If you’re using ES6, you can generate a sequence using Array.from() by passing in an array-like object for the length of the range, and a map function as a second argument to convert the array key of each item in the range into a character using String.fromCharCode():
Array.from({ length: 26 }, (_, i) => String.fromCharCode('A'.charCodeAt(0) + i));
You can also use the Array constructor (note: ES6 allows constructors to be invoked either with a function call or with the new operator) to initialize an array of the desired default length, fill it using Array.fill(), then map through it:
Array(26).fill().map((_, i) => String.fromCharCode('A'.charCodeAt(0) + i));
The same can be accomplished with the spread operator:
[...Array(26)].map((_, i) => String.fromCharCode('A'.charCodeAt(0) + i));
The above three examples will return an array with characters from A to Z. For custom ranges, you can adjust the length and starting character.
For browsers that don’t support ES6, you can use babel-polyfill or core-js polyfill (core-js/fn/array/from).
If you’re targeting ES5, I would recommend the Array.apply solution by @wires which is very similar to this one.
Lastly, Underscore/Lodash and Ramda have a range() function:
_.range('A'.charCodeAt(0), 'Z'.charCodeAt(0) + 1).map(i => String.fromCharCode(i));