# The `__generator` helper
The `__generator` helper is a function designed to support TypeScript's down-level emit for
async functions when targeting ES5 and earlier. But how, exactly, does it work?
Here's the body of the `__generator` helper:
```js
__generator = function (thisArg, body) {
var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
function verb(n) { return function (v) { return step([n, v]); }; }
function step(op) {
if (f) throw new TypeError("Generator is already executing.");
while (g && (g = 0, op[0] && (_ = 0)), _) try {
if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
if (y = 0, t) op = [op[0] & 2, t.value];
switch (op[0]) {
case 0: case 1: t = op; break;
case 4: _.label++; return { value: op[1], done: false };
case 5: _.label++; y = op[1]; op = [0]; continue;
case 7: op = _.ops.pop(); _.trys.pop(); continue;
default:
if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
if (t[2]) _.ops.pop();
_.trys.pop(); continue;
}
op = body.call(thisArg, _);
} catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
}
};
```
And here's an example of it in use:
```ts
// source
async function func(x) {
try {
await x;
}
catch (e) {
console.error(e);
}
finally {
console.log("finally");
}
}
// generated
function func(x) {
return __awaiter(this, void 0, void 0, function () {
var e_1;
return __generator(this, function (_a) {
switch (_a.label) {
case 0:
_a.trys.push([0, 1, 3, 4]);
return [4 /*yield*/, x];
case 1:
_a.sent();
return [3 /*break*/, 4];
case 2:
e_1 = _a.sent();
console.error(e_1);
return [3 /*break*/, 4];
case 3:
console.log("finally");
return [7 /*endfinally*/];
case 4: return [2 /*return*/];
}
});
});
}
```
There is a lot going on in this function, so the following will break down what each part of the
`__generator` helper does and how it works.
# Opcodes
The `__generator` helper uses opcodes which represent various operations that are interpreted by
the helper to affect its internal state. The following table lists the various opcodes, their
arguments, and their purpose:
| Opcode | Arguments | Purpose |
|----------------|-----------|--------------------------------------------------------------------------------------------------------------------------------|
| 0 (next) | *value* | Starts the generator, or resumes the generator with *value* as the result of the `AwaitExpression` where execution was paused. |
| 1 (throw) | *value* | Resumes the generator, throwing *value* at `AwaitExpression` where execution was paused. |
| 2 (return) | *value* | Exits the generator, executing any `finally` blocks starting at the `AwaitExpression` where execution was paused. |
| 3 (break) | *label* | Performs an unconditional jump to the specified label, executing any `finally` between the current instruction and the label. |
| 4 (yield) | *value* | Suspends the generator, setting the resume point at the next label and yielding the value. |
| 5 (yieldstar) | *value* | Suspends the generator, setting the resume point at the next label and delegating operations to the supplied value. |
| 6 (catch) | *error* | An internal instruction used to indicate an exception that was thrown from the body of the generator. |
| 7 (endfinally) | | Exits a finally block, resuming any previous operation (such as a break, return, throw, etc.) |
# State
The `_`, `f`, `y`, `t`, and `g` variables make up the persistent state of the `__generator` function. Each variable
has a specific purpose, as described in the following sections:
## The `_` variable
The `__generator` helper must share state between its internal `step` orchestration function and
the `body` function passed to the helper.
```ts
var _ = {
label: 0,
sent: function() {
if (t[0] & 1) // NOTE: true for `throw`, but not `next` or `catch`
throw t[1];
return sent[1];
},
trys: [],
ops: []
};
```
The following table describes the members of the `_` state object and their purpose:
| Name | Description |
|---------|---------------------------------------------------------------------------------------------------------------------------|
| `label` | Specifies the next switch case to execute in the `body` function. |
| `sent` | Handles the completion result passed to the generator. |
| `trys` | A stack of **Protected Regions**, which are 4-tuples that describe the labels that make up a `try..catch..finally` block. |
| `ops` | A stack of pending operations used for `try..finally` blocks. |
The `__generator` helper passes this state object to the `body` function for use with switching
between switch cases in the body, handling completions from `AwaitExpression`, etc.
