For the first, I frequently (for my own projects) change the Array prototype to have Array.prototype.toString = function () { return '[' + this.join('

For the first, I frequently (for my own projects) change the Array prototype to have Array.prototype.toString = function () { return '[' + this.join(', ') + ']'; }; It would be convenient if Object.prototype.toString by default threw an exception, since the default breaks the embedding rule I described. The second example is misleading, and it has nothing to do with the rules for addition: the {} at the beginning of the expression is parsed as a code block, played off the rules for what the value of a Javascript statement is. More realistically, we have ({}) + [] == "[object Object]" but the original is, effectively, { } console.log(+[]);