Object Spread
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Object Spread
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Summary : in this tutorial, you will learn how to use the JavaScript object spread ( ... ) in ES2018 to clone an object or merge objects into one.
In ES6, you use the spread operator ( ... ) to unpack elements of an array . The spread operator can be very useful to clone an array. For example:
In this example, the spread operator ( ... ) unpacks elements of the colors array and places them in a new array rgb . The spread operator ( ... ) can be used to merge two or more arrays into one as shown in the following example:
ES2018 expands the spread operator ( ... ) to make it works with own enumerable properties of an object. Suppose that you have a circle object with one property radius :
The following example uses the spread operator ( ... ) to create an coloredCircle object that has all properties of the circle object and an additional property color :
You can use the spread operator to clone the own enumerable properties of an object:
Note that the cloning is always shallow. For example:
Like arrays, you can use the spread operator ( ... ) to merge two objects:
The spread operator ( ... ) defines new properties in the target object while the Object.assign() method assigns them. It has two side effects.
The Object.assign() invokes setters on the target object while the spread operator doesn’t. The following illustrates how to clone an object using both Object.assign() and spread operator ( ... ). However, only the Object.assign() method triggers the setters:
If a target object has a read-only property, you cannot use Object.assign() method to assign a new value to that property. However, the spread operator ( ... ) can define a new property. Suppose you have an object called blueSquare whose the color property is readonly:
The following uses the spread operator ( ... ) to merge the style and blueSquare objects:
However, if you use the Object.assign() method, you will get an error:
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function todoApp ( state = initialState , action ) {
return Object . assign ( { } , state , {
function todoApp ( state = initialState , action ) {
return { ... state , visibilityFilter : action . filter }
return getAddedIds ( state . cart ) . map ( id =>
Object . assign ( { } , getProduct ( state . products , id ) , {
quantity : getQuantity ( state . cart , id )
return getAddedIds ( state . cart ) . map ( id => ( {
... getProduct ( state . products , id ) ,
quantity : getQuantity ( state . cart , id )
"presets" : [ "@babel/preset-env" ] ,
"plugins" : [ "@babel/plugin-proposal-object-rest-spread" ]
Copyright © 2015–2021 Dan Abramov and the Redux documentation authors.
Since one of the core tenets of Redux is to never mutate state, you'll often find yourself using Object.assign() to create copies of objects with new or updated values. For example, in the todoApp below Object.assign() is used to return a new state object with an updated visibilityFilter property:
While effective, using Object.assign() can quickly make simple reducers difficult to read given its rather verbose syntax.
An alternative approach is to use the object spread syntax recently added to the JavaScript specification. It lets you use the spread ( ... ) operator to copy enumerable properties from one object to another in a more succinct way. The object spread operator is conceptually similar to the ES6 array spread operator . We can simplify the todoApp example above by using the object spread syntax:
The advantage of using the object spread syntax becomes more apparent when you're composing complex objects. Below getAddedIds maps an array of id values to an array of objects with values returned from getProduct and getQuantity .
Object spread lets us simplify the above map call to:
While the object spread syntax is a Stage 4 proposal for ECMAScript and accepted for the 2018 specification release, you will still need to use a transpiler such as Babel to use it in production systems. You should use the env preset, install @babel/plugin-proposal-object-rest-spread and add it individually to the plugins array in your .babelrc .
Like the Array Spread Operator, the Object Spread Operator creates a shallow clone of the original object. In other words, for multidimensional source objects, elements in the copied object at a depth greater than one are mere references to the source object (with the exception of primitives , which are copied). Thus, you cannot reliably use the Object Spread Operator ( ... ) for deep cloning objects.
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