JS Coercion

January 18, 2020

Let’s talk about Coercion today. Simply put, coercion is the process of changing a value from one type to another. Let’s see a very simple example.

const number = 42
const string = String(number)

We’re talking about explicit coercion in this example. We explicitly tell JavaScript to coerce a number into a string. The same is possible for other values. Let’s see some more examples of explicit coercion.

const num = Number("10")
const bool = Boolean(0)
const float = parseFloat("10.2")
const anotherNum = parseInt("19")
const string = String(99)

But there are also cases (actually a lot of them) where implicit coercion will happen in JS.

const arr = []

   // do something

You’ve probably written some code like this before. In this case, implicit coercion happens in the if statement, because the if statement takes a boolean as an argument and arr.length returns a number.

Another example of implicit coercion is, for example, when you use a number in a string literal like this:

const num = 42
const str = `${num} pizzas for our office, please`

In the string literal, num is implicitly coerced into a string. Of course, we could also do this explicitly:

const num = 42
const str = `${String(num)} pizzas for our office, please`

Also, coercion always happens, when you use a double equal to compare two values. It doesn’t happen, when you use the triple equal:

console.log("42" == 42) // true
console.log("42" === 42) // false

In the "42" == 42 case, implicit coercion is done by JS, to compare equal types. That’s why the comparison is true. The triple equal === does no implicit coercion, that’s why this comparison is false.

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