trimPrefix() and trimSuffix() in TypeScript

string.spec.ts

import {} from 'jasmine';
import {trimSuffix, trimPrefix} from './string';
describe('string utils', () => {
  describe('trimSuffix', () => {
    for (const [from, trim, to] of [
      [null, 'abc', null],
      ['abc', 'c', 'ab'],
      ['abc', '', 'abc'],
      ['abc', null, 'abc'],
      ['abc', 'bc', 'a'],
      ['abc', 'abc', '' ],
      ['abc', 'abcd', 'abc'],
      ['abc', 'aabc', 'abc'],
      ['abcdabc', 'abc', 'abcd'],
      ['abcabc', 'abc', 'abc'],
      ['abcdabc', 'd', 'abcdabc'],
    ]) {
      it(`(from=${from}, trim=${trim}) => to=${to}`, () => {
        expect(trimSuffix(from,trim)).toEqual(to);
      });
    }
  });
  describe('trimPrefix', () => {
    for (const [from, trim, to] of [
      [null, 'abc', null],
      ['abc', 'a', 'bc'],
      ['abc', 'ab', 'c'],
      ['abc', '', 'abc'],
      ['abc', null, 'abc'],
      ['abc', 'd', 'abc'],
      ['abc', 'abc', ''],
      ['abc', 'abcd', 'abc'],
      ['abcdabc', 'abc', 'dabc'],
      ['abcabc', 'abc', 'abc'],
      ['abcdabc', 'd', 'abcdabc'],
    ]) {
      it(`(from=${from}, trim=${trim}) => to=${to}`, () => {
        expect(trimPrefix(from,trim)).toEqual(to);
      });
    }
  });
});

string.ts

// trimSuffix('abc', 'c') -> 'ab'
export function trimSuffix(toTrim: string, trim: string): string {
  if (!toTrim || !trim) {
    return toTrim;
  }
  const index = toTrim.lastIndexOf(trim);
  if (index === -1 || (index + trim.length) !== toTrim.length) {
    return toTrim;
  }
  return toTrim.substring(0, index);
}

// trimPrefix('abc', 'ab') -> 'c'
export function trimPrefix(toTrim: string, trim: string): string {
  if (!toTrim || !trim) {
    return toTrim;
  }
  const index = toTrim.indexOf(trim);
  if (index !== 0) {
    return toTrim;
  }
  return toTrim.substring(trim.length);
}

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