🍉 In JS, is an empty Array return false or true?

😃 General

When I was coding, there was a situation that if an array has values, it will return true. So I wrote like this bool = !!array. But, unexpectedly when array = [], bool is false, not true.

🤔 Why?

• First, I thought if empty string or empty Array is conversed to Boolean, it must return false.
• But it is not.

// Conversion
> Boolean("")
false
> Boolean([])
true
// Implied Conversion
> !!""
false
> !![]
true

• In Conversion, the result is like this, so I thought '' is false, [] is true.

// Loose Equality
> "" == false
true
> [] == false
true

• But here in Loose Equality, the result is '' is false(of course), [] is false too.
• I found a doc about this. Equality comparisons and sameness
• In Loose Equality, [] is Object, and false is Boolean. So in the table, it is applied that ToPrimitive([]) == ToNumber(false) and recursively ToNumber("") === 0. Eventually, it returns true.
• I certainly realized when I saw this table. plz memorize it.

toBoolean Table

🤖 Conclusion

• Because Array is type of object, the fact that an empty Array is conversed to true is correct.
• But in Loose Equality, [] == false is right.
• So, above bool = !!array code can be changed bool = array == false.
• Or, you can make function that checks array === undefined or array.length === 0. 🥝

📖 reference

• Equality comparisons and sameness
• Is A JavaScript Empty Array Falsey Or Not?
• toBoolean Table

Personal blog by Jon Jee.
I hope to explain with words and code.