What does "defaults" contain?

Question

I'm working with the Esprima parser, it outputs an AST format which is compatible with the Mozilla Spider Monkey Parser API.

In the Mozilla Docs, it specifies the Function node as:

interface Function <: Node {
    id: Identifier | null;
    params: [ Pattern ];
    defaults: [ Expression ];
    rest: Identifier | null;
    body: BlockStatement | Expression;
    generator: boolean;
    expression: boolean;
}

What will the defaults property contain? It always appears as just an empty array.

Answer 1

defaults of Mozilla JS AST contains ES6 default parameter values.

For example,

    function t(i = 20) { }

defaults will be [{ type: 'Literal', value: 20 }].

Because it is based on ES6 draft, Esprima master branch doesn't recognize it.