Js Spread Array
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Js Spread Array
Home / ES6 Tutorial / The Practical Usages of JavaScript Spread Operator
Note that the spread operator only copies the array itself to the new one, not the elements, meaning that the copy is shallow, not deep.
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Summary : in this tutorial, you will learn about the JavaScript spread operator that spreads out elements of an iterable object.
ES6 provides a new operator called spread operator that consists of three dots (...). The spread operator allows you to spread out elements of an iterable object such as an array ,a map , or a set . For example:
In this example, the three dots ( ... ) located in front of the odd array is the spread operator. The spread operator unpacks the elements of the odd array.
Note that ES6 also has the three dots ( ... ) which is a rest parameter that collects all remaining arguments of a function into an array.
In this example, the rest parameter ( ... ) collects the arguments 3,4 and 5 into an array args .
So the three dots ( ... ) represent both the spread operator and the rest parameter.
The rest parameters must be the last arguments of a function. However, the spread operator can be anywhere:
Note that ES2018 expands the spread operator to objects. It is known as the object spread .
Let’s take a look at some scenarios that using the spread operators can be very useful.
See the following compare() function compares two numbers:
In ES5, to pass an array of two numbers to the compare() function, you often use the apply() method as follows:
However, by using the spread operator, you can pass an array of two numbers to the compare() function:
The spread operator spreads out the elements of the array so a = 1 and b = 2 in this case.
Sometimes, a function may accept an indefinite number of arguments. Filling arguments from an array is not convenient.
For example, the push() method of an array object allows you to add one or more elements to an array. If you want to pass an array to the push() method, you need to use apply() method as follows:
The following example uses the spread operator to improve the readability of the code:
As you can see, using the spread operator is much cleaner.
The spread operator allows you to insert another array into the initialized array when you construct an array using the literal form. See the following example:
Also, you can use the spread operator to concatenate two or more arrays:
In addition, you can copy an array instance by using the spread operator:
In this example, we constructed the chars array from individual strings. When we applied the spread operator to the 'BC' string, it spreads out each individual character of the string 'BC' into individual characters.
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6 Use Cases of Spread with Array in JavaScript - DEV Community
JavaScript spread operator and array manipulation
javascript - spread operator converting objects to array - Stack Overflow
Understanding the JavaScript Spread Operator — Advanced Uses | Medium
JavaScript | Spread Operator - GeeksforGeeks
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Asked
3 years, 6 months ago
Active
1 year, 1 month ago
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are the numeric keys related to the indices of the array?
– Nina Scholz
Jul 24 '17 at 16:40
9,363 1 1 gold badge 20 20 silver badges 40 40 bronze badges
this is not what's going on nor is this the correct answer
– Christian Matthew
Dec 29 '19 at 15:42
@ChristianMatthew Care to elaborate? There are ~10 people, including the OP that would disagree with you
– mhodges
Dec 30 '19 at 16:56
your answer is literally correct but it doesn't explain anything the OP asked. He was inquiring why the spread operator did not work. If you can explain that in your answer I would revote your answer to correct.
– Christian Matthew
Dec 30 '19 at 17:04
@ChristianMatthew "Why doesn't this code work?" Is not an on-topic question. The real question was "Is there a way to use the spread operator to get the desired result?" And the answer was a resounding "no". To be more helpful, everyone provided alternate solutions to accomplish the same goal. I did, however, go above and beyond and address "why the spread operator didn't work in this case?". The OP is trying to spread an object onto an array , when the docs (that I referenced) clearly state that objects spread onto a new object not an array. Not sure what else you're looking for?
– mhodges
Dec 30 '19 at 17:14
That is fair and I will make sure to do that for the next time. Thanks for the edit.
– Christian Matthew
Dec 30 '19 at 18:21
136k 22 22 gold badges 162 162 silver badges 160 160 bronze badges
4,311 1 1 gold badge 19 19 silver badges 45 45 bronze badges
This gives the literal property "e" for each object. You need to add the property dynamically using bracket syntax to get the correct property name - arr.push({ [e]: data[e] });
– mhodges
Jul 24 '17 at 16:15
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I'm trying to convert a data structure like this:
Using the spread operator like this: [...data] returns any empty array.
Is there a way to use the spread operator to get the desired result? Also, why doesn't this approach work?
"Is there a way to use the spread operator to get the desired result?" Short answer, no. (see below for alternate solution to what you're trying to accomplish)
"Also, why doesn't this approach work?"
It doesn't work because according to the MDN docs
"The Rest/Spread Properties for ECMAScript proposal (stage 3) adds spread properties to object literals. It copies own enumerable properties from a provided object onto a new object ."
Like the docs say, according to the "Rest/Spread Properties proposal", you can't spread object properties onto an array, objects will always spread their properties onto a new object. Likewise, arrays will not spread onto an object, they will only spread onto a new array.
You can do this fairly easily with Object.keys().map() . Object.keys() will get an array of the keys of the object, and Array.map() will map them into an array of the desired structure, like so:
You can use Object.entries to get [key, value] pairs, and map them to an array of objects using computed property names :
I'm afraid you cant use to spread operator like in your example, however you can produce the desired output with reduce .
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