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ast-types-flow

Flow types for the Javascript AST. Based off of benjamn/ast-types.

# Usage

First install `ast-types-flow` via npm, then you can import any of the types that are exported.

/* @flow */
import type {Node} from 'ast-types-flow';
function getName(node: Node): string {
switch (node.type) {
case 'Identifier':
return node.name;
case 'ClassDeclaration':
return node.id.name; // Error, id could be null.
case 'FunctionDeclaration':
return node.id.name; // Fine if it's always there.
case 'FunctionExpression':
if (node.id) {
return node.id.name; // Can refine id to make sure it exists.
} else {
return 'Unknown';
}
case 'Literal':
return node.name; // Error, Literals don't have names, don't be silly.
}
return 'Unknown';
}

# How it works

A notion of "extends" is added to the Flow syntax via comments. A transform is included that will compile the source code into useful disjoint union types based on how the different types extend each other. For example:

type Node = {
common: string,
};
type Foo = {
// extends Node
foo: string,
};
type Bar = {
// extends Node
bar: number,
};

Will be transformed into:

type Node = {
type: 'Foo',
_Foo: void,
common: string,
foo: string,
} | {
type: 'Bar',
_Bar: void,
common: string,
bar: number,
};
type Foo = {
type: 'Foo',
_Foo: void,
common: string,
foo: string,
};
type Bar = {
type: 'Bar',
_Foo: void,
common: string,
bar: number,
};

A few things to note:

1. The type `Node` would more ideally be compiled into `Foo | Bar` but then the disjoint union cannot be properly refined. For now we have to duplicate the complete definitions.
2. Each entry in a disjoint union has to be structurally unique or Flow will have an error on the definition. That is why the private `_Foo: void` fields appear in the types.