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index.ts
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import type { NodePath, Visitor } from "@babel/traverse";
import {
LOGICAL_OPERATORS,
arrowFunctionExpression,
assignmentExpression,
binaryExpression,
booleanLiteral,
callExpression,
cloneNode,
conditionalExpression,
identifier,
isMemberExpression,
isOptionalCallExpression,
isOptionalMemberExpression,
isUpdateExpression,
logicalExpression,
memberExpression,
nullLiteral,
optionalCallExpression,
optionalMemberExpression,
sequenceExpression,
updateExpression,
} from "@babel/types";
import type * as t from "@babel/types";
import { willPathCastToBoolean } from "./util";
class AssignmentMemoiser {
private _map: WeakMap<t.Expression, { count: number; value: t.Identifier }>;
constructor() {
this._map = new WeakMap();
}
has(key: t.Expression) {
return this._map.has(key);
}
get(key: t.Expression) {
if (!this.has(key)) return;
const record = this._map.get(key);
const { value } = record;
record.count--;
if (record.count === 0) {
// The `count` access is the outermost function call (hopefully), so it
// does the assignment.
return assignmentExpression("=", value, key);
}
return value;
}
set(key: t.Expression, value: t.Identifier, count: number) {
return this._map.set(key, { count, value });
}
}
function toNonOptional(
path: NodePath<t.Expression>,
base: t.Expression,
): t.Expression {
const { node } = path;
if (isOptionalMemberExpression(node)) {
return memberExpression(base, node.property, node.computed);
}
if (path.isOptionalCallExpression()) {
const callee = path.get("callee");
if (path.node.optional && callee.isOptionalMemberExpression()) {
// object must be a conditional expression because the optional private access in object has been transformed
const object = callee.node.object as t.ConditionalExpression;
const context = path.scope.maybeGenerateMemoised(object);
callee
.get("object")
.replaceWith(assignmentExpression("=", context, object));
return callExpression(memberExpression(base, identifier("call")), [
context,
...path.node.arguments,
]);
}
return callExpression(base, path.node.arguments);
}
return path.node;
}
// Determines if the current path is in a detached tree. This can happen when
// we are iterating on a path, and replace an ancestor with a new node. Babel
// doesn't always stop traversing the old node tree, and that can cause
// inconsistencies.
function isInDetachedTree(path: NodePath) {
while (path) {
if (path.isProgram()) break;
const { parentPath, container, listKey } = path;
const parentNode = parentPath.node;
if (listKey) {
if (
container !==
// @ts-expect-error listKey must be a valid parent node key
parentNode[listKey]
) {
return true;
}
} else {
if (container !== parentNode) return true;
}
path = parentPath;
}
return false;
}
type Member = NodePath<t.OptionalMemberExpression | t.MemberExpression>;
const handle = {
memoise() {
// noop.
},
handle(this: HandlerState, member: Member, noDocumentAll: boolean) {
const { node, parent, parentPath, scope } = member;
if (member.isOptionalMemberExpression()) {
// Transforming optional chaining requires we replace ancestors.
if (isInDetachedTree(member)) return;
// We're looking for the end of _this_ optional chain, which is actually
// the "rightmost" property access of the chain. This is because
// everything up to that property access is "optional".
//
// Let's take the case of `FOO?.BAR.baz?.qux`, with `FOO?.BAR` being our
// member. The "end" to most users would be `qux` property access.
// Everything up to it could be skipped if it `FOO` were nullish. But
// actually, we can consider the `baz` access to be the end. So we're
// looking for the nearest optional chain that is `optional: true`.
const endPath = member.find(({ node, parent }) => {
if (isOptionalMemberExpression(parent)) {
// We need to check `parent.object` since we could be inside the
// computed expression of a `bad?.[FOO?.BAR]`. In this case, the
// endPath is the `FOO?.BAR` member itself.
return parent.optional || parent.object !== node;
}
if (isOptionalCallExpression(parent)) {
// Checking `parent.callee` since we could be in the arguments, eg
// `bad?.(FOO?.BAR)`.
// Also skip `FOO?.BAR` in `FOO?.BAR?.()` since we need to transform the optional call to ensure proper this
return (
// In FOO?.#BAR?.(), endPath points the optional call expression so we skip FOO?.#BAR
(node !== member.node && parent.optional) || parent.callee !== node
);
}
return true;
}) as NodePath<t.OptionalMemberExpression>;
// Replace `function (a, x = a.b?.#c) {}` to `function (a, x = (() => a.b?.#c)() ){}`
// so the temporary variable can be injected in correct scope
// This can be further optimized to avoid unnecessary IIFE
if (scope.path.isPattern()) {
endPath.replaceWith(
// The injected member will be queued and eventually transformed when visited
callExpression(arrowFunctionExpression([], endPath.node), []),
);
return;
}
const willEndPathCastToBoolean = willPathCastToBoolean(endPath);
const rootParentPath = endPath.parentPath;
if (rootParentPath.isUpdateExpression({ argument: node })) {
throw member.buildCodeFrameError(`can't handle update expression`);
}
const isAssignment = rootParentPath.isAssignmentExpression({
left: endPath.node,
});
const isDeleteOperation = rootParentPath.isUnaryExpression({
operator: "delete",
});
if (
isDeleteOperation &&
endPath.isOptionalMemberExpression() &&
endPath.get("property").isPrivateName()
) {
// @babel/parser will throw error on `delete obj?.#x`.
