What exactly is Type Coercion in Javascript?

Question

What exactly is type coercion in Javascript?

For example, on the use of == instead of ===?

Answer 1

Let’s start with a short intro to type systems which I think will help you understand the general idea of type coercion.

The type system of a language defines rules that tell us what types of data exist in that language and how they can be combined using different operators. For example, one such rule might specify that the plus (+) operator only acts on numbers. These rules exist primarily to prevent you from shooting yourself in the foot. But what happens when the programmer breaks that rule in the program? There’s nothing preventing the programmer from typing {} + {} or “hello” + 5 in a program even if the language doesn’t think those expressions make any sense.

What ultimately happens in those situations depends on how strict the language is about its type rules.

A languages type system often holds one of two positions about you breaking its rules:

1. Say “Hey, that’s not cool!” and immediately crash your program.
2. Say “I can’t do anything with {} … but I can do something with numbers” and attempt to convert {} to a number.

Languages with type systems that take the first position about its rules are colloquially referred to as “strongly typed” languages. They are strict about not letting you break its rules. Those that take the second approach (such as JavaScript) are referred to as “weakly typed” or “loosely typed” languages. Sure, you can break the rules, but don’t be surprised when it converts the type of data you described in your program by force in order to comply with its rules. That behavior is known as … (drum roll) ... type coercion.

Now let's look at some examples in JavaScript. First, let's start with an expression that does not lead to type coercion.

5 + 5

Using the + operator with two numbers which is perfectly valid. The program will treat + to mean “add” and happily add the two numbers. No conversion necessary.

But what about …

[] + 5

Uh oh. In JavaScript, + can mean add two numbers or concatenate two strings. In this case, we have neither two numbers nor two strings. We only have one number and an object. According to JavaScript's type rules, this makes no logical sense. Since it’s forgiving about you breaking its rules, instead of crashing it tries to make sense of it anyway. So what does JavaScript do? Well, it knows how to concatenate strings, so it converts both [] and 5 into strings and the result is string value “5”.

What’s the deal with the comparison operators == and ===? Why are there two comparison operators?

== is not immune to JavaScript’s type conversion behavior. Expressions such as 5 == “5” will evaluate to true because JavaScript will attempt to convert one of them so that it’s comparing the same type of data.

In many cases, that’s not desirable because you probably want to know if some data you’re comparing against is of a different type so that you can decide what to do about it. That’s where the === operator comes in. When you use ===, no type conversion will take place. Therefore, the expression 5 === “5” will evaluate to false.