Javascript difference between "=" and "==="

Question

I struggle to understand the function below. I didn't know why my script wasn't working until I changed = with === in the if statement, as shown below. Why does === work while = doesn't?

var testTest = function(answer) {
    if (answer === "doggies") {
       return "My favorite animal!";
    } else {
       return "Tested";
    }
};
testTest("doggies")

When I type doggies, it shows me My favorite animal! With anything else, it returns Tested as it should.

However, when I change the === in the if statement with =, the else part doesn't work.

var testTest = function(answer) {
    if (answer = "doggies") {
       return "My favorite animal!";
    } else {
       return "Tested";
    }
};
testTest("elephant")

`=` is an assignment operator and `===` is a comparison operator. Now look forward for these both you will find tons of articles. — Dhaval Marthak, Jun 18 '15 at 08:41
@AlexG No, this question is different. It is still a bad question, but different. — Ismael Miguel, Jun 18 '15 at 08:42
there are actually 3 different `=`, `==`, and `===`. the first is for *assignment*, the other two are for *comparison*. — Claies, Jun 18 '15 at 08:42

score 5 · Answer 1 · answered Jun 18 '15 at 08:42

5

You need to use == or === for equality checking. = is the assignment operator.

You can read about assignment operators here on MDN.

answered Jun 18 '15 at 08:42

Lloyd

29,197
4
84
98

score 3 · Accepted Answer · answered Jun 18 '15 at 09:44

3

As a quick reference as you are learning JS:

=   assignment operator
==  equal to
=== equal value and equal type

!=  not equal
!== not equal value or not equal type

answered Jun 18 '15 at 09:44

GibboK

71,848
143
435
658

score 0 · Answer 3 · answered Jun 18 '15 at 08:54

I assume you know that = is for assignment, after all, you are using assignment already in the first line:

var testTest = function(answer) {

and I don't think you think that this would compare anything here (or do you?).

The question remains though, why does = in if (answer = "doggies") "not work"?

An assignment is an expression. The result of that expression is the value that was assigned. Here, the result of answer = "doggies" is "doggies", i.e. you essentially running if ("doggies").

JavaScript performs type coercion. That means it automatically converts values of one data type to values of a different data type if necessary, according to specific rules.

The condition of an if statement has to resolve to a Boolean value. But here you are using a string value as condition. The String -> Boolean conversion rules are pretty simple:

The empty string converts to false.
A non-empty string converts to true.

So, after type conversion, the statement is equivalent to if (true), hence it will always execute the first block, never the else block.

Javascript difference between "=" and "==="

3 Answers3