Why is "[][[]]" undefined in JavaScript

Question

Our Beloved JavaScript can be very weird...

(![] + [])[+!+[] + +!+[]]                // 'l'
    + (![] + [])[+!+[]]                  // 'a'
    + ([![]] + [][[]])[+!+[] + [+[]]]    // 'i'
    + ([][[]] + [])[+!+[] + +!+[]]       // 'd'
 // Will output "laid"

The reason why JavaScript produces this output mainly depends on type casting of array and ability of + to cast 'things' into string. ![] // false while +[] // 0 my question is how did JavaScript successfully parse [][[]] to be undefined but reject [][][] as syntax error

Quentin · Accepted Answer · 2019-08-01 10:42:06Z

6

[] means different things depending on the context

[][[]]

The first [] creates a new array (array literal syntax).

Then that_array[[]] tries to read a property from that array (square bracket property accessor syntax).

The property name is [] (also array literal syntax), which being an array, gets converted to a string: ''

Since there is no property named an empty string, it resolves to undefined.

In other words:

const array = [];
const propertyName = [].toString();
console.log(array[propertyName]);

[][][]

This errors because while the first [] creates an array, the second [] tries to access a property from it but fails to include a value for the property name. It has ] before the value, so the second ] is unexpected. It never gets to the third [].

answered Aug 1, 2019 at 10:42

Quentin

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