Check if string contains only letters in javascript

With /^[a-zA-Z]/ you only check the first character:

• ^: Assert position at the beginning of the string
• [a-zA-Z]: Match a single character present in the list below:
  • a-z: A character in the range between "a" and "z"
  • A-Z: A character in the range between "A" and "Z"

If you want to check if all characters are letters, use this instead:

/^[a-zA-Z]+$/.test(str);

• ^: Assert position at the beginning of the string
• [a-zA-Z]: Match a single character present in the list below:
  • +: Between one and unlimited times, as many as possible, giving back as needed (greedy)
  • a-z: A character in the range between "a" and "z"
  • A-Z: A character in the range between "A" and "Z"
• $: Assert position at the end of the string (or before the line break at the end of the string, if any)

Or, using the case-insensitive flag i, you could simplify it to

/^[a-z]+$/i.test(str);

Or, since you only want to test, and not match, you could check for the opposite, and negate it:

!/[^a-z]/i.test(str);

The fastest way is to check if there is a non letter:

if (!/[^a-zA-Z]/.test(word))