Check for a sequence in JavaScript

sequence.js

var assert = require('chai').should();

function* mapProgressive(a, fn) {
	   for (let index = 0; index < a.length - 1; ++index) {
        yield  fn(a[index], a[index + 1]);
     }
}

function isArrayInSequence(a) {
 return Array.from(mapProgressive(a, (x,y) => {return x + 1 == y})).every(n => n == true)
}

describe('Array', function() {
  describe('isInSequence', function () {
    it('should return true when the array is in numerical sequence', function () {

        var a = [1,2,3];
        isArrayInSequence(a).should.be.true;

    });
    it('should return false when the array is not numerical sequence', function () {

        var a = [8,2,4];
        isArrayInSequence(a).should.be.false;

    });
  });
});