What do square brackets around an expression mean, e.g. `var x = a + [b]`?

Question

We have received some JavaScript from an agency that looks wrong, but works.

For some reason they are adding square brackets ([, ]) around v

Answer 1

b = "bar" + ["foo"]

This is syntactically correct, but indeed very, very, superfluous. This is how it works:

["foo"]

JavaScript takes the string "foo" and converts it to an array with one element, "foo":

"bar" + ["foo"]

when + is used, and one of the operands is a string, "bar" in this case, JavaScript converts the second one to a string. Since operand two is an Array, the Array.toString method is called, which, by default, returns all elements joined by a comma. We have one element, and the result will be equal to this element, i.e., in this context "foo" is equivalent to ["foo"].

If you redefine Array.toString you can see better what's going on:

alert("bar" + ["foo"])
Array.prototype.toString = function() { return "???"; }
alert("bar" + ["foo"])