Adding arrays and objects in javascript produces an error

Question

I get these outputs when the following operations are carried out

var t = {} + {}; // Alerts [object Object][object Object]
t = {} + 1 //Alerts [object Object]1
t = {} + "hello" //Alerts [object Object]hello
t = {} + function(){} //Alerts [object Object]function(){}
t = {} + [] //Alerts [object Object]

In the last case alone it alerts [Object object] shouldn't it display [object Object][object Object] for this too?

Tested in Firefox12.0.

Jon · Accepted Answer · 2012-05-17 10:21:54Z

7

No, because the second part of the output is the stringified form of the empty array []. Arrays are stringified as a comma-separated list of stringified values, so the empty array stringifies to the empty string. You can confirm this with console.log([] + "" === "").

Therefore, {} + [] results in the equivalent "[object Object]" + "".

answered May 17, 2012 at 10:21

Jon

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1 Comment

RobG Over a year ago

Just to add that when evaluating the addition operator +, each expression is converted to a primitive by calling its toString method (if it has one).

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