You attempted to assign a value to a function result. The result of a function can be assigned to a variable, but it cannot be used as a variable. If you want to assign a new value to the function itself, omit the parentheses (the function call operator). The following example demonstrates a situation in which this error is generated.
myFunction() = 42; // Attempting to assign the value 42 to the result of the function call.
To correct this error
myVar = myFunction(42);
myFunction = new Function("return 42;");