Question
Answer and Explanation
The message "unexpected use of 'self'" from a bug finder or linter typically means you're using the self keyword in a context where it's not expected or valid in JavaScript. This usually arises in one of the following situations:
1. Outside of a Class Method:
- In JavaScript, self (and equivalently this in most cases) typically refers to the current object instance of a class. If you use self within a function that is not part of a class's method or outside of any object's context, it becomes ambiguous, and that's when the bug finder flags it as unexpected.
2. In Arrow Functions within Class Methods:
- Arrow functions do not bind their own this. They inherit it from the enclosing scope. Therefore, if you intend to refer to the class instance inside an arrow function, and you are attempting to use self (which would be an attempt to access it through a different approach than inheritance), it will also be seen as unexpected. You should use this in the enclosing method instead.
3. Attempting to Use `self` in a non-browser environment:
- While in browsers self can refer to the global scope (`window`), in other environments like Node.js, self is not defined by default at the top level, and using it may produce unexpected results, hence the warning.
4. Confusion with Python's `self`:
- Sometimes programmers coming from Python may accidentally use self in JavaScript classes. While the concept of instance reference is similar to Python’s self, JavaScript uses this.
Here are a couple of code examples to illustrate:
Example of incorrect use of self:
function myFunction() {
console.log(self); // Unexpected use of 'self'
}
Example of using this in a class method with an arrow function:
class MyClass {
constructor(value){
this.value = value;
}
myMethod() {
setTimeout(() => {
console.log(this.value); // correct usage of this
}, 100);
}
}
Recommendations:
- Review your code carefully where the bug finder flags the self usage.
- If you're inside a method of a class and intend to refer to the instance, use this.
- If you're in a browser environment at the global level and that was your intention, you can leave it or use window to be explicit.
- In any non-browser environment, avoid using self at the top level; use the provided alternatives if available.
By understanding these contexts, you can adjust your JavaScript code to avoid the "unexpected use of 'self'" warning and ensure proper functionality.