I just came across this as a really nice and elegant solution:

Math.random().toString(36).slice(2)

Notes on this implementation:

• This will produce a string anywhere between zero and 12 characters long, usually 11 characters, due to the fact that floating point stringification removes trailing zeros.
• It won't generate capital letters, only lower-case and numbers.
• Because the randomness comes from Math.random(), the output may be predictable and therefore not necessarily unique.
• Even assuming an ideal implementation, the output has at most 52 bits of entropy, which means you can expect a duplicate after around 70M strings generated.

I loved the brievety of doubletap's Math.random().toString(36).substring(7) answer, but not that it had so many collisions as hacklikecrack correctly pointed out. It generated 11-chacter strings but has a duplicate rate of 11% in a sample size of 1 million.

Here's a longer (but still short) and slower alternative that had only 133 duplicates in a sample space of 1 million. In rare cases the string will still be shorter than 11 chars:

Math.abs(Math.random().toString().split('')
    .reduce(function(p,c){return (p<<5)-p+c})).toString(36).substr(0,11);

Short alternative using spread operator and .map()

Demo 1

const genRanHex = size => [...Array(size)].map(() => Math.floor(Math.random() * 16).toString(16)).join('');

console.log(genRanHex(6));
console.log(genRanHex(12));
console.log(genRanHex(3));

1. Pass in a number (size) for the length of the returned string.

2. Define an empty array (result) and an array of strings in the range of [0-9] and [a-f] (hexRef).

3. On each iteration of a for loop, generate a random number 0 to 15 and use it as the index of the value from the array of strings from step 2 (hexRef) -- then push() the value to the empty array from step 2 (result).

4. Return the array (result) as a join('')ed string.

Demo 2

const getRanHex = size => {
  let result = [];
  let hexRef = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'];

  for (let n = 0; n < size; n++) {
    result.push(hexRef[Math.floor(Math.random() * 16)]);
  }
  return result.join('');
}

console.log(getRanHex(6));
console.log(getRanHex(12));
console.log(getRanHex(3));