9 types of falsy values in JavaScript

You might have already come across (or may come across) the question below in a JavaScript/Frontend/Web Engineer interview.

What are the possible outcomes of the given code snippet?

function greet(value) {
    if (value) {
        return "Hello, World!";
    }
    return value;
}

To answer the above question, you first need to understand what is considered falsy in JavaScript. Before continuing with the post, take a while and try to answer this by yourself. Let me know your answers in the comments below.

Falsy values in JavaScript -

1. null

   The null value represents the absence of any value.

    Boolean(null)    // false
   

2. undefined

   The undefined property indicates that a variable has not been assigned a value, or not declared at all.

    Boolean(undefined)    // false
   

3. false

   The boolean value.

4. NaN

   The global property that represents Not-A-Number.

    Boolean(NaN)    // false
   

5. 0

   The number zero (including 0.0, 0x0).

    Boolean(0)    // false
   

6. -0

   The number negative zero (including -0.0, -0x0)

    Boolean(-0)    // false
   

7. 0n

   The BigInt zero (including 0x0n). BigInt values represent numeric values which are too large to be represented by the number.

    Boolean(0n)    // false
   

8. ""

   The empty string value (including '' and ``).

    Boolean("")    // false
   

9. document.all

   The only falsy object in JavaScript. This read-only property returns HTMLAllCollection rooted at the document node. This property is deprecated, some browsers might still support it.

    Boolean(document.all)    // false
   

So in total there are 9 falsy values in JavaScript and all other values are considered truthy.

Answer to the above question -

Above code snippet have 10 possible outcomes (9 falsy values and "Hello, World!" for all other values).