Daniel Matthew

Truthy and Falsy Values in JavaScript

A common activity when writing code is to check whether a result is true or false before using it. You may have seen the following pattern: used to assign a default value to a parameter, should it be omitted.

function chosenColour(colour) {
  colour = colour || "blue";

  // Colour related code goes here
}

When a non-boolean value is used in this logical OR operation, it is coerced to become one. If the value on the left is empty - or false - the value on the right will be used as a replacement.

It's possible to discover how a value will be interpreted by passing it as an argument to Boolean():

Boolean(3) // true
Boolean('foo') // true
Boolean({}) // true
Boolean([]) // true

Boolean(undefined) // false
Boolean(null) // false
Boolean(false) // false
Boolean('') // false
Boolean(NaN) // false

Daniel is a front-end developer at Talis, with an interest in accessibility, performance, and web components.