SyntaxError: test for equality (==) mistyped as assignment (=)?

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
}