[removed] What is the difference between `if (!x)` and `if (x == null)`?

Question

What is the difference between if (!x) and if (x == null); that is, when can their results be different?

Answer 1

Say x is a string.

x = undefined;
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

x = "";
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

x = null;
if(!x) {
   alert("X is not a truthy value");
}
if(x == null) {
   alert("X is null");
}

You'll notice that "X is not a truthy value" is shown in all three cases, but only in the case of X being undefined or null is "X is null" shown.

When X is a boolean value, then (!x) will be true when X is false but (x == null) will not be. For numbers 0 and NaN are considered false values, so not X is truthy.

See it in action, including the difference between == (equality using type conversion) and === (strict equality)