## The `f` variable
The `f` variable indicates whether the generator is currently executing, to prevent re-entry of
the same generator during its execution.
## The `y` variable
The `y` variable stores the iterator passed to a `yieldstar` instruction to which operations should be delegated.
## The `t` variable
The `t` variable is a temporary variable that stores one of the following values:
- The completion value when resuming from a `yield` or `yield*`.
- The error value for a catch block.
- The current **Protected Region**.
- The verb (`next`, `throw`, or `return` method) to delegate to the expression of a `yield*`.
- The result of evaluating the verb delegated to the expression of a `yield*`.
> NOTE: None of the above cases overlap.
## The `g` variable
The `g` variable is a temporary variable that holds onto the generator object for the purpose of attaching a
`Symbol.iterator` method (if its available), and holds onto that value until the generator is started, allowing
it to also act as the [`suspendedStart`](https://tc39.es/ecma262/#table-internal-slots-of-generator-instances) state.
# Protected Regions
A **Protected Region** is a region within the `body` function that indicates a
`try..catch..finally` statement. It consists of a 4-tuple that contains 4 labels:
| Offset | Description |
|--------|-----------------------------------------------------------------------------------------|
| 0 | *Required* The label that indicates the beginning of a `try..catch..finally` statement. |
| 1 | *Optional* The label that indicates the beginning of a `catch` clause. |
| 2 | *Optional* The label that indicates the beginning of a `finally` clause. |
| 3 | *Required* The label that indicates the end of the `try..catch..finally` statement. |
# The generator object
The final step of the `__generator` helper is the allocation of an object that implements the
`Generator` protocol, to be used by the `__awaiter` helper:
```ts
return g = { next: verb(0), "throw": verb(1), "return": verb(2) },
typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }),
g;
function verb(n) { return function (v) { return step([n, v]); }; }
```
This object translates calls to `next`, `throw`, and `return` to the appropriate Opcodes and
invokes the `step` orchestration function to continue execution. The `throw` and `return` method
names are quoted to better support ES3. In addition, a `Symbol.iterator` method is added to the
generator if the global `Symbol` constructor is available. Once we return, the object reference in
the `g` variable isn't used by the main [orchestration method](#orchestration), so we can use its
truthiness as a mechanism to determine whether the generator is in the `suspendedStart` state.
# Orchestration
The `step` function is the main orechestration mechanism for the `__generator` helper. It
interprets opcodes, handles **protected regions**, and communicates results back to the caller.
Here's a closer look at the `step` function:
```ts
function step(op) {
if (f) throw new TypeError("Generator is already executing.");
while (g && (g = 0, op[0] && (_ = 0)), _) try {
if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
if (y = 0, t) op = [op[0] & 2, t.value];
switch (op[0]) {
case 0: case 1: t = op; break;
case 4: _.label++; return { value: op[1], done: false };
case 5: _.label++; y = op[1]; op = [0]; continue;
case 7: op = _.ops.pop(); _.trys.pop(); continue;
default:
if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
if (t[2]) _.ops.pop();
_.trys.pop(); continue;
}
op = body.call(thisArg, _);
} catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
}
```
The main body of `step` exists in a `while` loop. This allows us to continually interpret
operations until we have reached some completion value, be it a `return`, `await`, or `throw`.
## Preventing re-entry
The first part of the `step` function is used as a check to prevent re-entry into a currently
executing generator:
```ts
if (f) throw new TypeError("Generator is already executing.");
```
## Running the generator
The main body of the `step` function consists of a `while` loop which continues to evaluate
instructions until the generator exits or is suspended:
```ts
while (g && (g = 0, op[0] && (_ = 0)), _) try ...
```
During the first call to `next()`, `return()`, or `throw()`, the generator will be in the `suspendedStart` state. This
is indicated by the `g` variable being "truthy" since it still holds a reference to the generator object. If `g` is
"truthy", then we reset it and check whether the first instruction sent to the generator (`op[0]`) is either a
`return` (`op[0] === 1`) or `throw` (`op[0] === 2`) Opcode, indicating an abrupt completion.
