javascript - Create Simple Dynamic Array

Question

What s the most efficient way to create this simple array dynamically   var arr      1    2    3    4    5    6    7    8    9    10      Let s say we can get the number 10 from a variable  var mynumber   10

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With ES2015  this can be achieved concisely in a single expression using the Array from method like so   Array from   length  10        idx    gt       idx      The first argument to from is an array like object that provides a length property  The second argument is a map function that allows us to replace the default undefined values with their adjusted index values as you requested  Checkout the specification here

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Update  micro-optimizations like this one are just not worth it  engines are so smart these days that I would advice in the 2020 to simply just go with var arr         Here is how I would do it  var mynumber   10  var arr   new Array mynumber    for  var i   0  i  lt  mynumber  i          arr i     i   1  toString       My answer is pretty much the same of everyone  but note that I did something different   It is better if you specify the array length and don t force it to expand every time  So I created the array with new Array mynumber

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updated    gt  june 2020 if are you using node js you can create or append a string value with  quot    quot  operator then  inside the loop you can for  var j   0  j  lt   data legth -1  j                          lang    data lang    quot    quot                         var langs   lang split       console log  quot Languages   gt  quot    lang  typeof lang   typeof langs   langs    console log lang 0      here access arrary by index value                you can see the type of  string and object

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I would do as follows    x000D   x000D  var num   10  x000D    dynar       Array num   map    i    gt    i      x000D  console log dynar   x000D   x000D   x000D

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Sounds like you just want to construct an array that contains the string versions of the integer values   A simple approach   var arr       for  var i   1  i  lt   mynumber  i    arr push    i     For a more interesting version you could do a generator     function tail i  maxval        return  i  concat i  lt  maxval   tail i 1  maxval            var arr   tail 1  mynumber

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The same way you would for all arrays you want to fill dynamically  A for loop  Suedo code is  arr  array   for i  i lt max  i      arr   i      that should help you on the way

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misread the question  corrected  Try   var myNumber   100      myarr    function arr i  return i   arr i-1  concat i     i   myNumber      Just for fun  if you extend Array like this   Array prototype mapx   function callback     return String this  split      map callback       You could use   var myNum   100       myarr   new Array myNum  mapx function el i  return i 1

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var arr       while mynumber--        arr mynumber    String mynumber 1

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I hope you have to get last element from array variable so my solution                var arr      1    2    3    4    5    6    7    8    9    10    var mynumber   arr  arr  length - 1     var mynumber   10

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If you are asking whether there is a built in Pythonic range-like function  there isn t   You have to do it the brute force way   Maybe rangy would be of interest to you

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This answer is about  how to dynamically create an array without loop    Literal operator    doesn t allow us to create dynamically  so let s look into Array  it s constructor and it s methods   In ES2015 Array has method  from    which easily allows us to create dynamic Array   Array from  length  10      - gt   undefined  undefined  undefined          When Array s constructor receives number as first parameter  it creates an Array with size of that number  but it is not iterable  so we cannot use  map     filter   etc     new Array 10     - gt   empty    10    But if we ll pass more than one parameter we will receive array from all parameters    new Array 1 2 3     - gt   1 2 3    If we would use ES2015 we can use spread operator which will spread empty Array inside another Array  so we will get iterable Array         new Array 10       - gt   undefined  undefined  undefined         But if we don t use ES2015 and don t have polyfills  there is also a way to create dynamic Array without loop in ES5  If we ll think about  apply   method  it spreads second argument array to params  So calling apply on Array s constructor will do the thing   Array apply null  new Array 10       - gt   undefined  undefined  undefined         After we have dynamic iterable Array  we can use map to assign dynamic values   Array apply null  new Array 10   map function el  i   return   i              1   2   3

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var arr       for var i 1  i lt  mynumber  i         arr push      i       This seems to be faster in Chrome  according to JSPerf  but please note that it is all very browser dependant   There s 4 things you can change about this snippet    Use for or while  Use forward or backward loop  with backward creating sparse array at beginning  Use push or direct access by index  Use implicit stringification or explicitly call toString    In each and every browser total speed would be combination of how much better each option for each item in this list performs in that particular browser   TL DR  it is probably not good idea to try to micro-optimize this particular piece

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I had a similar problem and a solution I found  forgot where I found it  is this   Array from Array mynumber    val  index    gt  index   1

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Some of us are referring to use from which is not good at the performance     x000D   x000D  function getArrayViaFrom input    x000D    console time  Execution Time    x000D    let output   Array from Array input    value  i    gt   i   1  toString    x000D    console timeEnd  Execution Time    x000D   x000D    return output  x000D    x000D   x000D  function getArrayViaFor input    x000D    console time  Execution Time 1    x000D    var output       x000D    for  var i   1  i  lt   input  i      x000D      output push i toString     x000D      x000D    console timeEnd  Execution Time 1    x000D   x000D    return output  x000D    x000D   x000D  console log getArrayViaFrom 10      Takes 10x more than for that is 0 220ms x000D  console log getArrayViaFor 10       Takes 10x less than From that is 0 020ms x000D   x000D   x000D

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var arr       for var i 1  i lt  mynumber  i         arr push i toString

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A little late to this game  but there is REALLY cool stuff you can do with ES6 these days  You can now fill an array of dynamic length with random numbers in one line of code      Array 10  keys    map      gt  Math floor Math random     100

[javascript] javascript - Create Simple Dynamic Array

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