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copied!<p>Why does <code>(![]+[])[+!![]+[]]</code> produce "a"</p> <p>step by step: this is parsed in: <code>(![]+[])</code> and <code>[+!![]+[]]</code>. First bit was already explained by artemb: <code>[]</code> is an array. Negating it, <code>![]</code> evaluates to a boolean, <code>false</code> - that's just how <code>!</code> works when it is applied to something that is not <code>null</code> or undefined. Again as pointed out by artemb, appending this <code>+[]</code> forces the boolean to be converted to a string. That's because the <code>+</code> is a string concatenation operator. The boolean <code>false</code> is then converted to its string representation, <code>"false"</code>.</p> <p>Then, the second bit, <code>[+!![]+[]]</code>. First of all, the outer <code>[</code> and <code>]</code> serve to treat the preceding string, which we just dervived is equal to <code>"false"</code> as an array of characters. By putting an integer index inside the <code>[</code> and <code>]</code>, you get the character at a particular index. So what remains is <code>+!![]+[]</code> This consists of 4 pieces: <code>+</code>, <code>!![]</code>, <code>+</code> and <code>[]</code>. First <code>!![]</code> is evaluated. We already saw that <code>![]</code> is a boolean <code>false</code> so prepending another <code>!</code> negates it, and yields <code>true</code>. Next thing what happens is that the <code>+</code> in <code>+!![]</code> gets applied, and by applying <code>+</code> it converts the boolean <code>true</code> into the number representation, which is <code>1</code> (so <code>+true</code> is <code>1</code>) The <code>+[]</code> that follows makes a string again from that <code>1</code> yielding <code>"1"</code> but it does not really make sense, the shorter expression <code>(![]+[])[+!![]]</code> already produces <code>a</code>. Appending <code>+[]</code> doesn't hurt either, the resulting expression is simply <code>["1"]</code> instead of <code>[1]</code>. My hunch is that when <code>[]</code> is applied to an array, whatever is inside the <code>[]</code> will be coerced into a number, which for <code>"1"</code> would give <code>1</code> again. So either way, <code>+!![]+[]</code> evaluates to <code>1</code>, making the final expression: <code>"false"[1]</code> which is saying: gimme the character at index 1 from the string <code>"false"</code>, and since by default, arrays start at <code>0</code> in javascript, this is the second character of <code>"false"</code>, and <code>a</code>.</p>

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