// This error serves as fallback when `delete obj?.#x` is constructed from babel types
throw member.buildCodeFrameError(
`can't delete a private class element`,
);
}
// Now, we're looking for the start of this optional chain, which is
// optional to the left of this member.
//
// Let's take the case of `foo?.bar?.baz.QUX?.BAM`, with `QUX?.BAM` being
// our member. The "start" to most users would be `foo` object access.
// But actually, we can consider the `bar` access to be the start. So
// we're looking for the nearest optional chain that is `optional: true`,
// which is guaranteed to be somewhere in the object/callee tree.
let startingOptional: NodePath<t.Expression> = member;
for (;;) {
if (startingOptional.isOptionalMemberExpression()) {
if (startingOptional.node.optional) break;
startingOptional = startingOptional.get("object");
continue;
} else if (startingOptional.isOptionalCallExpression()) {
if (startingOptional.node.optional) break;
startingOptional = startingOptional.get("callee");
continue;
}
// prevent infinite loop: unreachable if the AST is well-formed
throw new Error(
`Internal error: unexpected ${startingOptional.node.type}`,
);
}
// @ts-expect-error isOptionalMemberExpression does not work with NodePath union
const startingNode = startingOptional.isOptionalMemberExpression()
? // @ts-expect-error isOptionalMemberExpression does not work with NodePath union
startingOptional.node.object
: // @ts-expect-error isOptionalMemberExpression does not work with NodePath union
startingOptional.node.callee;
const baseNeedsMemoised = scope.maybeGenerateMemoised(startingNode);
const baseRef = baseNeedsMemoised ?? startingNode;
// Compute parentIsOptionalCall before `startingOptional` is replaced
// as `node` may refer to `startingOptional.node` before replaced.
const parentIsOptionalCall = parentPath.isOptionalCallExpression({
callee: node,
});
// here we use a function to wrap `parentIsOptionalCall` to get type
// for parent, do not use it anywhere else
// See https://github.com/microsoft/TypeScript/issues/10421
const isOptionalCall = (
parent: t.Node,
): parent is t.OptionalCallExpression => parentIsOptionalCall;
// if parentIsCall is true, it implies that node.extra.parenthesized is always true
const parentIsCall = parentPath.isCallExpression({ callee: node });
startingOptional.replaceWith(toNonOptional(startingOptional, baseRef));
if (isOptionalCall(parent)) {
if (parent.optional) {
parentPath.replaceWith(this.optionalCall(member, parent.arguments));
} else {
parentPath.replaceWith(this.call(member, parent.arguments));
}
} else if (parentIsCall) {
// `(a?.#b)()` to `(a == null ? void 0 : a.#b.bind(a))()`
member.replaceWith(this.boundGet(member));
} else if (
(process.env.BABEL_8_BREAKING || this.delete) &&
parentPath.isUnaryExpression({ operator: "delete" })
) {
parentPath.replaceWith(this.delete(member));
} else if (parentPath.isAssignmentExpression()) {
// `a?.#b = c` to `(a == null ? void 0 : a.#b = c)`
handleAssignment(this, member, parentPath);
} else {
member.replaceWith(this.get(member));
}
let regular: t.Expression = member.node;
for (let current: NodePath = member; current !== endPath; ) {
const parentPath = current.parentPath as NodePath<t.Expression>;
// skip transforming `Foo.#BAR?.call(FOO)`
if (
parentPath === endPath &&
isOptionalCall(parent) &&
parent.optional
) {
regular = parentPath.node;
break;
}
regular = toNonOptional(parentPath, regular);
current = parentPath;
}
let context: t.Identifier;
const endParentPath = endPath.parentPath as NodePath<t.Expression>;
if (
isMemberExpression(regular) &&
endParentPath.isOptionalCallExpression({
callee: endPath.node,
optional: true,
})
) {
const { object } = regular;
context = member.scope.maybeGenerateMemoised(object);
if (context) {
regular.object = assignmentExpression(
"=",
context,
// object must not be Super when `context` is an identifier
// eslint-disable-next-line @typescript-eslint/no-unnecessary-type-assertion
object as t.Expression,
);
}
}
let replacementPath: NodePath = endPath;
if (isDeleteOperation || isAssignment) {
