Javascript confusion with types and results of adding empty arrays and objects

Question

I have came across this comparisons:

[] + []   === ''; // true
[] * 1    === 0;  // true
false + 1 === 1;  // true

The last comparison makes sense, but what I am wondering is and can't find a resource about why [] + [] equals empty string and why [] * 1 equals 0?

For [] + [], see this explanation http://2ality.com/2012/01/object-plus-object.html — Fredrik Widerberg, Aug 17 '18 at 10:41
you can't do mathematics with an array so javascript convert the `[]` to `0` — jonatjano, Aug 17 '18 at 10:42

OliverRadini · Accepted Answer · 2018-08-17T11:00:11.047

The answer linked from the comment posted by hardik-shah would certainly give you a better understanding of how these type of evaluations get handled in javascript. That answer points to the ecma docs, which is also useful. I'll try and give a limited explanation of how the examples you posted work.

Below I've quoted the article I used as the basis to interpreting how these instances of the + operator work. I believe I've interpreted the article correctly, but I'm no expert on these things so I would be happy to be corrected.

[] + []   === ''; // true

This is true because both operands are objects; [].valueOf doesn't return a primitive, so [].toString is used, which evaluates to ''

[] * 1    === 0;  // true

This is true because, as one of the operands is a primitive, arithmetic is used rather than concatenation. '' * 1 evaluates to 0 * 1, which equals 0

false + 1 === 1;  // true

Both operands here are primitives, so arithmetic will be used. Number(false) gives us 0, so 0 + 1 equals 1

From this article

If at least one operand is an object, it is converted to a primitive value (string, number or boolean)

After conversion, if at least one operand is string type, the second operand is converted to string and the concatenation is executed

In other case both operands are converted to numbers and arithmetic addition is executed.

The object to primitive conversion:

If object type is Date, then toString() method is used;

In other case valueOf() method is used, if it returns a primitive value;

In other case (if valueOf() doesn't exist or doesn't return a primitive value), then toString() method is used. This happens most of the times.

Javascript confusion with types and results of adding empty arrays and objects

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