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?

Answer 1

.concat不会改变原始数组，您必须重新分配：

a = a.concat(b);

Answer 2

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)); 

Answer 3

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

you should try:

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

Answer 4

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);

Answer 5

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 ...

Answer 6

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.