I’m going to have an occasional series about weird stuff in JavaScript. Let’s kick this off with a weird question:
Why is it that when you have [] + [], you get an empty string ('') ? Isn’t this against the law?
In JavaScript, the + operator has two jobs
Job #1: Addition
console.log(2 + 3); // 5 console.log(256 + 256);// 512
Here you see that the addition operator has produced a sum of numeric operands.
Job #2: String Concatenation
console.log('foo' + 'bar') // 'foobar
console.log('hello' + ' ' + 'world'); // 'hello world'
Here, the + has concatenated strings.
This by the way is common in many languages. Java, C#, and Python all have the + perform double-duty as a numeric sum operator and a string concatenator.
JavaScript will coerce values in order to avoid errors
What happens when you try to concatenate something that isn’t a string?
This one might feel obvious:
console.log(2 + 'foo'); // 2foo
That feels obvious because one thing is definitely a number and another definitely isn’t.
But here’s one that can feel confusing:
console.log(2 + '3'); // '23'
The reasoning for this one is the exact same as the reasoning for the first one: JavaScript coerced the non-string into a string, and then concatenated.
This is not common to other languages. Other languages throw errors when you mix in types.
So is [] + [] addition or concatenation?
It’s not addition — it’s concatenation.
Before JavaScript can concatenate, it has to make sure everything is a string.
So each array gets coerced into a string, and then the two strings are concatenated.
It would be more obvious if those arrays had values:
console.log(['foo'] + ['bar']); // 'foobar' console.log(['foo', 'bar'] + ['baz']) ; // 'foo,barbaz'
Now that you can see what happens when arrays with values get concatenated, it should be a little more clear what happens when empty arrays get concatenated, too:
console.log([] + []); // ''
TL;DR
You’re not adding empty arrays. You’re concatenating them. JavaScript will coerce non-strings into strings in order to concatenate successfully. Two empty strings concatenated still make an empty string.