How to concat two javascript arrays?

Question

I seem to have trouble to concat two javascript arrays. Here is the code:

console.log("\n\n");
a = [[1,2]];
b = [[1,2,3]];
console.table(a);
console.table(b);
a.concat(b);
console.table(a);

in which I create two arrays 'a' and 'b' (with elements being arrays as well, but who cares), with the goal of adding the single element of 'b' (the array [1,2,3] to the array 'a'. I expect 'b' to have two elements now (the array [1,2] and the array [1,2,3], but it does not look so. I get the output as follows:

I expected the last output of console.table to have two rows with the content

0   1   2
1   1   2   3

What am I doing wrong here?

tymeJV · Accepted Answer · 2017-07-21 19:06:29Z

2

.concat doesn't mutate the original array, you have to re-assign:

a = a.concat(b);

answered Jul 21, 2017 at 19:06

tymeJV

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Comments

Roman · Accepted Answer · 2017-07-21 19:07:28Z

1

Instead of manipulate an exisiting array a.concat(b) returns a new one.

you should try:

var c = a.concat(b)
console.table(c)

answered Jul 21, 2017 at 19:07

Roman

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Comments

Dij · Accepted Answer · 2017-07-21 19:06:51Z

1

concat() does not change the existing arrays, but returns a new array, containing the values of the joined arrays. you will have to assign it to some variable.

console.log("\n\n");
a = [[1,2]];
b = [[1,2,3]];
console.log(a);
console.log(b);
console.log(a.concat(b));

answered Jul 21, 2017 at 19:06

Dij

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Comments

seescode · Accepted Answer · 2017-07-21 19:06:32Z

0

If you are using ES6 you can use the spread operator to get the desired result:

console.log("\n\n");
a = [[1,2]];
b = [[1,2,3]];
console.table(a);
console.table(b);
c = [...a, ...b];
console.table(c);

answered Jul 21, 2017 at 19:06

seescode

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Comments

Jarek Kulikowski · Accepted Answer · 2017-07-21 19:06:52Z

0

If arrays a and b have one element each then this will give you the answer

var c = a[0].concat(b[0]);

otherwise you would have to use a map or easily do it in ES6 with ...

answered Jul 21, 2017 at 19:06

Jarek Kulikowski

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Comments

vol7ron · Accepted Answer · 2017-07-23 05:17:18Z

0

If you are avoiding the reassignment, you could use Array.push.apply():

var a = [[1,2]],
    b = [[1,2,3]];

a.push.apply(a,b);

console.log(a);

Note: This was offered for completeness, my opinion would be to use concat assign as it's easier for me to understand what is occurring.

answered Jul 23, 2017 at 5:17

vol7ron

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