replacementPath = endParentPath;
regular = endParentPath.node;
}
const baseMemoised = baseNeedsMemoised
? assignmentExpression(
"=",
// When base needs memoised, the baseRef must be an identifier
cloneNode(baseRef as t.Identifier),
cloneNode(startingNode),
)
: cloneNode(baseRef);
if (willEndPathCastToBoolean) {
let nonNullishCheck;
if (noDocumentAll) {
nonNullishCheck = binaryExpression("!=", baseMemoised, nullLiteral());
} else {
nonNullishCheck = logicalExpression(
"&&",
binaryExpression("!==", baseMemoised, nullLiteral()),
binaryExpression(
"!==",
cloneNode(baseRef),
scope.buildUndefinedNode(),
),
);
}
replacementPath.replaceWith(
logicalExpression("&&", nonNullishCheck, regular),
);
} else {
let nullishCheck;
if (noDocumentAll) {
nullishCheck = binaryExpression("==", baseMemoised, nullLiteral());
} else {
nullishCheck = logicalExpression(
"||",
binaryExpression("===", baseMemoised, nullLiteral()),
binaryExpression(
"===",
cloneNode(baseRef),
scope.buildUndefinedNode(),
),
);
}
replacementPath.replaceWith(
conditionalExpression(
nullishCheck,
isDeleteOperation
? booleanLiteral(true)
: scope.buildUndefinedNode(),
regular,
),
);
}
// context and isDeleteOperation can not be both truthy
if (context) {
const endParent = endParentPath.node as t.OptionalCallExpression;
endParentPath.replaceWith(
optionalCallExpression(
optionalMemberExpression(
endParent.callee,
identifier("call"),
false,
true,
),
[cloneNode(context), ...endParent.arguments],
false,
),
);
}
return;
}
// MEMBER++ -> _set(MEMBER, (ref = _get(MEMBER), ref2 = ref++, ref)), ref2
// ++MEMBER -> _set(MEMBER, (ref = _get(MEMBER), ++ref))
if (isUpdateExpression(parent, { argument: node })) {
if (this.simpleSet) {
member.replaceWith(this.simpleSet(member));
return;
}
const { operator, prefix } = parent;
// Give the state handler a chance to memoise the member, since we'll
// reference it twice. The second access (the set) should do the memo
// assignment.
this.memoise(member, 2);
const ref = scope.generateUidIdentifierBasedOnNode(node);
scope.push({ id: ref });
const seq: t.Expression[] = [
// ref = _get(MEMBER)
assignmentExpression("=", cloneNode(ref), this.get(member)),
];
if (prefix) {
seq.push(updateExpression(operator, cloneNode(ref), prefix));
// (ref = _get(MEMBER), ++ref)
const value = sequenceExpression(seq);
parentPath.replaceWith(this.set(member, value));
return;
} else {
const ref2 = scope.generateUidIdentifierBasedOnNode(node);
scope.push({ id: ref2 });
seq.push(
assignmentExpression(
"=",
cloneNode(ref2),
updateExpression(operator, cloneNode(ref), prefix),
),
cloneNode(ref),
);
// (ref = _get(MEMBER), ref2 = ref++, ref)
const value = sequenceExpression(seq);
parentPath.replaceWith(
sequenceExpression([this.set(member, value), cloneNode(ref2)]),
);
return;
}
}
// MEMBER = VALUE -> _set(MEMBER, VALUE)
// MEMBER += VALUE -> _set(MEMBER, _get(MEMBER) + VALUE)
// MEMBER ??= VALUE -> _get(MEMBER) ?? _set(MEMBER, VALUE)
if (parentPath.isAssignmentExpression({ left: node })) {
handleAssignment(this, member, parentPath);
return;
}
// MEMBER(ARGS) -> _call(MEMBER, ARGS)
if (parentPath.isCallExpression({ callee: node })) {
parentPath.replaceWith(this.call(member, parentPath.node.arguments));
return;
}
// MEMBER?.(ARGS) -> _optionalCall(MEMBER, ARGS)
if (parentPath.isOptionalCallExpression({ callee: node })) {
// Replace `function (a, x = a.b.#c?.()) {}` to `function (a, x = (() => a.b.#c?.())() ){}`
// so the temporary variable can be injected in correct scope
// This can be further optimized to avoid unnecessary IIFE
if (scope.path.isPattern()) {
parentPath.replaceWith(
// The injected member will be queued and eventually transformed when visited
callExpression(arrowFunctionExpression([], parentPath.node), []),
);
return;
}
parentPath.replaceWith(
this.optionalCall(member, parentPath.node.arguments),
);
return;
}
// delete MEMBER -> _delete(MEMBER)
if (
(process.env.BABEL_8_BREAKING || this.delete) &&
parentPath.isUnaryExpression({ operator: "delete" })
) {
parentPath.replaceWith(this.delete(member));
return;
