JavaScript 权威指南第七版（GPT 重译）（五）(3)

function fetchSequentially(urls) {
    // We'll store the URL bodies here as we fetch them
    const bodies = [];
    // Here's a Promise-returning function that fetches one body
    function fetchOne(url) {
        return fetch(url)
            .then(response => response.text())
            .then(body => {
                // We save the body to the array, and we're purposely
                // omitting a return value here (returning undefined)
                bodies.push(body);
            });
    }
    // Start with a Promise that will fulfill right away (with value undefined)
    let p = Promise.resolve(undefined);
    // Now loop through the desired URLs, building a Promise chain
    // of arbitrary length, fetching one URL at each stage of the chain
    for(url of urls) {
        p = p.then(() => fetchOne(url));
    }
    // When the last Promise in that chain is fulfilled, then the
    // bodies array is ready. So let's return a Promise for that
    // bodies array. Note that we don't include any error handlers:
    // we want to allow errors to propagate to the caller.
    return p.then(() => bodies);
}

fetchSequentially(urls)
    .then(bodies => { /* do something with the array of strings */ })
    .catch(e => console.error(e));

// This function takes an array of input values and a "promiseMaker" function.
// For any input value x in the array, promiseMaker(x) should return a Promise
// that will fulfill to an output value. This function returns a Promise
// that fulfills to an array of the computed output values.
//
// Rather than creating the Promises all at once and letting them run in
// parallel, however, promiseSequence() only runs one Promise at a time
// and does not call promiseMaker() for a value until the previous Promise
// has fulfilled.
function promiseSequence(inputs, promiseMaker) {
    // Make a private copy of the array that we can modify
    inputs = [...inputs];
    // Here's the function that we'll use as a Promise callback
    // This is the pseudorecursive magic that makes this all work.
    function handleNextInput(outputs) {
        if (inputs.length === 0) {
            // If there are no more inputs left, then return the array
            // of outputs, finally fulfilling this Promise and all the
            // previous resolved-but-not-fulfilled Promises.
            return outputs;
        } else {
            // If there are still input values to process, then we'll
            // return a Promise object, resolving the current Promise
            // with the future value from a new Promise.
            let nextInput = inputs.shift(); // Get the next input value,
            return promiseMaker(nextInput)  // compute the next output value,
                // Then create a new outputs array with the new output value
                .then(output => outputs.concat(output))
                // Then "recurse", passing the new, longer, outputs array
                .then(handleNextInput);
        }
    }
    // Start with a Promise that fulfills to an empty array and use
    // the function above as its callback.
    return Promise.resolve([]).then(handleNextInput);
}

// Given a URL, return a Promise that fulfills to the URL body text
function fetchBody(url) { return fetch(url).then(r => r.text()); }
// Use it to sequentially fetch a bunch of URL bodies
promiseSequence(urls, fetchBody)
    .then(bodies => { /* do something with the array of strings */ })
    .catch(console.error);

let response = await fetch("/api/user/profile");
let profile = await response.json();

async function getHighScore() {
    let response = await fetch("/api/user/profile");
    let profile = await response.json();
    return profile.highScore;
}

displayHighScore(await getHighScore());

getHighScore().then(displayHighScore).catch(console.error);

async function getJSON(url) {
    let response = await fetch(url);
    let body = await response.json();
    return body;
}

let value1 = await getJSON(url1);
let value2 = await getJSON(url2);

let [value1, value2] = await Promise.all([getJSON(url1), getJSON(url2)]);

async function f(x) { /* body */ }

function f(x) {
    return new Promise(function(resolve, reject) {
        try {
            resolve((function(x) { /* body */ })(x));
        }
        catch(e) {
            reject(e);
        }
    });
}

const fs = require("fs");
async function parseFile(filename) {
    let stream = fs.createReadStream(filename, { encoding: "utf-8"});
    for await (let chunk of stream) {
        parseChunk(chunk); // Assume parseChunk() is defined elsewhere
    }
}

const urls = [url1, url2, url3];

const promises = urls.map(url => fetch(url));

for(const promise of promises) {
    response = await promise;
    handle(response);
}

