mod.ts

function preserveCamelCase(input: string | string[]) {
  let isLastCharLower = false;
  let isLastCharUpper = false;
  let isLastLastCharUpper = false;

  for (let i = 0; i < input.length; i++) {
    const c = input[i];

    if (isLastCharLower && /[a-zA-Z]/.test(c) && c.toUpperCase() === c) {
      input = input.slice(0, i) + "-" + input.slice(i);
      isLastCharLower = false;
      isLastLastCharUpper = isLastCharUpper;
      isLastCharUpper = true;
      i++;
    } else if (
      isLastCharUpper &&
      isLastLastCharUpper &&
      /[a-zA-Z]/.test(c) &&
      c.toLowerCase() === c
    ) {
      input = input.slice(0, i - 1) + "-" + input.slice(i - 1);
      isLastLastCharUpper = isLastCharUpper;
      isLastCharUpper = false;
      isLastCharLower = true;
    } else {
      isLastCharLower = c.toLowerCase() === c;
      isLastLastCharUpper = isLastCharUpper;
      isLastCharUpper = c.toUpperCase() === c;
    }
  }

  return input;
}

export interface CamelCaseOptions {
  pascalCase: boolean;
}

export function camelCase(
  input: string | string[],
  options: CamelCaseOptions = { pascalCase: false }
) {
  function postProcess(x: string) {
    return options.pascalCase ? x.charAt(0).toUpperCase() + x.slice(1) : x;
  }

  if (Array.isArray(input)) {
    input = input
      .map(x => x.trim())
      .filter(x => x.length)
      .join("-");
  } else {
    input = input.trim();
  }

  if (input.length === 0) {
    return "";
  }

  if (input.length === 1) {
    return options.pascalCase ? input.toUpperCase() : input.toLowerCase();
  }

  if (/^[a-z\d]+$/.test(input)) {
    return postProcess(input);
  }

  const hasUpperCase = input !== input.toLowerCase();

  if (hasUpperCase) {
    input = preserveCamelCase(input);
  }

  input = (input as string)
    .replace(/^[_.\- ]+/, "")
    .toLowerCase()
    .replace(/[_.\- ]+(\w|$)/g, (m, p1) => p1.toUpperCase());

  return postProcess(input);
}