It's become a common pattern in the JavaScript world to use the || (called the "logical or") operator for the purpose of setting a default value for a function argument. While it works quite well in most cases, but there are two cases where troubles may arise.

Common use cases

The || operator is often used to give an argument of a function a default value in the case that the argument isn't passed.

function getUserMessages(opts) {
  const options = opts || {}
  const headers = options.headers || {}

  axios.get('/api/users/messages', { headers })
}

This ensures that there is no Cannot read property 'headers' of undefined error when accessing headers on options because options is guaranteed to never be undefined or null.

How it works

This gist of the || operator is this: Is the left operand truthy?

If yes, return the left operand. If no, return the right operand.

console.log(undefined || { name: 'Jim' }) // { name: 'Jim' }
console.log(null || { name: 'Jim' }) // { name: 'Jim' }
console.log(0 || { name: 'Jim' }) // { name: 'Jim' }

console.log(42 || { name: 'Jim' }) // 42

Interestingly, if the left operand is truthy, the right operand is never evaluated. To illustrate this point, if the right operand is a function call, e.g. 42 || getUserAge(), then getUserAge would never be called and || operation would return 42.

// getUserAge is never run
console.log(42 || getUserAge()) // 42
Falsey values
The falsey values are null, undefined, NaN, 0, false, and empty strings. All other values are considered truthy.

This pattern works well with function parameters because parameters default to undefined when they are not passed.

function getUserMessages(opts) {
  const options = opts || {}
  const headers = options.headers || {}

  axios.get('/api/users/messages', { headers })
}

With the above example, if getUserMessages is called with no arguments, opts is automatically set to undefined. Since opts is undefined, and undefined is a falsey value, the || operator would yield the right operand.

This also works with options.headers || {} because properties return undefined when they don't exist in an object.

The gotcha

The problem comes when using || with numbers and strings, where at the same time 0 and "" should be considered valid values.

function stringify(data, options) {
  const indent = options.indent || 2

  return JSON.stringify(data, null, indent)
}

const json = stringify({ lastName: 'Halpert' }, { indent: 0 })
console.log(json) // ​​​​'{"​​​​\n  "lastName": "Halpert"​​​​​​​​​\n}'
//                        ▲
// The json is indented with 2 spaces instead of 0

The goal above was to default the indent to 2 in the case that the indent option wasn't passed. However, 0 is a falsey value, and thus the || operator returns the right operand's value, resulting in an indent of 2 when it should be 0.

There are a safer ways

There are two ways to achieve default values. One way is using the newer ES2015 features called object destructuring along and default value assignment, and the other is to just use good old if statements or ternaries. Let's take a look at the ES2015 solution.

function stringify(data, { indent: indent=0 } = {}) {
  return JSON.stringify(data, null, indent)
}

const json = stringify({ lastName: 'Halpert' })
console.log(json) // '{"lastName":"Halpert"}'

There are two parts to the syntax used in the function parameters above, which are object destructuring and default value assignment. "Object destructuring" is the process of assigning an object's property to a variable. By itself, it takes on the syntax of { propertyName: variableName }, and looks as follows when used inside function parameters:

//       the property we want to assign to a variable
//                           ▼
function stringify(data, { indent: indent }) {
//                                    ▲
//       the name of the variable we want create

// the above is just another way of doing this
function stringify(data, options) {
  const indent = options.indent

This syntax says, "I expect the second argument to be an object, and I want to assign the indent property of that object to a variable named indent". There is also a shorthand for this syntax that allows the omission of the variable name if the variable and property have the same name.

function stringify(data, { indent }) {

// the above is still just the equivalent of
function stringify(data, options) {
  const indent = options.indent

Along with destructuring, we can default our indent variable with a value of 2 using default value assignment. Destructuring default assignment takes on the following syntax { propertyName: variableName = defaultValue }, where defaultValue can be a number, a string, NaN, undefined, an array, a function call — well, you get the point. I would highly recommend keeping the default values simple for readabilities sake, though.

A note on destructuring
Destructuring isn't limited to being used in function paramaters, but can also be used with normal variable assignments such as:
const { indent: indent } = options;

In the original example, I also used what are called "parameter defaults" (another newish ES2015 feature) in order to default the second parameter to an object. This allows the complete omission of the second argument when calling stringify(data), and is done to avoid the destructuring from throwing a Cannot read property 'indent' of undefined error.

Parameter defaults take the form of:

function (parameterName = defaultValue) {

but, we can use destructuring in tandem with parameter defaults to create the full desired result.

//                 The second parameter will default to an object
//                                        ▼
function stringify(data, { indent = 0 } = {}) {

function stringify(data, options) {
  options = options === undefined ? {} : options

It's also worth noting that parameter defaults don't work the same as in Python or Ruby, where the default value is reused each time the function runs. Instead, the default value is evaluated every time the function runs. Yippee!

Caveat

The first caveat is that default value assignment and default parameters only work when the argument's ultimate value is undefined. It doesn't work with null.

function stringify(data, { indent=0 } = {}) {
  console.log(indent)
}

stringify({}, {}) // 0
stringify({}, { indent: undefined }) // 0

stringify({}, { indent: false }) // false
stringify({}, { indent: null }) // null

The second is that they are newish features that aren't supported by all browsers, mainly IE, older versions of Node, and some mobile browsers. But, this is why Babel was invented.

"That all looks complex"

You might think parameter defaults and destructuring look complicated, or maybe you're not completely understanding it right now. It definitely takes a while to understand and get used to. I would heavily suggest reading up more on destructuring and parameter defaults, since the syntax provides a much more succinct of writing code.