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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
import generate from "@babel/generator";
import * as t from "@babel/types";
import { hasNode, replaceNode } from "./utils/ast";
import { isTopLevel } from "./utils/helpers";
function hasTopLevelAwait(ast) {
const hasAwait = hasNode(
ast,
(node, ancestors) => t.isAwaitExpression(node) && isTopLevel(ancestors)
);
return hasAwait;
}
// translates new bindings `var a = 3` into `a = 3`.
function translateDeclarationIntoAssignment(node) {
return node.declarations.reduce((acc, declaration) => {
// Don't translate declaration without initial assignment (e.g. `var a;`)
if (!declaration.init) {
return acc;
}
acc.push(
t.expressionStatement(
t.assignmentExpression("=", declaration.id, declaration.init)
)
);
return acc;
}, []);
}
/**
* Given an AST, modify it to return the last evaluated statement's expression value if possible.
* This is to preserve existing console behavior of displaying the last executed expression value.
*/
function addReturnNode(ast) {
const statements = ast.program.body;
const lastStatement = statements.pop();
// if the last expression is an awaitExpression, strip the `await` part and directly
// return the argument to avoid calling the argument's `then` function twice when the
// mapped expression gets evaluated (See Bug 1771428)
if (t.isAwaitExpression(lastStatement.expression)) {
lastStatement.expression = lastStatement.expression.argument;
}
// NOTE: For more complicated cases such as an if/for statement, the last evaluated
// expression value probably can not be displayed, unless doing hacky workarounds such
// as returning the `eval` of the final statement (won't always work due to CSP issues?)
// or SpiderMonkey support (See Bug 1839588) at which point this entire module can be removed.
statements.push(
t.isExpressionStatement(lastStatement)
? t.returnStatement(lastStatement.expression)
: lastStatement
);
return statements;
}
function getDeclarations(node) {
const { kind, declarations } = node;
const declaratorNodes = declarations.reduce((acc, d) => {
const declarators = getVariableDeclarators(d.id);
return acc.concat(declarators);
}, []);
// We can't declare const variables outside of the async iife because we
// wouldn't be able to re-assign them. As a workaround, we transform them
// to `let` which should be good enough for those case.
return t.variableDeclaration(
kind === "const" ? "let" : kind,
declaratorNodes
);
}
function getVariableDeclarators(node) {
if (t.isIdentifier(node)) {
return t.variableDeclarator(t.identifier(node.name));
}
if (t.isObjectProperty(node)) {
return getVariableDeclarators(node.value);
}
if (t.isRestElement(node)) {
return getVariableDeclarators(node.argument);
}
if (t.isAssignmentPattern(node)) {
return getVariableDeclarators(node.left);
}
if (t.isArrayPattern(node)) {
return node.elements.reduce(
(acc, element) => acc.concat(getVariableDeclarators(element)),
[]
);
}
if (t.isObjectPattern(node)) {
return node.properties.reduce(
(acc, property) => acc.concat(getVariableDeclarators(property)),
[]
);
}
return [];
}
/**
* Given an AST and an array of variableDeclaration nodes, return a new AST with
* all the declarations at the top of the AST.
*/
function addTopDeclarationNodes(ast, declarationNodes) {
const statements = [];
declarationNodes.forEach(declarationNode => {
statements.push(getDeclarations(declarationNode));
});
statements.push(ast);
return t.program(statements);
}
/**
* Given an AST, return an object of the following shape:
* - newAst: {AST} the AST where variable declarations were transformed into
* variable assignments
* - declarations: {Array<Node>} An array of all the declaration nodes needed
* outside of the async iife.
*/
function translateDeclarationsIntoAssignment(ast) {
const declarations = [];
t.traverse(ast, (node, ancestors) => {
const parent = ancestors[ancestors.length - 1];
if (
t.isWithStatement(node) ||
!isTopLevel(ancestors) ||
t.isAssignmentExpression(node) ||
!t.isVariableDeclaration(node) ||
t.isForStatement(parent.node) ||
t.isForXStatement(parent.node) ||
!Array.isArray(node.declarations) ||
node.declarations.length === 0
) {
return;
}
const newNodes = translateDeclarationIntoAssignment(node);
replaceNode(ancestors, newNodes);
declarations.push(node);
});
return {
newAst: ast,
declarations,
};
}
/**
* Given an AST, wrap its body in an async iife, transform variable declarations
* in assignments and move the variable declarations outside of the async iife.
* Example: With the AST for the following expression: `let a = await 123`, the
* function will return:
* let a;
* (async => {
* return a = await 123;
* })();
*/
function wrapExpressionFromAst(ast) {
// Transform let and var declarations into assignments, and get back an array
// of variable declarations.
let { newAst, declarations } = translateDeclarationsIntoAssignment(ast);
const body = addReturnNode(newAst);
// Create the async iife.
newAst = t.expressionStatement(
t.callExpression(
t.arrowFunctionExpression([], t.blockStatement(body), true),
[]
)
);
// Now let's put all the variable declarations at the top of the async iife.
newAst = addTopDeclarationNodes(newAst, declarations);
return generate(newAst).code;
}
export default function mapTopLevelAwait(expression, ast) {
if (!ast) {
// If there's no ast this means the expression is malformed. And if the
// expression contains the await keyword, we still want to wrap it in an
// async iife in order to get a meaningful message (without this, the
// engine will throw an Error stating that await keywords are only valid
// in async functions and generators).
if (expression.includes("await ")) {
return `(async () => { ${expression} })();`;
}
return expression;
}
if (!hasTopLevelAwait(ast)) {
return expression;
}
return wrapExpressionFromAst(ast);
}
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