Js Object Spread
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Js 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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In JavaScript, spread syntax refers to the use of an ellipsis of three dots ( … ) to expand an iterable object into the list of arguments.
“When ...arr is used in the function call, it ‘expands’ an iterable object arr into the list of arguments.” — JavaScript.info
The spread operator was added to JavaScript in ES6 (ES2015), just like the rest parameters , which have the same syntax: three magic dots … .
“Spread operator to the rescue! It looks similar to rest parameters, also using ... , but does quite the opposite.” — JavaScript.info
Take trying to find the largest number in an array with Math.max() :
Trying to pass an array to a JavaScript function expecting separate arguments does not work. In this case it produces a NaN result. Enter … :
The spread syntax “spreads” the array into separate arguments.
T he … spread operator is useful for many different routine tasks in JavaScript, including the following:
In each case, the spread syntax expands an iterable object, usually an array, though it can be used on any interable, including a string.
H ere are a couple basic examples of using … in JavaScript, where I demonstrate copying an array, splitting a string into characters, and combining the properties of two JavaScript objects:
In the next section, I explore each of the above uses of the spread syntax.
U sing the … spread operator is a convenient way to copy an array or combine arrays, and it can even add new items:
As seen in the last example, the spread operator can quickly combine two arrays, an operation known as array concatenation :
“The Math object's set of functions are a perfect example of the spread operator as the only argument to a function.” — @davidwalshblog on his blog
O ne of the best ways to understand the use of spread operator in JavaScript is to look at the the built-in functions Math.min() and Math.max() , which both expect a list of arguments, not an array.
S ince the spread operator “spreads” an array into different arguments, any functions that accepts multiple any number of arguments can benefit from use of the spread operator.
As noted in the last example, the spread operator can add an item to an another array with a natural, easy-to-understand syntax:
A dding an item to an array in React state is easily accomplished using the spread operator. Take the following example adapted from my article on how to add to an array in React State :
T he spread syntax is useful for combining the properties and methods on objects into a new object:
T he spread operator can convert NodeList and arguments objects to arrays, such as when selecting HTML elements on the page:
W hen programming to support Internet Explorer and browsers on older mobile devices, the spread operator is not going to work.
Here is the current browser compatibility chart:
In that case, the function Function.prototype.apply() will have the same effect as the spread syntax:
Note that the first argument to .apply() is the target for this , which in this case does not matter, so I passed in null as the first argument.
Another option would be using the tool Babel to compile the JavaScript code along with the plugin babel-plugin-transform-spread .
O ne of the benefits of using the spread operator is that it will create a new reference to its primitive values, copying them.
That means that changes to the original array will not affect the copied array, which is what would happen if the array had been linked to the original with the assignment operator = :
As you can see, the spread operator is useful for creating new instances of arrays that do not behave unexpectedly due to old references.
On the other hand, when JavaScript objects including arrays are deeply nested, the spread operator only copies the first level with a new reference, but the deeper values are still linked together.
This type of problem is called creating a deep copy, as opposed to a shallow copy. Deep copies can be made using lodash or the R.clone() method from the Ramda functional programming library.
Thanks to Michał Bargiel for pointing out that the spread operator makes a shallow copy but not a deep copy.
“The spread operator can expand another item by split an iterable element like a string or an array into individual elements:” — CodinGame .com
T he spread operator … is useful for working with arrays and objects in JavaScript. It is a convenient feature added in ES6 (ES2015).
One of my favorite uses of the spread syntax is when combining arrays such as when adding an item to React State .
I also like that it can quickly combine the properties of objects into a new object, though any properties whose names conflict will be lost.
Knowing the spread syntax definitely saves me time when coding, and I recommend using it to all JavaScript developers.
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