}
// for (MEMBER of ARR)
// for (MEMBER in ARR)
// { KEY: MEMBER } = OBJ -> { KEY: _destructureSet(MEMBER) } = OBJ
// { KEY: MEMBER = _VALUE } = OBJ -> { KEY: _destructureSet(MEMBER) = _VALUE } = OBJ
// {...MEMBER} -> {..._destructureSet(MEMBER)}
//
// [MEMBER] = ARR -> [_destructureSet(MEMBER)] = ARR
// [MEMBER = _VALUE] = ARR -> [_destructureSet(MEMBER) = _VALUE] = ARR
// [...MEMBER] -> [..._destructureSet(MEMBER)]
if (
// for (MEMBER of ARR)
// for (MEMBER in ARR)
parentPath.isForXStatement({ left: node }) ||
// { KEY: MEMBER } = OBJ
(parentPath.isObjectProperty({ value: node }) &&
parentPath.parentPath.isObjectPattern()) ||
// { KEY: MEMBER = _VALUE } = OBJ
(parentPath.isAssignmentPattern({ left: node }) &&
parentPath.parentPath.isObjectProperty({ value: parent }) &&
parentPath.parentPath.parentPath.isObjectPattern()) ||
// [MEMBER] = ARR
parentPath.isArrayPattern() ||
// [MEMBER = _VALUE] = ARR
(parentPath.isAssignmentPattern({ left: node }) &&
parentPath.parentPath.isArrayPattern()) ||
// {...MEMBER}
// [...MEMBER]
parentPath.isRestElement()
) {
member.replaceWith(this.destructureSet(member));
return;
}
if (parentPath.isTaggedTemplateExpression()) {
// MEMBER -> _get(MEMBER).bind(this)
member.replaceWith(this.boundGet(member));
} else {
// MEMBER -> _get(MEMBER)
member.replaceWith(this.get(member));
}
},
};
function handleAssignment(
state: HandlerState,
member: NodePath<t.MemberExpression | t.OptionalMemberExpression>,
parentPath: NodePath<t.AssignmentExpression>,
) {
if (state.simpleSet) {
member.replaceWith(state.simpleSet(member));
return;
}
const { operator, right: value } = parentPath.node;
if (operator === "=") {
parentPath.replaceWith(state.set(member, value));
} else {
const operatorTrunc = operator.slice(0, -1);
if (LOGICAL_OPERATORS.includes(operatorTrunc)) {
// Give the state handler a chance to memoise the member, since we'll
// reference it twice. The first access (the get) should do the memo
// assignment.
state.memoise(member, 1);
parentPath.replaceWith(
logicalExpression(
operatorTrunc as t.LogicalExpression["operator"],
state.get(member),
state.set(member, value),
),
);
} else {
// Here, the second access (the set) is evaluated first.
state.memoise(member, 2);
parentPath.replaceWith(
state.set(
member,
binaryExpression(
operatorTrunc as t.BinaryExpression["operator"],
state.get(member),
value,
),
),
);
}
}
}
export interface Handler<State> {
memoise?(
this: HandlerState<State> & State,
member: Member,
count: number,
): void;
destructureSet(
this: HandlerState<State> & State,
member: Member,
): t.Expression;
boundGet(this: HandlerState<State> & State, member: Member): t.Expression;
simpleSet?(this: HandlerState<State> & State, member: Member): t.Expression;
get(this: HandlerState<State> & State, member: Member): t.Expression;
set(
this: HandlerState<State> & State,
member: Member,
value: t.Expression,
): t.Expression;
call(
this: HandlerState<State> & State,
member: Member,
args: t.CallExpression["arguments"],
): t.Expression;
optionalCall(
this: HandlerState<State> & State,
member: Member,
args: t.OptionalCallExpression["arguments"],
): t.Expression;
delete(this: HandlerState<State> & State, member: Member): t.Expression;
}
export interface HandlerState<State = {}> extends Handler<State> {
handle(
this: HandlerState<State> & State,
member: Member,
noDocumentAll?: boolean,
): void;
memoiser: AssignmentMemoiser;
}
// We do not provide a default traversal visitor
// Instead, caller passes one, and must call `state.handle` on the members
// it wishes to be transformed.
// Additionally, the caller must pass in a state object with at least
// get, set, and call methods.
// Optionally, a memoise method may be defined on the state, which will be
// called when the member is a self-referential update.
export default function memberExpressionToFunctions<CustomState = {}>(
path: NodePath,
visitor: Visitor<HandlerState<CustomState>>,
state: Handler<CustomState> & CustomState,
) {
path.traverse(visitor, {
...handle,
...state,
memoiser: new AssignmentMemoiser(),
});
}