为什么我的JavaScript OR语句未返回正确的值？

Question

I'm stuck with the following:

foo = properties[field] || overrides.default || config.widget.default || undefined;

with

console.log(properties[field])     // undefined
console.log(overrides.default)     // 0
console.log(config.widget.default) // undefined

So as I understand, foo should be 0 . Yet... it's undefined

This still works:

foo = properties[field] || overrides.default;

But as soon as I add another OR option, it does not work anymore.

Question :
Why is my 2nd operand with value=0 not overriding the undefined operands?

Thanks!

Answer 1

The || operator returns its left-hand operand if it is truthy, otherwise it returns the right-hand operand.

In your example, all operands have falsey values, so the compound || statement returns the right-most value.

In JS, the following are all falsey: 0 , "" (empty string), undefined , null , NaN . And of course false . All other values are truthy (including strings like "0" or "false" because they're not empty strings).