Most of us think that the difference between == equality operator and === equality operator in JavaScript is that == equality operator compares two values while === equality operator compares the value and it’s type. Basically, this is a wrong concept. What actually is the difference then? Before I tell you about the difference, you first have to know about “coercion” in JavaScript. What exactly is “coercion”? “coercion” is just a fancy term for type conversion. For example, you want to convert a number into a string this conversion is called “coercion”.
var a = “11”;
var b = Number(a);
console.log(a); // “11”
console.log(b); // 11
typeof a; // ”string”
typeof b; // “number”
As you know what “coercion” is, now let’s get back to our topic, What is the difference between == equality operator and === equality operator in JavaScript? The best way to explain is that == compares two values with coercion allowed, and === compares two values without coercion allowed.
For example:
var a = “11”;
var b = 11;
a == b; //true
a === b; //false
In above example a == b JavaScript finds out that the types don’t match as a is of “string” type and b is of “number” type, then it coerces one or both values until both type match and then simple value comparison is done. In above example a == b, to give the result true, you can think JavaScript coerce value and end up with either 11 == 11 or “11” == “11”.
The a === b produces false as there is no coercion allowed, hence failing simple value comparison.
Note: Reference is taken from book by Kyle Simpson You Don’t Know JS: Up and Going