What are false and true in Javascript?

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Falsy in Javascript

In Javascript, these are considered falsy …

• boolean false
• ” – empty string
• 0 – number zero
• NaN – Not a Number
• undefined
• null

When placed in an if-branch, it results in a false condition…

However, null is not equivalent to false …

In fact null and undefined are not equivalent to anything except for themselves…

NaN is not equivalent to anything including itself…

Truthy in Javascript

Everything else is truthy.  So that means that some things that seem like they should be falsy are actually truthy.

For example, these all are truthy…

• {} – empty object
• “0” – the string “0”
• “false” – the string “false”

A function definition (empty or not and with return statement or not) is truthy.

A function call that returns undefined is falsy …

A function that has just a return statement, actually returns an undefined.

Is Empty Array Truthy or Falsy?

An empty array by itself is truthy…

Unless it is in a comparison …

in which case it compares positively to false.  This is because the comparison causes all elements of the array to be convert to string and then concatenated resulting in a empty string “” for an empty array, which is falsy.

 

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