Flattening

Use the reduce method in combination with the concat method to “flatten”
an array of arrays into a single array that has all the elements of the input
arrays

come again

 [[1,1,1], [2,2,2], [3,3,3]].reduce(function(a,b){ return a.concat(b)}) 

Result: [1, 1, 1, 2, 2, 2, 3, 3, 3]

//tested and trusted

function singleArr(arr){
var ret = [], arr = arr;

for(i=0;i<arr.length;i++){
//if the item is an array
if(arr[i] instanceof Array){
//loop through and add it to the 'ret' array variable
for(c=0;c<arr[i].length;c++) ret.push(arr[i][c]);
} else ret.push(arr[i]);
}
return ret;
}

console.log( singleArr([0,[1,2,3,4],5,[6,7,8,9],10,"end"]) ); //[0,1,2,3,4,5,6,"end"]