你可以用正则表达式得到这两个部分：

const arr = [
  '123',
  '123.331',
  '123.331abc',
  '12331abc123'
];

const splitBeforeLetter = str => str.match(/([^A-Za-z]+)(.*)/).slice(1);

console.log(...arr.map(splitBeforeLetter));

字符串

const test = "123.887azee7";
const firstLetterIndex = test.search(/[a-zA-Z]/); // return the index of the first letter found, "-1" otherwise

字符串
那么你就用String.prototype.substring()

if (firstLetterIndex !== -1) {
    const firstSubstring = test.substring(0, firstLetterIndex); //  out put 123.887
    const secondSubstring = test.substring(firstLetterIndex); // output azee7
} else {
    console.log('no letters founed');
    // or do whatever you want
}

型

匹配第一部分，然后.slice()得到第二部分：

const [firstPart] = string.match(/^[^a-z]*/i);
const secondPart = string.slice(firstPart.length);

字符串
^[^a-z]*表示开头有0个或多个非字母。由于*量词的性质，这将始终匹配，从而导致第一个元素是第一部分的匹配数组。
试试看：

console.config({ maximize: true });

const testcases = [
    '12331abc123',
    '123.331abc',
    'abc123',
    'abc',
    '123',
];

function splitBeforeFirstLetter(string) {
  const [firstPart] = string.match(/^[^a-z]*/i);
  const secondPart = string.slice(firstPart.length);
  
  return [firstPart, secondPart];
}

testcases.forEach(
  testcase => console.log(testcase, splitBeforeFirstLetter(testcase))
);

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的数据