javascript - Why does points.sort(function(a, b){return a-b}); return -1, 0 or 1?

Question

My difficulty here could be my mathematical illiteracy, but I was trying to sort some numbers in a JavaScript array and this is the solution I found online. It does indeed work, but my question is why?! I would really like to understand this piece of code properly.

The site, W3 Schools says:

You can fix this by providing a function that returns -1, 0, or 1:

var points = [40, 100, 1, 5, 25, 10];

points.sort(function(a,b){return a-b});

Why would only -1, 0 or 1 be returned? I have Googled, and return can return pretty much any value you want.

Again, if this is an incredibly dumb question I apologise.

Answer 1

The sort callback has to return

• a negative number if a < b
• 0 if a === b
• a positive number if a > b

Three possible return values are needed because the sort function needs to know whether a is smaller than, equal to, or larger than b in order to correctly position a in the result array.

It is very common to just return -1, 0 and 1 if you working with non-numerical data (I guess that's why W3Schools mentions it). But if you use numerical data, you can simply subtract the values because

• if a < b then a - b < 0, i.e. a negative number
• if a === b then a - b === 0, i.e. 0
• if a > b then a - b > 0, i.e. a positive number

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W3Schools is not very precise which is one of the reason why you should avoid it. Use MDN instead.