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Javascript ES6 spread operator on undefined

Posted by: admin November 1, 2017 Leave a comment


While developing my react App, I needed to send a conditional prop to a component so I found somewhere a pattern to do so, although it seems really weird to me and I couldn’t understand how and why it worked.

If I type:

console.log(...undefined)   // Error 
console.log([...undefined]) // Error
console.log({...undefined}) // Work

When the spread operator is activated on undefined an error is raised, although when the undefined is inside an object, an empty object returned.

I’m quite surprised regarding this behavior, is that really how it supposed to be, can I rely on this and is that a good practice?


Seems like it was a mistake in the language or a rather “unplanned” feature. Take a look at this