for await (const response of promises) {
    handle(response);
}

// A Promise-based wrapper around setTimeout() that we can use await with.
// Returns a Promise that fulfills in the specified number of milliseconds
function elapsedTime(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
}
// An async generator function that increments a counter and yields it
// a specified (or infinite) number of times at a specified interval.
async function* clock(interval, max=Infinity) {
    for(let count = 1; count <= max; count++) { // regular for loop
        await elapsedTime(interval);            // wait for time to pass
        yield count;                            // yield the counter
    }
}
// A test function that uses the async generator with for/await
async function test() {                       // Async so we can use for/await
    for await (let tick of clock(300, 100)) { // Loop 100 times every 300ms
        console.log(tick);
    }
}

function clock(interval, max=Infinity) {
    // A Promise-ified version of setTimeout that we can use await with.
    // Note that this takes an absolute time instead of an interval.
    function until(time) {
        return new Promise(resolve => setTimeout(resolve, time - Date.now()));
    }
    // Return an asynchronously iterable object
    return {
        startTime: Date.now(),  // Remember when we started
        count: 1,               // Remember which iteration we're on
        async next() {          // The next() method makes this an iterator
            if (this.count > max) {     // Are we done?
                return { done: true };  // Iteration result indicating done
            }
            // Figure out when the next iteration should begin,
            let targetTime = this.startTime + this.count * interval;
            // wait until that time,
            await until(targetTime);
            // and return the count value in an iteration result object.
            return { value: this.count++ };
        },
        // This method means that this iterator object is also an iterable.
        [Symbol.asyncIterator]() { return this; }
    };
}

/**
 * An asynchronously iterable queue class. Add values with enqueue()
 * and remove them with dequeue(). dequeue() returns a Promise, which
 * means that values can be dequeued before they are enqueued. The
 * class implements [Symbol.asyncIterator] and next() so that it can
 * be used with the for/await loop (which will not terminate until
 * the close() method is called.)
 */
class AsyncQueue {
    constructor() {
        // Values that have been queued but not dequeued yet are stored here
        this.values = [];
        // When Promises are dequeued before their corresponding values are
        // queued, the resolve methods for those Promises are stored here.
        this.resolvers = [];
        // Once closed, no more values can be enqueued, and no more unfulfilled
        // Promises returned.
        this.closed = false;
    }
    enqueue(value) {
        if (this.closed) {
            throw new Error("AsyncQueue closed");
        }
        if (this.resolvers.length > 0) {
            // If this value has already been promised, resolve that Promise
            const resolve = this.resolvers.shift();
            resolve(value);
        }
        else {
            // Otherwise, queue it up
            this.values.push(value);
        }
    }
    dequeue() {
        if (this.values.length > 0) {
            // If there is a queued value, return a resolved Promise for it
            const value = this.values.shift();
            return Promise.resolve(value);
        }
        else if (this.closed) {
            // If no queued values and we're closed, return a resolved
            // Promise for the "end-of-stream" marker
            return Promise.resolve(AsyncQueue.EOS);
        }
        else {
            // Otherwise, return an unresolved Promise,
            // queuing the resolver function for later use
            return new Promise((resolve) => { this.resolvers.push(resolve); });
        }
    }
    close() {
        // Once the queue is closed, no more values will be enqueued.
        // So resolve any pending Promises with the end-of-stream marker
        while(this.resolvers.length > 0) {
            this.resolvers.shift()(AsyncQueue.EOS);
        }
        this.closed = true;
    }
    // Define the method that makes this class asynchronously iterable
    [Symbol.asyncIterator]() { return this; }
    // Define the method that makes this an asynchronous iterator. The
    // dequeue() Promise resolves to a value or the EOS sentinel if we're
    // closed. Here, we need to return a Promise that resolves to an
    // iterator result object.
    next() {
        return this.dequeue().then(value => (value === AsyncQueue.EOS)
                                   ? { value: undefined, done: true }
                                   : { value: value, done: false });
    }
}
// A sentinel value returned by dequeue() to mark "end of stream" when closed
AsyncQueue.EOS = Symbol("end-of-stream");

