The JavaScript warning "test for equality (==) mistyped as assignment (=)?" occurs when
there was an assignment (=) when you would normally expect a test for
equality (==).
Message
Warning: SyntaxError: test for equality (==) mistyped as assignment (=)?
Error type
(Firefox only) SyntaxError warning which is reported only if
javascript.options.strict preference is set to true.
What went wrong?
There was an assignment (=) when you would normally expect a test for
equality (==). To help debugging, JavaScript (with strict warnings enabled)
warns about this pattern.
Examples
Assignment within conditional expressions
It is advisable to not use simple assignments in a conditional expression (such as
if...else),
because the assignment can be confused with equality when glancing over the code. For
example, do not use the following code:
if (x = y) {
// do the right thing
}
If you need to use an assignment in a conditional expression, a common practice is to put additional parentheses around the assignment. For example:
if ((x = y)) {
// do the right thing
}
Otherwise, you probably meant to use a comparison operator (e.g. == or
===):
if (x === y) {
// do the right thing
}