The behavior of a Javascript compiler is doubly rewarding when working with an array of empty arrays or an array of zeros, as the

Question

The behavior of a Javascript compiler is doubly rewarding when working with an array of empty arrays or an array of zeros, as the

I've been looking for an answer to my question, but so far I haven't had any luck finding it through search engines. It seems like using the plus sign '+' in a search query is not very effective with Google or Bing.

Nevertheless, why does this result in zero?

+[[]][0] // = 0

while this results in one?

++[[]][0] // = 1

UPDATE: Michael Berkowski provided a good explanation, but there's still one part that confuses me.

If [[]][0] evaluates to an empty array, then why does ++[] yield a

ReferenceError: Invalid left-hand side expression in prefix operation

?

UPDATE 2: I think I understand now...it seems like I was mistakenly trying to type ++0 in the console and getting an error, when actually I should be using var a = 0; ++a.

javascript

Answer 1

Answer №1

The most effective way to understand this is by analyzing how each part of it is calculated and evaluated.

When you have [[]][0], it just results in an empty array []. Adding a unary plus + before it converts the string representation into an integer 0, similar to +4 or -3. So, +0 equals 0.

Using ++ as a numeric operator also changes the empty string into an integer 0, but with the prefix increment operation, it becomes 1.

[[]][0]
// [] empty array
[[]][0].toString()
// ""

// Unary + converts the empty string to an integer
+("")
// 0

// Prefix increment on an empty string gives 1 (increments the previous 0)
var emptyString = "";
++emptyString;
// 1

Answer 2

The most effective way to understand this is by analyzing how each part of it is calculated and evaluated.

When you have [[]][0], it just results in an empty array []. Adding a unary plus + before it converts the string representation into an integer 0, similar to +4 or -3. So, +0 equals 0.

Using ++ as a numeric operator also changes the empty string into an integer 0, but with the prefix increment operation, it becomes 1.

[[]][0]
// [] empty array
[[]][0].toString()
// ""

// Unary + converts the empty string to an integer
+("")
// 0

// Prefix increment on an empty string gives 1 (increments the previous 0)
var emptyString = "";
++emptyString;
// 1

The behavior of a Javascript compiler is doubly rewarding when working with an array of empty arrays or an array of zeros, as the

Answer №1

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