ecmascript 6 - javascript es6 array feature [...data, 0] "spread operator" - Stack Overflow

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I came across this in some example code and I am completely lost.

const addCounter = (list) => {
    return [...list, 0];  // This is the bit I am lost on, and I don't know about [...list, 0]
}

Apparently the above is equal to the following:

const addCounter = (list) => {
    return list.concat([0]);
}

Any suggestion or explaination is much appreciated.

I came across this in some example code and I am completely lost.

const addCounter = (list) => {
    return [...list, 0];  // This is the bit I am lost on, and I don't know about [...list, 0]
}

Apparently the above is equal to the following:

const addCounter = (list) => {
    return list.concat([0]);
}

Any suggestion or explaination is much appreciated.

• javascript
• ecmascript-6

Share Improve this question Follow edited Jun 14, 2018 at 20:44 goWithSwagger 1,77211 gold badge1919 silver badges2020 bronze badges asked Nov 24, 2015 at 9:59 BillBill 19.2k2121 gold badges8888 silver badges135135 bronze badges 1

• Does this answer your question? Spread Syntax vs Rest Parameter in ES2015 / ES6 – Henke - Нава́льный П с м Commented Mar 15, 2021 at 14:55

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2 Answers 2

Sorted by: Reset to default 28

...list is using the spread syntax to spread the elements of list. Let's assume the list is [1, 2, 3]. Therefore [...list, 0] becomes:

[1, 2, 3, 0]

Which has the same result as doing list.concat([0]);

This is not a feature of the array in ES6, it's just been used for array concatenation. It has other uses. Read more on MDN, or see this question.

...list spreads (lays) out all the elements in the array list.

so [...list, 0] is all of the elements of list with a 0 at the end

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