Why it is not good to use "!= null" to test if argument is passed
javascript
Solution
`!= null` is false for `undefined`, it basically means `x !== null && x !== undefined`. `!=` does type coercion, and one of the effects of that is that it treats `null` and `undefined` as equivalent (`null == undefined` is true).
So unless you want that, some people recommend not using `!=` with `null` (or `undefined`).
And you're quite right that the code defends against two (well actually three) possibilities:
Nothing was passed for the argument at all.
`null` was passed.
`undefined` was passed.
But the comment says it's a bad way to test whether an argument was passed, and it is: Because it doesn't catch passing `null` or `undefined`. (Passing in `null` and not passing in anything are different cases.) That doesn't necessarily mean it's a bad way to check the argument for a specific function.
Problem
From the book Maintainable JavaScript it mentioned : ``` // Bad: Testing to see if an argument was passed function doSomething(arg1, arg2, arg3, arg4){ if (arg4 != null){ doSomethingElse(); } } ``` but I feel using `!= null` here is quite effective, it filtered both the case of argument not passed or it is passed as null any reason why the author suggest it is bad ?