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Title: Enter a positive integer n, return 1-n of n random, and do not repeat

For example, enter 4 and return 3, 2, 4, 1

1. Basic version

const a = 12 
let b = new Array(a) 
for(let i = 0; i < b.length; i++) { 
    const c = parseInt(Math.random() * a) + 1 b.indexOf(c) === -1 ? b[i] = c : i-- 
} 
console.log(b)
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function ccf(e) {
    let ary = [];
    let newAry = [];
    for (let i=0; i<e;) {let origin = Math.round(Math.random()*e) % e + 1;
      if(!ary[origin]){
        ary[origin] = origin;
        newAry.push(origin);
        i++;
      }
    }
    return newAry
}

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2. Array subscript method

/ / method
const randomNum1 = n= > {
  let arr = [];
  let temp;
  for (let i = 0; i < n; i++) {
    // Generate an array of 1-n
    arr[i] = i + 1;
  }
  for (let j = 0; j < n; j++) {
    // Generate a random subscript
    let index = Math.floor(Math.random() * n);
    // Swap with the last number in the array, and subtract the total length of the array by one
    // The index may be the same each time, but the number in the corresponding array is different
    temp = arr[n - 1];
    arr[n - 1] = arr[index];
    arr[index] = temp;
    n--;
  }
  return arr;
}
console.log(randomNum1(5));


/ / method 2
const randomNum2 = n= > {
  let arr = [];
  for (let i = 0; i < n; i++) {
    // Generate an array of 1-n
    arr[i] = i + 1;
  }
  // sort to generate a random decimal
  arr.sort(function () {
    return 0.5 - Math.random()
  })
  return arr;
}
console.log(randomNum2(5));

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3. After the optimization

const fn = (n) => {
  let i, range, num = new Array(n).fill(1).map((val, inx) => inx + 1);
  for (i = n - 1; i > 0; i--) {
    range = Math.floor(Math.random() * i);
    [num[i], num[range]] = [num[range], num[i]];
  }
  return num;
}
fn(10);

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