Javascript Fallback Values on Variables and Booleans - a hard lesson

I discovered the other day that a feature I had written may have broken for months without me realizing it, and wanted to share it here (mostly as a note to my future self).

I had a user setting that was stashed in local storage - it could be either true or false, but if it wasn't set at all, I wanted that setting to be considered true. I wrote something like this:

const settingFromLocalStorage = window.localStorage.getItem('setting');

const settingValue = settingFromLocalStorage || true;

if (settingValue) { ... }

This was very neat and tidy, and I thought covered my bases by providing a fallback value.

This does not do what you think it does

Here is where I went wrong: I wanted to know if the setting was true or false and do different things based on that. I only wanted to default to true if there was no setting at all (i.e. it was undefined).

The problem with fallback variable assignment like this:

const settingValue = settingFromLocalStorage || true;

Is that it will fallback if the first option is falsy, not just undefined.


const settingValue = true || true; // evaluates to true


const settingValue = false || true; // ALSO evaluates to true

So my feature would never perform the behavior it expected when the setting was false.

I had fallen into this trap because of this pattern's use for passing parameters to a function:

// you'll see a lot of examples like this
const someFunction = (param) => {
  const someVar = param || `fallbackValue`;

I had gotten used to const someVar = param || 'fallbackValue'; as a way to deal with undefined values and supplying a default. This works when you're dealing with strings, but it does not do what you want if you're dealing with booleans.

Hope that helps you!


Help is on the way!

There is a proposal to add "Nullish Coalescing" to JavaScript to deal with this exact use case.

You can see the proposal here.

The gist of it is that the following syntax will do what I thought I was doing:

const settingValue = settingFromLocalStorage ?? true;

It has already landed in Safari Technology Preview, and there is a babel plugin to support it.