What exactly is Type Coercion in Javascript?

Question

Type coercion means that when the operands of an operator are different types, one of them will be converted to an "equivalent" value of the other operand's type. For instance, if you do:

boolean == integer

the boolean operand will be converted to an integer: false becomes 0, true becomes 1. Then the two values are compared.

However, if you use the non-converting comparison operator ===, no such conversion occurs. When the operands are of different types, this operator returns false, and only compares the values when they're of the same type.

You can find a good explanation of JavaScript's coercion rules in You Don't Know JS and more reference-oriented documentation in MDN.

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.

In Python if you try to add, say, strings and integers, you get an error:

>>> "hi" + 10
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects

Yet in JavaScript, you don't. The 10 gets converted to a string:

> "hi" + 10
"hi10"

"Type coercion" is just a fancy misnomer for the above. In actuality, neither language has "types" in the sense of Java or C or other languages with static type systems. How the languages treat interactions between the various non-statically-typed values is a matter of choice and convention.

let me explain type coercion with the following example

Type Coercion means Javascript automatically (on-the-fly) converts a variable from one datatype to another

Ex: 123 + "4" generally raises an error but in Javascript due to type coercion, it results in 1234 a string

if(23 == "23"){
    console.log(" this line is inside the loop and is executed ");
}

In the above code, because of type coercion - JavaScript thinks 23 (number) and "23" (string) are the same thing. this makes the condition true and prints the console.log

In the other case

if(23 === "23"){
   console.log(" this line is inside the loop and is NOT executed");
}

In === case Javascript doesn't do Type Coercion, and since 23 is a number and "23" is String and because of === these two datatypes are different and that leads to the false in condition. It does not print the console.log

In simple words

In this case = it is an assignment operator - which assigns values such as var a = 3;, etc

(below operators are for comparison)

In this case == Javascript converts/coerces the datatype to another and then compares it.

In this case === Javascript doesn't convert/coerces the datatype

In order to avoid bugs and for debugging purposes === is mostly used

Please let me know the accuracy of the above information.

What is coercion:

Type coercion in javascript occurs when the Javascript engine has to perform a certain operation for which it needs data to be in a certain type. When the engine encounters data in a certain type that is not applicable for the operation it then coerces the data into a certain type. This is needed because variables in javascript are dynamically typed, which means that a given variable can be assigned a value of any type.

Example:

---

if(1){
  // 1 gets coerced to true
}


if(4 > '3') {
  // 3 gets coerced into a number
}


44 == "44"  // true, the string 44 gets converted to a nr

Boolean coercion:

In javascript coercion, all values are converted to true except for the following values which are coerced to false:

console.log(!!"");         // false
console.log(!!0);          // false
console.log(!!null);       // false
console.log(!!undefined);  // false
console.log(!!NaN);        // false
console.log(!!false);      // false

Also notice that in the above example that the double ! operator is used. The ! mark operator coerces a value into a boolean with the opposite value. We can use this operator twice to convert any value into a boolean.