Boolean Conversion in JavaScript // Boolean(value) VS !!value

There are two key ways people convert truthy and falsy values to true and false in #JavaScript. This tutorial discusses the two methods. We also present which one we recommend, so that it should be your default (with reasons) 🌹

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#basarat

0:00 The plan
0:22 The ways
1:27 The better way
4:06 Smash that like

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I'm Basarat, and I love helping developers. More about me:

Microsoft MVP for TypeScript
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Book Author : TypeScript Deep Dive
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[IainSimmons]

Great little video. I had always wondered this, but preferred the double bangs!! 🙂

[timocov]

In this case I'd say that it's better to use val !== null over implicit casting to boolean value because this check is the way more explicit and clearly shows your intension of this check, especially if a value might be a string of a number. Explicit is better than implicit.

As for the topic I'd say that you need to use any of this (I prefer Boolean() since it's more explicit and a minifier will replace it with !! anyway) only in case of converting a result to boolean, i.e. when the result of an expression should be a boolean, e.g. when you return something from a function or declare a variable.

[danieljanjanocha]

Nice video. What I'd personally add is a use case that Boolean looks better in, that is in array.prototype functions such as array.prototype.filter.
values.filter(v=>!!v) vs values.filter(Boolean)

[avi]

This seems like a bug in the TypeScript interpreter. Why does TS treat them differently if they are JS runtime equivalent?

[ficoprieto]

I think the second use case of `!!` is redundant, as neither `!!` or `Boolean()` is required in this case. It seems that JS/TS automatically cast a value to its truthy form when used as a condition in boolean logic.

e.g. `valueFromNetwork && use(valueFromNetwork)` is valid, providing both the expected runtime result & accurate type linting.