Most efficient way to prepend a value to an array

Question

Assuming I have an array that has a size of N  where N  gt  0   is there a more efficient way of prepending to the array that would not require O N   1  steps   In code  essentially  what I currently am doing is  function prependArray value  oldArray      var newArray   new Array value      for var i   0  i  lt  oldArray length    i        newArray push oldArray i           return newArray

User · Answer

With ES6  you can now use the spread operator to create a new array with your new elements inserted before the original elements    x000D   x000D     Prepend a single item  x000D  const a    1  2  3   x000D  console log  0     a    x000D   x000D   x000D     x000D   x000D     Prepend an array  x000D  const a    2  3   x000D  const b    0  1   x000D  console log     b     a    x000D   x000D   x000D    Update 2018-08-17  Performance  I intended this answer to present an alternative syntax that I think is more memorable and concise  It should be noted that according to some benchmarks  see this other answer   this syntax is significantly slower  This is probably not going to matter unless you are doing many of these operations in a loop

User · Answer

I m not sure about more efficient in terms of big-O but certainly using the unshift method is more concise   var a    1  2  3  4   a unshift 0   a       gt   0  1  2  3  4     Edit   This jsPerf benchmark shows that unshift is decently faster in at least a couple of browsers  regardless of possibly different big-O performance if you are ok with modifying the array in-place   If you really can t mutate the original array then you would do something like the below snippet  which doesn t seem to be appreciably faster than your solution   a slice   unshift 0      Use  slice  to avoid mutating  a      Edit 2   For completeness  the following function can be used instead of OP s example prependArray      to take advantage of the Array unshift      method   function prepend value  array      var newArray   array slice      newArray unshift value     return newArray     var x    1  2  3   var y   prepend 0  x   y       gt   0  1  2  3   x       gt   1  2  3

User · Answer

Example of prepending in-place    x000D   x000D  var A    7 8 9  x000D  var B    1 2 3  x000D   x000D  A unshift    B  x000D   x000D  console log A      1 2 3 7 8 9  x000D   x000D   x000D

User · Answer

f you need to preserve the old array   slice the old one and unshift the new value s   to the beginning of the slice   var oldA  4 5 6   newA oldA slice 0   newA unshift 1 2 3   oldA   n  newA      returned value  4 5 6 1 2 3 4 5 6

User · Answer

If you would like to prepend array  a1 with an array a2  you could use the following   var a1    1  2   var a2    3  4   Array prototype unshift apply a1  a2   console log a1        gt   3  4  1  2

User · Answer

There is special method   a unshift value     But if you want to prepend several elements to array it would be faster to use such a method   var a    1  2  3       b    4  5    function prependArray a  b        var args   b      args unshift 0       args unshift 0       Array prototype splice apply a  args      prependArray a  b   console log a      - gt   4  5  1  2  3

User · Answer

Calling unshift only returns the length of the new array  So  to add an element in the beginning and to return a new array  I did this   let newVal    someValue   let array     hello    world      newVal   concat array     or simply with spread operator     newVal     array     This way  the original array remains untouched

User · Answer

I have some fresh tests of different methods of prepending  For small arrays   lt 1000 elems  the leader is for cycle coupled with a push method  For huge arrays  Unshift method becomes the leader   But this situation is actual only for Chrome browser  In Firefox unshift has an awesome optimization and is faster in all cases   ES6 spread is 100  times slower in all browsers   https   jsbench me cgjfc79bgx 1

User · Answer

If you are prepending an array to the front of another array  it is more efficient to just use concat  So   var newArray   values concat oldArray     But this will still be O N  in the size of oldArray  Still  it is more efficient than manually iterating over oldArray  Also  depending on the details  it may help you  because if you are going to prepend many values  it s better to put them into an array first and then concat oldArray on the end  rather than prepending each one individually   There s no way to do better than O N  in the size of oldArray  because arrays are stored in contiguous memory with the first element in a fixed position  If you want to insert before the first element  you need to move all the other elements  If you need a way around this  do what  GWW said and use a linked list  or a different data structure

[javascript] Most efficient way to prepend a value to an array

Update 2018-08-17: Performance

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