# Binder Functions

Two critical binder functions are `bindSourceFile` and `mergeSymbolTable`. We will take a look at these next.

## `bindSourceFile`

Basically checks if `file.locals` is defined, if not it hands over to (a local function) `bind`.

Note: `locals` is defined on `Node` and is of type `SymbolTable`. Note that `SourceFile` is also a `Node` (in fact a root node in the AST).

TIP: local functions are used heavily within the TypeScript compiler. A local function very likely uses variables from the parent function (captured by closure). In the case of `bind` (a local function within `bindSourceFile`) it (or a function it calls) will setup the `symbolCount` and `classifiableNames` among others, that are then stored on the returned `SourceFile`.

## `bind`

Bind takes any `Node` (not just `SourceFile`). First thing it does is assign the `node.parent` (if `parent` variable has been setup ... which again is something the binder does during its processing within the `bindChildren` function), then hands off to `bindWorker` which does the *heavy* lifting. Finally it calls `bindChildren` (a function that simply stores the binder state e.g. current `parent` within its function local vars, then calls `bind` on each child, and then restores the binder state). Now let's look at `bindWorker` which is the more interesting function.

## `bindWorker`

This function switches on `node.kind` (of type `SyntaxKind`) and delegates work to the appropriate `bindFoo` function (also defined within `binder.ts`). For example if the `node` is a `SourceFile` it calls (eventually and only if its an external file module) `bindAnonymousDeclaration`

## `bindFoo` functions

There are a few patterns common to `bindFoo` functions as well as some utility functions that these use. One function that is almost always used is the `createSymbol` function. It is presented in its entirety below:

```typescript
function createSymbol(flags: SymbolFlags, name: string): Symbol {
    symbolCount++;
    return new Symbol(flags, name);
}
```

As you can see it is simply keeping the `symbolCount` (a local to `bindSourceFile`) up to date and creating the symbol with the specified parameters.


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