Javascript: Sort by date, the tricky part

Think about the below trivial problem: Sort the activities by date in ascending order:

const activities = [
  { title: 'Hiking', date: new Date('2019-06-28') },
  { title: 'Shopping', date: new Date('2019-06-10') },
  { title: 'Trekking', date: new Date('2019-06-22') }
]

Sounds trivial right?

var output = activities.sort( (a, b) => a.date > b.date);

Emm, you tried this in Node.js, in npm runkit, all looks right. The sorted result is

Shopping --> Trekking --> Hiking

However, if you run this code in the Chrome browser, you will see a different result! The result is still the same as the input, which is

Hiking --> Shopping --> Trekking

Why is that?

The reason is, the sort function accepts a comparator (the lambda or arrow function above) to be an integer, not a boolean!

Below is the documentation fromMozilla：

If compareFunction is supplied, all non-undefined array elements are sorted according to the return value of the compare function (all undefined elements are sorted to the end of the array, with no call to compareFunction). If a and b are two elements being compared, then:

• If compareFunction(a, b) returns less than 0, sort a to an index lower than b (i.e. a comes first).
• If compareFunction(a, b) returns 0, leave a and b unchanged with respect to each other, but sorted with respect to all different elements. Note: the ECMAscript standard does not guarantee this behavior, thus, not all browsers (e.g. Mozilla versions dating back to at least 2003) respect this.
• If compareFunction(a, b) returns greater than 0, sort b to an index lower than a (i.e. b comes first).
• compareFunction(a, b) must always return the same value when given a specific pair of elements a and b as its two arguments. If inconsistent results are returned, then the sort order is undefined.

Soif you run below code：Note the difference of

const output = activities.sort( (a, b) => a.date - b.date);
//(a, b) => a.date > b.date ➜ (a, b) => a.date - b.date

That is it!

So to recap, the Javascript in the browser may be different from the one ran on node.js. Read the official documents to make sure we are using the correct one.