Home » Javascript difference between “=” and “===” [duplicate]

Javascript difference between “=” and “===” [duplicate]

Solutons:


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

You can read about assignment operators here on MDN.

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

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.

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