// Push events of the specified type on the specified document element
// onto an AsyncQueue object, and return the queue for use as an event stream
function eventStream(elt, type) {
    const q = new AsyncQueue();                  // Create a queue
    elt.addEventListener(type, e=>q.enqueue(e)); // Enqueue events
    return q;
}
async function handleKeys() {
    // Get a stream of keypress events and loop once for each one
    for await (const event of eventStream(document, "keypress")) {
        console.log(event.key);
    }
}

// Returns {value: 1, writable:true, enumerable:true, configurable:true}
Object.getOwnPropertyDescriptor({x: 1}, "x");
// Here is an object with a read-only accessor property
const random = {
    get octet() { return Math.floor(Math.random()*256); },
};
// Returns { get: /*func*/, set:undefined, enumerable:true, configurable:true}
Object.getOwnPropertyDescriptor(random, "octet");
// Returns undefined for inherited properties and properties that don't exist.
Object.getOwnPropertyDescriptor({}, "x")        // => undefined; no such prop
Object.getOwnPropertyDescriptor({}, "toString") // => undefined; inherited

let o = {};  // Start with no properties at all
// Add a non-enumerable data property x with value 1.
Object.defineProperty(o, "x", {
    value: 1,
    writable: true,
    enumerable: false,
    configurable: true
});
// Check that the property is there but is non-enumerable
o.x            // => 1
Object.keys(o) // => []
// Now modify the property x so that it is read-only
Object.defineProperty(o, "x", { writable: false });
// Try to change the value of the property
o.x = 2;      // Fails silently or throws TypeError in strict mode
o.x           // => 1
// The property is still configurable, so we can change its value like this:
Object.defineProperty(o, "x", { value: 2 });
o.x           // => 2
// Now change x from a data property to an accessor property
Object.defineProperty(o, "x", { get: function() { return 0; } });
o.x           // => 0

let p = Object.defineProperties({}, {
    x: { value: 1, writable: true, enumerable: true, configurable: true },
    y: { value: 1, writable: true, enumerable: true, configurable: true },
    r: {
        get() { return Math.sqrt(this.x*this.x + this.y*this.y); },
        enumerable: true,
        configurable: true
    }
});
p.r  // => Math.SQRT2

/*
 * Define a new Object.assignDescriptors() function that works like
 * Object.assign() except that it copies property descriptors from
 * source objects into the target object instead of just copying
 * property values. This function copies all own properties, both
 * enumerable and non-enumerable. And because it copies descriptors,
 * it copies getter functions from source objects and overwrites setter
 * functions in the target object rather than invoking those getters and
 * setters.
 *
 * Object.assignDescriptors() propagates any TypeErrors thrown by
 * Object.defineProperty(). This can occur if the target object is sealed
 * or frozen or if any of the source properties try to change an existing
 * non-configurable property on the target object.
 *
 * Note that the assignDescriptors property is added to Object with
 * Object.defineProperty() so that the new function can be created as
 * a non-enumerable property like Object.assign().
 */
Object.defineProperty(Object, "assignDescriptors", {
    // Match the attributes of Object.assign()
    writable: true,
    enumerable: false,
    configurable: true,
    // The function that is the value of the assignDescriptors property.
    value: function(target, ...sources) {
        for(let source of sources) {
            for(let name of Object.getOwnPropertyNames(source)) {
                let desc = Object.getOwnPropertyDescriptor(source, name);
                Object.defineProperty(target, name, desc);
            }
            for(let symbol of Object.getOwnPropertySymbols(source)) {
                let desc = Object.getOwnPropertyDescriptor(source, symbol);
                Object.defineProperty(target, symbol, desc);
            }
        }
        return target;
    }
});
let o = {c: 1, get count() {return this.c++;}}; // Define object with getter
let p = Object.assign({}, o);                   // Copy the property values
let q = Object.assignDescriptors({}, o);        // Copy the property descriptors
p.count   // => 1: This is now just a data property so
p.count   // => 1: ...the counter does not increment.
q.count   // => 2: Incremented once when we copied it the first time,
q.count   // => 3: ...but we copied the getter method so it increments.

// Create a sealed object with a frozen prototype and a non-enumerable property
let o = Object.seal(Object.create(Object.freeze({x: 1}),
                                  {y: {value: 2, writable: true}}));