extend interface & `<:`

[xp44mm]

Can't understand what extend interface and <: mean, can anybody explain it?

the extend interface

extend interface Function {
    generator: boolean;
}

and <:

interface ForOfStatement <: ForInStatement {
    type: "ForOfStatement";
}

[bradzacher]

extend interface means that you take the original Function interface from an older version of the spec, and you augment it with the changes listed.

In your example it's saying that "the AST for this ES version adds the property generator: boolean to the Function AST node".

---

X <: Y means that X inherits from Y, and it lists the changes / additions.

In your example it's saying that the FoOfStatement AST node inherits its properties from the ForInStatement AST node, except the type property's value should be 'ForOfStatement' (instead of 'ForInStatement')

[nzakas]

Nice description 👍