JavaScript - Copy item reference of an array

Question

The answer might be obvious but I didn't find out. I have this code :

const arr = ["Hello"];
let currentStr = arr[0];
currentStr += " world";
console.log(currentStr); // prints "Hello world"
console.log(arr); // prints ["Hello"]

I just want to copy the string reference so when I change the currentStr value, it also change its reference in the array (here the first item of the arr array).

const arr = ["Hello"];
let currentStr = arr[0];
currentStr += " world";
console.log(currentStr); // prints "Hello world"
console.log(arr); // expect print ["Hello world"]

you are not pushing into the array - you are just joint two strings together - hence why the print [Hello] from the original arr — Always Helping
– Always Helping, Commented Sep 1, 2020 at 10:46
arr[0] += " world" did you try like this or you want entirely different things,? — Murat Çimen
– Murat Çimen, Commented Sep 1, 2020 at 10:47
@Always No I just want to concat the first item, not pushing into the array — user10458813
– user10458813, Commented Sep 1, 2020 at 10:48
string is immutable. When you change it, its reference changes. — Mustafa Arslan
– Mustafa Arslan, Commented Sep 1, 2020 at 10:53

Faktor 10 · Accepted Answer · 2020-09-01 11:20:57Z

1

In Javascript strings are immutable. I.e. when you make a change to a string behind the scenes Javascript is making a new copy of that string. Arrays are slightly different. You can mutate arrays. I.e. change what they are storing.

To achieve what you want you can use the splice method on the array to replace the 'Hello' string with a new string called 'Hello' + ' World'. Here is the code for it.

const arr = ['Hello']
arr.splice(0,1, arr[0] + ' World') 
console.log(arr) // ['Hello World']

See the MDN docs on Arrays, and the splice method.

answered Sep 1, 2020 at 11:20

Faktor 10

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Comments

Zain · Accepted Answer · 2020-09-01 10:59:13Z

0

const arr = ["Hello"];
var currentStr=[];
let newStr= arr[0]+" World"; 
currentStr.push(newStr)
console.log(currentStr); //
console.log(arr); //

answered Sep 1, 2020 at 10:59

Zain

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1 Comment

Thomas C. Over a year ago

Leaving it open but please add an explanatory line.

LilFlower · Accepted Answer · 2020-09-01 12:13:35Z

0

You can do this by re-assigning currentStr to arr[0].

const arr = ["Hello"];
let currentStr = arr[0];
currentStr += " world";
console.log(currentStr); // prints "Hello world"
arr[0] = currentStr;
console.log(arr); // prints ["Hello world"]

answered Sep 1, 2020 at 12:13

LilFlower

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