merge unknown number of sub-arrays in array

Question

I have an array like this:

arr = [ [[x,x],[x,x]], [[x,x],[x,x],[x,x]], [[x,x]] ]

and I want to turn it into an array like this:

arr = [  [x,x],[x,x] ,  [x,x],[x,x],[x,x],   [x,x]  ]

so I have tried this:

for (var i=1; i< arr.length; i++){ arr[0].concat(arr[i]); }

but it does not work. How can I 'merge' this intermediate level of array?

How does that "not work"? What output does it give? Does it give any errors? Why are you starting with var i = 1? — gen_Eric
– gen_Eric, Commented Mar 31, 2017 at 14:14
Starting with i=1 because i=0 would be arr[0] to which I want to add the other sub-arrays. — grateful
– grateful, Commented Mar 31, 2017 at 14:16

Nenad Vracar · Accepted Answer · 2017-03-31 14:32:07Z

11

With ES6 you can use spread syntax with concat()

var arr = [ [['x','x'],['x','x']], [['x','x'],['x','x'],['x','x']], [['x','x']] ]

var merged = [].concat(...arr)
console.log(JSON.stringify(merged))

For older versions of ecmascript the same can be done using concat() and apply().

var arr = [ [['x','x'],['x','x']], [['x','x'],['x','x'],['x','x']], [['x','x']] ]

var merged = [].concat.apply([], arr)
console.log(JSON.stringify(merged))

edited Mar 31, 2017 at 14:32

answered Mar 31, 2017 at 14:18

Nenad Vracar

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2 Comments

grateful Over a year ago

Great! Just the kind of efficient code and new thing to learn I was looking for. What does dot dot dot (...) mean?

Nenad Vracar Over a year ago

It's developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Aaron Dunigan AtLee · Accepted Answer · 2023-05-24 19:35:14Z

2

This is exactly what Array.prototype.flat() does.

var arr = [ [['x','x'],['x','x']], [['x','x'],['x','x'],['x','x']], [['x','x']] ]

var merged = arr.flat()
console.log(JSON.stringify(merged))

[["x","x"],["x","x"],["x","x"],["x","x"],["x","x"],["x","x"]]

answered May 24, 2023 at 19:35

Aaron Dunigan AtLee

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Comments

gen_Eric · Accepted Answer · 2017-03-31 14:20:26Z

1

The array.concat() doesn't change the array you call it on, but rather returns a new array - a new array you are ignoring.

You should create a result array and then append everything to that, instead of trying to modify arr.

var arr = [ [['x','x'],['x','x']], [['x','x'],['x','x'],['x','x']], [['x','x']] ];
var new_arr = [];

for(var i = 0; i < arr.length; i++){
    new_arr = new_arr.concat(arr[i]);
}

answered Mar 31, 2017 at 14:20

gen_Eric

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