Return index of greatest value in an array

Question

I have this   var arr    0  21  22  7     What s the best way to return the index of the highest value into another variable

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EDIT  Years ago I gave an answer to this that was gross  too specific  and too complicated  So I m editing it  I favor the functional answers above for their neat factor but not their readability  but if I were more familiar with javascript then I might like them for that  too      Pseudo code   Track index that contains largest value  Assume index 0 is largest initially  Compare against current index  Update index with largest value if necessary      Code   var mountains    3  1  5  9  4    function largestIndex array     var counter   1    var max   0     for counter  counter  lt  array length  counter         if array max   lt  array counter            max   counter              return max     console log  index with largest value is     largestIndex mountains       index with largest value is  3

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To complete the work of  VFDan  I benchmarked the 3 methods  the accepted one  custom loop   reduce  and find max arr   on an array of 10000 floats  Results on chromimum 85 linux  higher is better    custom loop  100  reduce  94 36  indexOf max   70   Results on firefox 80 linux  higher is better    custom loop  100  reduce  96 39  indexOf max   31 16   Conclusion  If you need your code to run fast  don t use indexOf max   reduce is ok but use the custom loop if you need the best performances  You can run this benchmark on other browser using this link  https   jsben ch wkd4c

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A stable version of this function looks like this      not defined for empty array function max index elements        var i   1      var mi   0      while  i  lt  elements length            if    elements i   lt  elements mi                mi   i          i    1            return mi

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function findIndicesOf haystack  needle        var indices            var j   0      for  var i   0  i  lt  haystack length    i            if  haystack i     needle              indices j      i            return indices      pass array to haystack and Math max    array  to needle  This will give all max elements of the array  and it is more extensible  for example  you also need to find min values

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Another solution of max using reduce   1 2 5 0 4  reduce  a b i    gt  a 0   lt  b    b i    a   Number MIN VALUE -1      5 2   This returns  5e-324  -1  if the array is empty   If you want just the index  put  1  after  Min via  Change to  gt  and MAX VALUE    1 2 5 0 4  reduce  a b i    gt  a 0   gt  b    b i    a   Number MAX VALUE -1      0  3

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This is probably the best way  since it   s reliable and works on old browsers   function indexOfMax arr        if  arr length     0            return -1             var max   arr 0       var maxIndex   0       for  var i   1  i  lt  arr length  i              if  arr i   gt  max                maxIndex   i              max   arr i                        return maxIndex      There   s also this one-liner   let i   arr indexOf Math max    arr      It performs twice as many comparisons as necessary and will throw a RangeError on large arrays  though  I   d stick to the function

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If you are utilizing underscore  you can use this nice short one-liner     indexOf arr    max arr     It will first find the value of the largest item in the array  in this case 22  Then it will return the index of where 22 is within the array  in this case 2

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In one line and probably faster then arr indexOf Math max apply Math  arr      x000D   x000D  var a    0  21  22  7   x000D  var indexOfMaxValue   a reduce  iMax  x  i  arr    gt  x  gt  arr iMax    i   iMax  0   x000D   x000D  document write  indexOfMaxValue       indexOfMaxValue      prints  indexOfMaxValue   2  x000D   x000D   x000D    Where    iMax - the best index so far  the index of the max element so far  on the first iteration iMax   0 because the second argument to reduce   is 0  we can t omit the second argument to reduce   in our case  x - the currently tested element from the array i - the currently tested index arr - our array    0  21  22  7     About the reduce   method  from  JavaScript  The Definitive Guide  by David Flanagan       reduce   takes two arguments  The first is the function that performs the reduction operation  The task of this reduction function is to somehow combine or reduce two values into a single value  and to return that reduced value       Functions used with  reduce   are different than the functions used with  forEach   and map    The familiar value  index  and array values are passed as the second  third  and fourth arguments  The first argument is the accumulated result of the reduction so far  On the first call to the function  this first argument is the initial value you passed as the   second argument to  reduce    On subsequent calls  it is the value returned by the previous invocation of the function       When you invoke  reduce   with no initial value  it uses the first element of the array as the initial value  This means that the first call to the reduction function will have the first and second array elements as its   first and second arguments

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x000D   x000D   var arr  0 6 7 7 7   x000D   var largest  0   x000D     find the largest num  x000D   for var i 0 i lt arr length i     x000D     var comp  arr i -largest 0   gt 0  x000D        if comp   x000D     largest      x000D     largest push arr i    x000D       x000D     x000D   alert largest    7 x000D    x000D     find the index of  arr  x000D   var arrIndex     x000D   for var i 0 i lt arr length i     x000D      var comp arr i -largest 0   0  x000D   if comp   x000D   arrIndex push i   x000D     x000D     x000D   alert arrIndex     2 3 4  x000D   x000D   x000D

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Unless I m mistaken  I d say it s to write your own function   function findIndexOfGreatest array      var greatest    var indexOfGreatest    for  var i   0  i  lt  array length  i          if   greatest    array i   gt  greatest          greatest   array i         indexOfGreatest   i              return indexOfGreatest

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Here is another solution  If you are using ES6 using spread operator   var arr    0  21  22  7    const indexOfMaxValue   arr indexOf Math max    arr

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If you create a copy of the array and sort it descending  the first element of the copy will be the largest  Than you can find its index in the original array   var sorted       arr  sort  a b    gt  b - a  arr indexOf sorted 0     Time complexity is O n  for the copy  O n log n   for sorting and O n  for the indexOf    If you need to do it faster  Ry s answer is O n

[javascript] Return index of greatest value in an array

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