jsrt/lib/stdlib/promise.js

// Promises, as the spec says, except:
// - A throw in a handler or an executor does not reject, because there is no
//   `try`. `reject()` and a returned rejected promise do
// - An unhandled rejection does nothing. Node prints it and stops
// - `Promise.all` takes an array, not any iterable
//
// Written for type inference, which names an object by the instr that makes
// it and the call place of the call it is made in (see compiler/src/infer):
// - A promise is an object literal, made in the fn that user code calls, not
//   in a helper. So each place in user code gets its own promise objects, and
//   their fields are always there
// - The closures that run a reaction are made in `then`, for the same reason
// - Fulfilment and rejection have their own fns and fields, so that a result
//   and a reason never share a type
// - A fn that makes a closure keeps out of the common path: a captured arg is
//   in a box, which every call from one place shares

// The states of a promise
const __PENDING = 0;
const __FULFILLED = 1;
const __REJECTED = 2;

class Promise {
    constructor(executor) {
        const p = { __proto__: this.__proto__, state: __PENDING, value: undefined, reason: undefined, resolved: false, reactions: [] };

        if (executor !== undefined) {
            executor((value) => __resolve(p, value), (reason) => __reject(p, reason));
        }

        return p;
    }

    then(onFulfilled, onRejected) {
        const derived = { __proto__: this.__proto__, state: __PENDING, value: undefined, reason: undefined, resolved: false, reactions: [] };
        let value;
        let reason;

        const fulfilled = () => {
            if (typeof onFulfilled === "function") __resolve(derived, onFulfilled(value));
            else __resolve(derived, value);
        };

        const rejected = () => {
            if (typeof onRejected === "function") __resolve(derived, onRejected(reason));
            else __reject(derived, reason);
        };

        // What `__fulfil()` and `__settle_rejected()` call: each runs its
        // handler in a job of its own
        const reaction = {
            fulfil: (v) => {
                value = v;
                __queue_job(fulfilled);
            },
            reject: (r) => {
                reason = r;
                __queue_job(rejected);
            },
        };

        if (this.state === __PENDING) {
            this.reactions.push(reaction);
        } else if (this.state === __FULFILLED) {
            reaction.fulfil(this.value);
        } else {
            reaction.reject(this.reason);
        }

        return derived;
    }

    catch(onRejected) {
        return this.then(undefined, onRejected);
    }

    finally(f) {
        return this.then(
            (value) => {
                f();
                return value;
            },
            (reason) => {
                f();
                return Promise.reject(reason);
            },
        );
    }

    static resolve(value) {
        if (__is_promise(value)) return value;
        const p = { __proto__: this.prototype, state: __PENDING, value: undefined, reason: undefined, resolved: false, reactions: [] };
        __resolve(p, value);
        return p;
    }

    static reject(reason) {
        const p = { __proto__: this.prototype, state: __PENDING, value: undefined, reason: undefined, resolved: false, reactions: [] };
        __reject(p, reason);
        return p;
    }

    static all(promises) {
        const p = { __proto__: this.prototype, state: __PENDING, value: undefined, reason: undefined, resolved: false, reactions: [] };
        const values = [];
        let left = promises.length;

        if (left === 0) __resolve(p, values);

        for (let i = 0; i < promises.length; i++) {
            Promise.resolve(promises[i]).then(
                (value) => {
                    values[i] = value;
                    left--;
                    if (left === 0) __resolve(p, values);
                },
                (reason) => __reject(p, reason),
            );
        }

        return p;
    }
}

function __is_promise(x) {
    return typeof x === "object" && x !== null && x.reactions !== undefined;
}

// Resolve `p` with `value`, unless it was resolved or rejected before
function __resolve(p, value) {
    if (p.resolved) return;
    p.resolved = true;
    __fulfil_or_adopt(p, value);
}

// Reject `p` with `reason`, unless it was resolved or rejected before
function __reject(p, reason) {
    if (p.resolved) return;
    p.resolved = true;
    __settle_rejected(p, reason);
}

// Fulfil `p` with `value`, or if `value` is a thenable, adopt its state. That
// is in a job of its own, as the spec says, so it takes a tick more
function __fulfil_or_adopt(p, value) {
    if ((typeof value === "object" && value !== null) || typeof value === "function") {
        const then = value.then;

        if (typeof then === "function") {
            __queue_adopt(p, value);
            return;
        }
    }

    __fulfil(p, value);
}

// A closure boxes the args that it captures, and the box is one for every
// call from one place. So only this fn captures `p` and `value`, and only
// thenables share its boxes
function __queue_adopt(p, thenable) {
    __queue_job(() => __adopt(p, thenable));
}

// The reactions are cleared in place: a new array here would be one array
// for every promise
function __fulfil(p, value) {
    p.state = __FULFILLED;
    p.value = value;

    const reactions = p.reactions;
    for (let i = 0; i < reactions.length; i++) reactions[i].fulfil(value);
    reactions.length = 0;
}

function __settle_rejected(p, reason) {
    p.state = __REJECTED;
    p.reason = reason;

    const reactions = p.reactions;
    for (let i = 0; i < reactions.length; i++) reactions[i].reject(reason);
    reactions.length = 0;
}

// Make `p` follow the thenable `thenable`
function __adopt(p, thenable) {
    thenable.then(
        (value) => {
            if (p.state === __PENDING) __fulfil_or_adopt(p, value);
        },
        (reason) => {
            if (p.state === __PENDING) __settle_rejected(p, reason);
        },
    );
}

function queueMicrotask(f) {
    __queue_job(f);
}