Source

Girl Develop It San Francisco, Teaching Materials, Exercises: Strings: MixUp

Description

Given two strings s1 and s2 and an integer n, swap the first n characters of s1 and s2 and concatenate the results, separated by a space. If either string is less than or equal to n characters in length, return them unmodified but concatenated, separated by a space.

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'foo', 'bar', 3 ⟹ 'foo bar'
'foobar', 'bazqux', 3 ⟹ 'bazbar fooqux'

My solution

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const headswap = (s1, s2, n) =>
  s1.length <= n || s2.length <= n
      ? s1 + ' ' + s2
      : s2.substring(0, n) + s1.substring(n) + ' ' +
        s1.substring(0, n) + s2.substring(n);


module.exports = headswap;

Tests

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const assert = require('assert')
    , headswap = require('./headswap.js');


describe('#headswap()', () => {
    it('Should leave strings unmodified', () => {
        assert.strictEqual(headswap('foo', 'bar', 3), 'foo bar');
    });
    it('Should modify strings', () => {
        assert.strictEqual(headswap('foobar', 'bazqux', 3), 'bazbar fooqux');
    });
});