Message
Warning: SyntaxError: test for equality (==) mistyped as assignment (=)?
Error type
SyntaxError warning in strict mode only.
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
}