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

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

class Promise {
    constructor(executor) {
        this.state = __PENDING;
        this.value = undefined;
        this.resolved = false; // `resolve()` or `reject()` was called, maybe with a thenable
        this.reactions = [];

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

    then(onFulfilled, onRejected) {
        const derived = new Promise();
        const reaction = { onFulfilled: onFulfilled, onRejected: onRejected, derived: derived };

        if (this.state === __PENDING) {
            this.reactions.push(reaction);
        } else {
            __queue_reaction(reaction, this.state, this.value);
        }

        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 = new Promise();
        __resolve(p, value);
        return p;
    }

    static reject(reason) {
        const p = new Promise();
        __reject(p, reason);
        return p;
    }

    static all(promises) {
        const p = new Promise();
        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);
}

// 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_job(() => __adopt(p, value));
            return;
        }
    }

    __settle(p, __FULFILLED, value);
}

function __reject(p, reason) {
    if (p.resolved) return;
    p.resolved = true;
    __settle(p, __REJECTED, reason);
}

function __settle(p, state, value) {
    p.state = state;
    p.value = value;

    const reactions = p.reactions;
    p.reactions = [];

    for (let i = 0; i < reactions.length; i++) {
        __queue_reaction(reactions[i], state, value);
    }
}

function __queue_reaction(reaction, state, value) {
    const handler = state === __FULFILLED ? reaction.onFulfilled : reaction.onRejected;
    const derived = reaction.derived;
    __queue_job(() => __react(handler, value, derived, state));
}

// Call `handler` with the value of a promise that is in `state`, and resolve
// `derived` with the result. With no handler, `derived` gets the same state
function __react(handler, value, derived, state) {
    if (typeof handler === "function") {
        __resolve(derived, handler(value));
    } else if (state === __FULFILLED) {
        __resolve(derived, value);
    } else {
        __reject(derived, value);
    }
}

// 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(p, __REJECTED, reason);
        },
    );
}

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