If the first instruction is abrupt, we can emulate [GeneratorResumeAbrupt](https://tc39.es/ecma262/#sec-generatorresumeabrupt)
by setting the state variable (`_`) to a falsy value, which will skip the loop body and
[complete the generator](#handling-a-completed-generator).
If this is _not_ the first instruction sent to the generator, or if the first instruction was `next()`, we will proceed
to evaluate the loop body.
When the generator has run to completion, the `_` state variable will be cleared, forcing the loop
to exit.
## Evaluating the generator body
```ts
try {
...
op = body.call(thisArg, _);
}
catch (e) {
op = [6, e];
y = 0;
}
finally {
f = t = 0;
}
```
Depending on the current operation, we re-enter the generator body to start or continue execution.
Here we invoke `body` with `thisArg` as the `this` binding and the `_` state object as the only
argument. The result is a tuple that contains the next Opcode and argument.
If evaluation of the body resulted in an exception, we convert this into an Opcode 6 ("catch")
operation to be handled in the next spin of the `while` loop. We also clear the `y` variable in
case it is set to ensure we are no longer delegating operations as the exception occurred in
user code *outside* of, or at the function boundary of, the delegated iterator (otherwise the
iterator would have handled the exception itself).
After executing user code, we clear the `f` flag that indicates we are executing the generator,
as well as the `t` temporary value so that we don't hold onto values sent to the generator for
longer than necessary.
Inside of the `try..finally` statement are a series of statements that are used to evaluate the
operations of the transformed generator body.
The first thing we do is mark the generator as executing:
```ts
if (f = 1, ...)
```
Despite the fact this expression is part of the head of an `if` statement, the comma operator
causes it to be evaluated and the result thrown out. This is a minification added purely to
reduce the overall footprint of the helper.
## Delegating `yield*`
The first two statements of the `try..finally` statement handle delegation for `yield*`:
```ts
if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
if (y = 0, t) op = [op[0] & 2, t.value];
```
If the `y` variable is set, and `y` has a `next`, `throw`, or `return` method (depending on the
current operation), we invoke this method and store the return value (an IteratorResult) in `t`.
If `t` indicates it is a yielded value (e.g. `t.done === false`), we return `t` to the caller.
If `t` indicates it is a returned value (e.g. `t.done === true`), we mark the operation with the
`next` Opcode, and the returned value.
If `y` did not have the appropriate method, or `t` was a returned value, we reset `y` to a falsy
value and continue processing the operation.
## Handling operations
The various Opcodes are handled in the following switch statement:
```ts
switch (op[0]) {
case 0: case 1: t = op; break;
case 4: _.label++; return { value: op[1], done: false };
case 5: _.label++; y = op[1]; op = [0]; continue;
case 7: op = _.ops.pop(); _.trys.pop(); continue;
default:
if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
if (t[2]) _.ops.pop();
_.trys.pop(); continue;
}
```
The following sections describe the various Opcodes:
### Opcode 0 ("next") and Opcode 1 ("throw")
```ts
case 0: // next
case 1: // throw
t = op;
break;
```
Both Opcode 0 ("next") and Opcode 1 ("throw") have the same behavior. The current operation is
stored in the `t` variable and the `body` function is invoked. The `body` function should call
`_.sent()` which will evaluate the appropriate completion result.
### Opcode 4 ("yield")
```ts
case 4: // yield
_.label++;
return { value: op[1], done: false };
```
When we encounter Opcode 4 ("yield"), we increment the label by one to indicate the point at which
the generator will resume execution. We then return an `IteratorResult` whose `value` is the
yielded value, and `done` is `false`.
### Opcode 5 ("yieldstar")
```ts
case 5: // yieldstar
_.label++;
y = op[1];
op = [0];
continue;
```
When we receive Opcode 5 ("yieldstar"), we increment the label by one to indicate the point at which
the generator will resume execution. We then store the iterator in `op[1]` in the `y` variable, and
set the operation to delegate to Opcode 0 ("next") with no value. Finally, we continue execution at
the top of the loop to start delegation.
