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Let's talk about "falsiness"
If you write a statement where you are testing if something is true or not--like an
if statement that uses a logical operator like ||, !, or &&--you expect that statement to return a boolean value of true or false. Sometimes it doesn't resolve to a boolean value--true or false--yet, nevertheless, it gets interpreted like a boolean value. We call those values "truthy" or "falsey".In Javascript, examples of falsy values are:
nullundefinedNaN (Not a Number)"" (empty string)false0Every other value--every value that is not one of the above--will evaluate to
true in a Boolean evaluation in Javascript. Let's go through them one by one. First, I'll demonstrate a variable assignment to each falsy value. Next, I'll show how that falsy value is is interpreted as
false using the Boolean() function, which takes a value or expression and always returns either true or false.A variable that resolves to
null is a variable that has been deliberately assigned an empty valuelet foo = null
console.log(foo) // nullAn
undefined variable is a variable that has not yet been assigned a value.let foo
console.log(foo) // undefined
console.log(Boolean(foo)) // falseA variable that resolves to
NaN can be the result of an invalid math operation, such as 0/0 or sqrt(-1). A number is expected from a math operation, but the expression is invalid, so the result is not a (valid) number.let foo = 0/0
console.log(foo) // undefined
console.log(Boolean(foo)) // falseJust what it sounds like: a variable that has been assigned to a string with no content.
let foo = ""
console.log(foo) // (actually, nothing shows up)
console.log(Boolean(foo)) // falseJust what it sounds like: a variable that has been assigned to a the boolean value
false.let foo = false
console.log(foo) // false
console.log(Boolean(foo)) // falseJust what it sounds like: a variable that has been assigned to
0 or that resolves to 0.let foo = 5 - 5
console.log(foo) // 0
console.log(Boolean(foo)) // false25