reverse sentense

reverse.js

const assert = require('assert');

function reverse(sentences) {
  const tokens = sentences.split(' ');
  const half = Math.floor(tokens.length / 2)
  for(let i = 0; i < half; i++) {
    const a = tokens[i];
    const b = tokens[tokens.length - (i + 1)];
    if (b === undefined) {
      return;
    }
    const ab = a + b;
    tokens[tokens.length - (i + 1)] = ab.substring(0, ab.length - b.length);
    tokens[i] = ab.substring(a.length, ab.length);
  }
  return tokens.join(' ');
}

const cases = [{
  input: '1 2 3 4 5 6',
  output: '6 5 4 3 2 1',
}, {
  input: '1 2 3 4 5 6 7',
  output: '7 6 5 4 3 2 1',
}];

cases.forEach(c => {
  assert.equal(c.output, reverse(c.input));
});