### Opcode 7 ("endfinally")
```ts
case 7:
op = _.ops.pop();
_.trys.pop();
continue;
```
Opcode 7 ("endfinally") indicates that we have hit the end of a `finally` clause, and that the last
operation recorded before entering the `finally` block should be evaluated.
### Opcode 2 ("return"), Opcode 3 ("break"), and Opcode 6 ("catch")
```ts
default:
if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) {
_ = 0;
continue;
}
if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) {
_.label = op[1];
break;
}
if (op[0] === 6 && _.label < t[1]) {
_.label = t[1];
t = op;
break;
}
if (t && _.label < t[2]) {
_.label = t[2];
_.ops.push(op);
break;
}
if (t[2])
_.ops.pop();
_.trys.pop();
continue;
}
```
The handling for Opcode 2 ("return"), Opcode 3 ("break") and Opcode 6 ("catch") is more
complicated, as we must obey the specified runtime semantics of generators. The first line in this
clause gets the current **Protected Region** if found and stores it in the `t` temp variable:
```ts
if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && ...) ...
```
The remainder of this statement, as well as the following by several `if` statements test for more
complex conditions. The first of these is the following:
```ts
if (!(t = ...) && (op[0] === 6 || op[0] === 2)) {
_ = 0;
continue;
}
```
If we encounter an Opcode 6 ("catch") or Opcode 2 ("return"), and we are not in a protected region,
then this operation completes the generator by setting the `_` variable to a falsy value. The
`continue` statement resumes execution at the top of the `while` statement, which will exit the loop
so that we continue execution at the statement following the loop.
```ts
if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) {
_.label = op[1];
break;
}
```
The `if` statement above handles Opcode 3 ("break") when we are either not in a **protected region**, or
are performing an unconditional jump to a label inside of the current **protected region**. In this case
we can unconditionally jump to the specified label.
```ts
if (op[0] === 6 && _.label < t[1]) {
_.label = t[1];
t = op;
break;
}
```
The `if` statement above handles Opcode 6 ("catch") when inside the `try` block of a **protected
region**. In this case we jump to the `catch` block, if present. We replace the value of `t` with
the operation so that the exception can be read as the first statement of the transformed `catch`
clause of the transformed generator body.
```ts
if (t && _.label < t[2]) {
_.label = t[2];
_.ops.push(op);
break;
}
```
This `if` statement handles all Opcodes when in a **protected region** with a `finally` clause.
As long as we are not already inside the `finally` clause, we jump to the `finally` clause and
push the pending operation onto the `_.ops` stack. This allows us to resume execution of the
pending operation once we have completed execution of the `finally` clause, as long as it does not
supersede this operation with its own completion value.
```ts
if (t[2])
_.ops.pop();
```
Any other completion value inside of a `finally` clause will supersede the pending completion value
from the `try` or `catch` clauses. The above `if` statement pops the pending completion from the
stack.
```ts
_.trys.pop();
continue;
```
The remaining statements handle the point at which we exit a **protected region**. Here we pop the
current **protected region** from the stack and spin the `while` statement to evaluate the current
operation again in the next **protected region** or at the function boundary.
## Handling a completed generator
Once the generator has completed, the `_` state variable will be falsy. As a result, the `while`
loop will terminate and hand control off to the final statement of the orchestration function,
which deals with how a completed generator is evaluated:
```ts
if (op[0] & 5)
throw op[1];
return { value: op[0] ? op[1] : void 0, done: true };
```
If the caller calls `throw` on the generator it will send Opcode 1 ("throw"). If an exception
is uncaught within the body of the generator, it will send Opcode 6 ("catch"). As the generator has
completed, it throws the exception. Both of these cases are caught by the bitmask `5`, which does
not collide with the only two other valid completion Opcodes.
If the caller calls `next` on the generator, it will send Opcode 0 ("next"). As the generator has
completed, it returns an `IteratorResult` where `value` is `undefined` and `done` is true.
If the caller calls `return` on the generator, it will send Opcode 2 ("return"). As the generator
has completed, it returns an `IteratorResult` where `value` is the value provided to `return`, and
`done` is true.
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