Function resolver

pub fn resolver(
    unresolved_mark: Mark,
    top_level_mark: Mark,
    typescript: bool,
) -> impl Pass + VisitMut + 'static

Available on crate feature __ecma and (crate features __ecma_transforms or __testing_transform) only.

See Ident for know how does swc manages identifiers.

When to run

The resolver expects ‘clean’ ast. You can get clean ast by parsing, or by removing all syntax context in ast nodes.

What does it do

Firstly all scopes (fn, block) has it’s own SyntaxContext. Resolver visits all identifiers in module, and look for binding identifies in the scope. Those identifiers now have the SyntaxContext of scope (fn, block). While doing so, resolver tries to resolve normal identifiers (no hygiene info) as a reference to identifier of scope. If the resolver find suitable variable, the identifier reference will have same context as the variable.

Panics

top_level_mark should not be root.

Example

let a = 1;
{
    let a = 2;
    use(a);
}
use(a)

resolver does

1. Define a with top level context.

2. Found a block, so visit block with a new syntax context.

3. Defined a with syntax context of the block statement.

4. Found usage of a, and determines that it’s reference to a in the block. So the reference to a will have same syntax context as a in the block.

5. Found usage of a (last line), and determines that it’s a reference to top-level a, and change syntax context of a on last line to top-level syntax context.

Parameters

unresolved_mark

Mark applied to unresolved references.

A pass should accept this Mark if it’s going to generate a refernce to globals like require.

e.g. common_js pass generates calls to require, and this should not be shadowed by a declaration named require in the same file. So it uses this value.

top_level_mark

Mark applied to top-level bindings.

NOTE: This is not globals. This is for top level items declared by users.

A pass should accept this Mark if it requires user-defined top-level items.

e.g. jsx pass requires to call React imported by the user.

import React from 'react';

In the code above, React has this Mark. jsx passes need to reference this Mark, so it accpets this.

This Mark should be used for referencing top-level bindings written by user. If you are going to create a binding, use private_ident instead.

In other words, this Mark should not be used for determining if a variable is top-level. This is simply a configuration of the resolver pass.

typescript

Enable this only if you are going to strip types or apply type-aware passes like decorators pass.

FAQ

Does a pair (JsWord, SyntaxContext) always uniquely identifiers a

variable binding?

Yes, but multiple variables can have the exactly same name.

In the code below,

var a = 1, a = 2;

both of them have the same name, so the (JsWord, SyntaxContext) pair will be also identical.