How to understand this code about js permutation and combination?

Code: 

/**
  * 
  **/
bonusCombination (arr, num, fun) {
  if (arr.length < num || num > 10) {
    return []
  }
  let variable = ["a", "b", "c", "d", "e", "f", "g", "h", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u"]
  let replaceStr = "-sharp$-sharp"
  let str = "var arr=arguments[0]; var fun=arguments[1];  var ret=[]; for (var a = 0; a < arr.length; aPP) { " + replaceStr + " } return ret;"
  for (let i = 1; i < num; iPP) {
    str = str.replace(replaceStr, " for (var " + variable[i] + " =" + variable[i - 1] + "+ 1; " + variable[i] + " < arr.length; " + variable[i] + "PP) { " + replaceStr + "  }")
  }
  let temp = "var temp= [];"
  for (let i = 0; i < num; iPP) {
    temp += "temp.push(arr[" + variable[i] + "]);"
  }
  if (fun) {
    temp += "ret.push(fun(temp)); "
  } else {
    temp += "ret.push(temp);"
  }
  str = str.replace(replaceStr, temp)
  return (new Function(str)).apply(null, [arr, fun])
}

figure:

  1. doesn"t understand why strings are used as variables. Is there any other function of this method?
  2. Why did you define a variable; in the first place
  3. Please explain step by step.
Javascript es6--series algorithm permutation-and-combination
May.11,2021

explain the source code one by one, which is probably not easy to understand; let's look at this from another point of view.

< H2 > returns a dynamic function < / H2 >

first look at the return statement of the bonusCombination function: return (new Function (str)). Apply (null, [arr, fun]) . The decomposition of this statement is as follows::

  • new Function (str) : it means to dynamically construct a function using the str string. Let's call the constructed function strFn (that is, var strFn = new Function (str) )
    • .
  • so there is strFn.apply (null, [arr, fun]) , which is transformed into a form that we can usually understand as strFn (arr, fun) .

from this we can see that the purpose of this bonusCombination is to dynamically build a function strFn , which accepts two input parameters arr and fun -- which are the input parameters we passed to bonusCombination ;

corresponding to the example you gave, the arr here is . , while fun is null

. < H2 > input the function of num and dynamically create the function body < / H2 >

We know that bonusCombination has another input parameter, which is num . Its function is to forge the specific content of the above dynamic function strFn , and use this num input parameter to influence that large segment of character operation, thus affecting the specific content of the dynamic function:

.

for reading such a function, the best way to understand what it means is to enumerate.

for example, if we let num be 2 : then the dynamically built function strFn has the following structure: 

  
 is it so slow to run?

Previous: Wechat official account obtains the geographical location part of the mobile phone authorization prompt is abnormal.

Next: Cross-domain problem of vue project access background interface

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1bdbc49-3129d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1bdbc49-3129d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?