3'}
    

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