JavaScript - duplicating array doesn't work

Your for loop never ends, because the length is increasing for every iteration.

For getting only one copy, you could store the length in advance and loop only this length.

var myArray = ["one", "two", "five"];

for (var i = 0, l = myArray.length; i < l; i++){
    myArray.push(myArray[i]);
}

console.log(myArray);

When you use add items to the array, the length grows, which makes the for loop add more items, which increase the length, and so on...

A simple solution is to concat the array to itself:

var myArray = ["one", "two", "five"];

var result = myArray.concat(myArray);

console.log(result);

Or push all the items at once using spread, if you want to mutate the array:

var myArray = ["one", "two", "five"];

myArray.push(...myArray);

console.log(myArray);

If you change myArray.length to a constant number, this problem does not happen. However, the reason for the crashing is every call to .push() increases your array length by 1 anyway, I think this crash is interesting and could be an unhandled bug maybe!

var myArray = ["one", "two", "five"];
var l=myArray.length;
for(var i = 0; i < l; i++){// for example l=3 here
  myArray.push(myArray[i]);

}

An ES6 solution would be use the spread operator. Which behaves like a concat.

let myArray = ["one" ,"two","five"];
myArray = [ ...myArray, ...myArray];