How to convert a string into a Cairo felt (field element)?

Here's how you can convert a string to a felt array. It's recommended to use a felt array as a normal felt can only carry 31 characters. If 31 characters is enough for you, then you can just use the first element of the array as your result : https://github.com/immutable/imx-starknet/blob/main/tests/utils/starknetUtils.ts#L60

/**
 * Splits a string into an array of short strings (felts). A Cairo short string (felt) represents up to 31 utf-8 characters.
 * @param {string} str - The string to convert
 * @returns {bigint[]} - The string converted as an array of short strings as felts
 */
export function strToFeltArr(str: string): BigInt[] {
  const size = Math.ceil(str.length / 31);
  const arr = Array(size);

  let offset = 0;
  for (let i = 0; i < size; i++) {
    const substr = str.substring(offset, offset + 31).split("");
    const ss = substr.reduce(
      (memo, c) => memo + c.charCodeAt(0).toString(16),
      ""
    );
    arr[i] = BigInt("0x" + ss);
    offset += 31;
  }
  return arr;
}

And vice versa

 * Converts an array of utf-8 numerical short strings into a readable string
 * @param {bigint[]} felts - The array of encoded short strings
 * @returns {string} - The readable string
 */
export function feltArrToStr(felts: bigint[]): string {
  return felts.reduce(
    (memo, felt) => memo + Buffer.from(felt.toString(16), "hex").toString(),
    ""
  );
}