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This code returns an array that consists of two copies of the input array.

Can you review this algorithm?

//input: [1, 2, 3]  output: [1, 2, 3, 1, 2, 3]              
function duplicate(arr) {
 let copy = [];
 for(let i = 0; i < arr.length; i++) {
  copy.push(arr[i])
  }
 return copy.concat(arr)
 }
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3
  • \$\begingroup\$ Just use return arr.concat(arr) \$\endgroup\$ Commented May 16, 2018 at 4:15
  • 1
    \$\begingroup\$ Or the ES6+ way return [...arr, ...arr]; \$\endgroup\$ Commented May 16, 2018 at 4:51
  • \$\begingroup\$ Javascript ... duplicate an array. A good SO thread & starting place for looking into whys and wherefores of array duplication variations. \$\endgroup\$ Commented May 22, 2018 at 17:57

1 Answer 1

Reset to default
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The alternative approaches in the comments return arr.concat(arr); and return [...arr, ...arr]; are good replacements.

However, if you wrote this as part of a course, you should consider the following:

  • Your indentation is off, this is rule 1 in writing code: indent it
  • You are missing a semicolon after copy.push(arr[i])
  • In such a small function, I would have declared i up top in that let statement
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