What is the best way to concat arrays in mutating way in JS? My problem:
var a = [1,2,3]
var b = a;
a = a.concat([4,5,6]) //<-- typical way of array concatenation in JS
console.log(a) //[1,2,3,4,5,6]
console.log(b) //[1,2,3] <-- I'd like it to be the same like the first array
I can use some loop of course, but I wonder if there is some faster and cleaner way to achieve it.
push mutates an array, but it expects sequence of arguments, so we use apply:
var a = [1,2,3]
var b = a
a.push.apply(a, [4,5,6])
console.log(a) //=> [1, 2, 3, 4, 5, 6]
console.log(b) //=> [1, 2, 3, 4, 5, 6]
In ES6 you could use spread operator ...:
a.push(...[4, 5, 6])
a.push(4, 5, 6);You can use push.apply here to update a without creating a new variable:
a.push.apply(a, [4,5,6]);
Or perhaps the more easier to understand:
Array.prototype.push.apply(a, [4,5,6]);
In short apply calls a function push with the first argument set as your this (object context), and the second argument an array. It's equivalent to a.push(element1, ..., element10), for example.
You can use the fact that push can take a variable number of arguments, and will push all of them to the end of the array. You can then write:
a.push.apply(a, [1, 2, 3]);
As apply will transform an array of arguments into an argument list for the function.
You can keep "a" as a property so that it can be accessed by reference:
var myObj = {};
myObj.a = [1,2,3]
var b = myObj;
myObj.a = myObj.a.concat([4,5,6])
console.log(myObj.a);
console.log(b.a);
