What values are truthy and falsy in JavaScript?

20 Apr 2019

In most languges, conditional control statements often require expressions that evaluate to boolean values: one of true or false. In JavaScript however, non-boolean values can also be used in conditional control statements. Values that are treated as false but are not actually false themselves are said to be falsy. The same goes with truthy values with respect to true.

Here’s a simple example, where we have a string value being used in place of a boolean.

  
    const test = 'hello there'
    if (test) {
      console.log('This value is truthy') // <---- This will run
    } else {
      console.log('This value is falsy)
    }
  

Since non-empty strings are considered truthy in JavaScript, the first console.log statement runs.

Use cases

There are a few common cases in which people tend to use non-boolean values as booleans.

Default values

Since null and undefined are falsy, they’re often used in conditional statements to check if a variable has been assigned a value, and then assign one if it hasn’t.

  
    let test
    if (!test) {
      test = 'this is now assigned'
    }

    console.log(test) // => 'this is now assigned'
  

We can also rewrite this example to be a little more brief, like so:

  
    let test
    test = test || 'this is now assigned'

    console.log(test) // => 'this is now assigned'
  

Non-zero number checking

0 is also falsy in JavaScript, so when dealing with numbers we can use if statements and similar language constructs to both test if a number is 0.

Non-empty string checking

Since the empty string ('') is considered falsy, we can easily use it in conditionals when dealing with strings in our application logic. For example, imagine a file that has been read into memory in an array and split by newline, with the ending \n stripped out. If we wanted to filter out empty lines, we could do:

  
    const allLines = ['this is a line', '', 'another line', 'this is also a line']
    const nonEmptyLines = allLines.filter(line => line)

    console.log(nonEmptyLines) // => ['this is a line', 'another line', 'this is also a line']
  

Finally, here’s a handy reference of what JavaScript considers truthy and falsy.

List of truthy values

• All objects, including {} and []
• Empty arrays
• Non-empty strings, including ' '
• All numbers, including negative ones
• true
• functions

List of falsy values

• ''
• 0
• null
• undefined
• NaN
• false