How about this compact little trick?

var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; var stringLength = 5; var randomString = Array.apply(null, new Array(stringLength)).map(function () { return possible[Math.floor(Math.random() * possible.length)]; }).join(''); 

You need the Array.apply there to trick the empty array into being an array of undefineds.

If you're coding for ES2015, then building the array is a little simpler:

const possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; const stringLength = 5; const randomString = Array.from({ length: stringLength }, () => {  return possible[Math.floor(Math.random() * possible.length)]; }).join(''); 

How about this compact little trick?

var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; var stringLength = 5; function pickRandom() { return possible[Math.floor(Math.random() * possible.length)]; } var randomString = Array.apply(null, Array(stringLength)).map(pickRandom).join(''); 

You need the Array.apply there to trick the empty array into being an array of undefineds.

If you're coding for ES2015, then building the array is a little simpler:

var randomString = Array.from({ length: stringLength }, pickRandom).join(''); 

How about this compact little trick?

var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; var stringLength = 5; var randomString = Array.apply(null, new Array(stringLength)).map(function () { return possible[Math.floor(Math.random() * possible.length)]; }).join(''); 

You need the Array.apply there to trick the empty array into being an array of undefineds.

How about this compact little trick?

var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; var stringLength = 5; var randomString = Array.apply(null, new Array(stringLength)).map(function () { return possible[Math.floor(Math.random() * possible.length)]; }).join(''); 

You need the Array.apply there to trick the empty array into being an array of undefineds.

If you're coding for ES2015, then building the array is a little simpler:

const possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'; const stringLength = 5; const randomString = Array.from({ length: stringLength }, () => { return possible[Math.floor(Math.random() * possible.length)]; }).join('');