When do I get this error?

The "Expected an assignment or function call and instead saw an expression" error is thrown when JSLint, JSHint or ESLint encounters an expression with no effect. In the following example we have a conditional expression that will evaluate to true but has no other effect on the program:

var x = 1;
x === 1; // Evaluates to 'true'

Why do I get this error?

This error is raised to highlight a piece of useless and unnecessary code. The code will work as expected but since a lone floating expression has no effect on anything there is no point in it being there at all.

In general you would expect to see a statement which has an effect, such as assigning a value to a variable or invoking a function:

var x = 1;
x = 2; // Assignment instead of unused expression

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W030. This means you can tell JSHint to not issue this warning with the /*jshint -W030 */ directive.

In ESLint the rule that generates this warning is named no-unused-expressions. You can disable it by setting it to 0, or enable it by setting it to 1.