有没有办法以功能方式生成一个新的可变长度数组？

Question

我试图编写一个简单的函数式语句来生成指定长度的任意字符串（它不需要真正随机）。

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var charSet = '0123456789' +
              'ABCDEFGHIJKLMNOPQRSTUVWXZ' +
              'abcdefghijklmnopqrstuvwxyz'

// I'm trying for something like:
var randomStringT1 = new Array(10)         // I didn't actually expect this to work 
                     .map(v => Math.random())
                     .map(v => v * (charSet.length - 1))
                     .map(v => Math.floor(v))
                     .map(v => charSet.charAt(v))
                     .join('')

console.log('Take 1: new Array created inline')
console.log('randomStringT1 -->', randomStringT1, '<--')
// However, the `new Array(10)` does not appear to be created to the specified
// length.


// I've also tried creating the array first, e.g.:
var randomStringsArrayT2 = new Array(10)          // I did expect this
var randomStringT2 = randomStringsArrayT2         // approach to work
                     .map(v => Math.random())
                     .map(v => v * (charSet.length - 1))
                     .map(v => Math.floor(v))
                     .map(v => charSet.charAt(v))
                     .join('')

console.log('Take 2: randomStringsArray from new Array')
console.log('randomStringT2 -->', randomStringT2, '<--')
// But it still returns an empty string.

// The only thing that's worked so far, is to hardcode a new array
// with bogus starter values, like this:
var randomStringArrayT3 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] // !YUCK! !BAD! !EVIL!
var randomStringT3 = randomStringArrayT3
                     .map(v => Math.random())
                     .map(v => v * (charSet.length - 1))
                     .map(v => Math.floor(v))
                     .map(v => charSet.charAt(v))
                     .join('')

console.log('Take 3: manually generating a new array with bogus values')
console.log('randomStringT3 -->', randomStringT3, '<--')
// Which does actually result in the desired output, but requires 
// 
// var randomStringArrayT3 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 
// 
// !!YUCK!! !!BAD ON TOO MANY LEVELS OF HELL TO COUNT!! !!EVIL!!

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有没有办法以功能方式生成新的可变长度数组，即避免显式循环和像var randomStringArray = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]那样的黑客攻击？

编辑添加以下内容。

我也在离线问这个问题的朋友，他的回答是：

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// Take a symbol set
function string (ss) {

  function recurse (len, str) {
    // If the count is zero or less, we're done!
    if (len <= 0) {

      // Base case
      return str
    }

    // Find a random index
    var index = Math.floor(Math.random() * ss.length)

    // Grab the character
    var char = ss[index]

    // Recurse! No mutation here!
    return recurse(len - 1, str + char)
  }

  // Return another function which takes the length of the string to generate.
  return function (len) {

    // No for loop here, we're using immutable recursion!
    return recurse(len, '')
  }
}

var charSet = '0123456789' +
		      'ABCDEFGHIJKLMNOPQRSTUVWXZ' +
		      'abcdefghijklmnopqrstuvwxyz'

var generateString = string(charSet)

console.log(generateString(10));

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