How to convert letters to numbers with Javascript?

How could I convert a letter to its corresponding number in JavaScript?

For example:

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a = 0
b = 1
c = 2
d = 3

I found this question on converting numbers to letters beyond the 26 character alphabet, but it is asking for the opposite.

Is there a way to do this without a huge array?

If you want to be able to handle Excel like letters past 26 like AA then you can use the following function adapted from this question:

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function convertLetterToNumber(str) {
  var out = 0, len = str.length;
  for (pos = 0; pos < len; pos++) {
    out += (str.charCodeAt(pos) - 64) * Math.pow(26, len - pos - 1);
  }
  return out;
}

###Demo in Stack Snippets:

<!-- begin snippet: js hide: true --> <!-- language: lang-js -->

function convertLetterToNumber(str) {
  var out = 0, len = str.length;
  for (pos = 0; pos < len; pos++) {
    out += (str.charCodeAt(pos) - 64) * Math.pow(26, len - pos - 1);
  }
  return out;
}

var testCase = ["A","B","C","Z","AA","AB","BY"];

var converted = testCase.map(function(obj) {
  return {
    letter: obj,
    number: convertLetterToNumber(obj)
  };

});

console.table(converted);

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