TypeScript - null versus undefined

I made some research few months ago and I came out with the fact that undefined must be prioritize using tslint rule 'no-null-keyword'.

I tried to change my codebase and I had some issues. Why? Because I'm using an API that return null for empty fields.

I struggled because of the tslint triple-equals rule.

if (returnedData === undefined) // will be false because returnedData is null

Which let you 2 options:

1) Add some parameters to your triple-equals rules.

"triple-equals": [true, "allow-null-check"] and do If (returnedData == null)

allow-null-check allow "==" for null

2) Use If (returnedData) instead but it checks if null/undefined/empty string or a zero

No rationale is necessary for guidelines, the requirement could be chosen at random in order to keep the code base consistent.

As for this guideline, undefined takes more characters to type but doesn't need to be explicitly assigned to nullable variable or property:

class Foo {
  bar: number|undefined;
}

function foo(bar: number|undefined) {}

vs.

class Foo {
  bar: number|null = null;
}

function foo(bar: number|null = null) {}

Also, it's less convenient to check the type of null value at runtime because typeof val === 'object' for both null and object value, while it's typeof val === 'undefined' for undefined.

There is relevant TSLint rule that addresses this concern, no-null-keyword.