awesome-string/src/query/starts_with.js

The ultimate JavaScript string library

import clipNumber from 'helper/number/clip_number';
import coerceToString from 'helper/string/coerce_to_string';
import isNil from 'helper/object/is_nil';
import toInteger from 'helper/number/to_integer';
import toString from 'helper/string/to_string';

/**
 * Checks whether `subject` starts with `start`.
 *
 * @function startsWith
 * @static
 * @since 1.0.0
 * @memberOf Query
 * @param {string} [subject=''] The string to verify.
 * @param {string} start The starting string.
 * @param {number} [position=0] The position to start searching.
 * @return {boolean} Returns `true` if `subject` starts with `start` or `false` otherwise.
 * @example
 * as.startsWith('say hello to my little friend', 'say hello');
 * // => true
 *
 * as.startsWith('tony', 'on', 1);
 * // => true
 *
 * as.startsWith('the world is yours', 'world');
 * // => false
 */
export default function startsWith(subject, start, position) {
  const subjectString = coerceToString(subject);
  const startString = toString(start);
  if (startString === null) {
    return false;
  }
  if (startString === '') {
    return true;
  }
  position = isNil(position) ? 0 : clipNumber(toInteger(position), 0, subjectString.length);
  return subjectString.substr(position, startString.length) === startString;
}