What condition is necessary to show that `sale` is `true`?

You need to know that = (a single equals sign) is an assignment operator and literally assigns a value to a variable.
To check if a conditions is true for now use == (double equals).
if you had typed

if (sale == true) {

}

that kinda would have been logically correct. But doing if(sale) {…} is a SUPER common shorthand that will result in a valid output. And this will always check for the condition to evaluate to TRUE.
I hope this helps.

When using an if statement, the statement is being evaluated if it’s true. And because we set the sale variable (let sale= true), the statement will be evaluated as true. Once our if statement is evaluated as true, the code block containing the console.log will be executed.

If we set the sale variable to false (let sale = false), the if condition is evaluated to false and therefore the code block will not be executed. Hope this helps!