Understanding the JavaScript Interview Question: console.log(([][[]] + [])[+!![]]+([]+{})[+!![]+ + !![]])

The interview question asks what the output of console.log(([][[]] + [])[+!![]]+([]+{})[+!![]+ + !![]]) is. It tests understanding of type conversion, implicit coercion, and operator behavior in JavaScript.

Knowledge required to solve this problem

The unary + operator performs the same conversion as Number().

When + is used for addition and either operand is an object, number, or boolean, the engine calls toString() on the operands.

The logical NOT operator ! always returns a boolean.

Double NOT !! forces a value to a boolean.

Solving the expression ([][[]] + [])[+!![]]+([]+{})[+!![]+ + !![]]

First, evaluate ([][[]] + []). Inside this, [][[]] accesses an empty array with a key of []. The empty array is converted to an empty string, so the property name becomes "", which does not exist, yielding undefined. Adding [] to undefined triggers implicit conversion: undefined becomes the string "undefined" and [] becomes "", resulting in the string "undefined".

Next, evaluate [+!![]]. !![] converts an empty array to true; the unary + then converts true to the number 1. Thus [+!![]] is [1], and ([][[]] + [])[+!![]] accesses the character at index 1 of "undefined", which is "n".

Then evaluate ([]+{})[+!![]+ + !![]]. The sub‑expression []+{} concatenates an empty array and an empty object, producing the string "[object Object]". The index expression +!![]+ + !![] evaluates to 1+1, i.e., 2. Accessing index 2 of "[object Object]" yields the character "b".

Finally, concatenate the two characters obtained: "n" + "b" = "nb". Therefore the console logs nb.

In summary, the difficulty of this question often stems from not understanding the evaluation order and how expressions like [][[]] are interpreted: the empty array’s key [] is coerced to an empty string, leading to undefined, which then participates in further implicit conversions.