如何编写异步函数以在 javascript 中查找字符串枚举？

Question

I did a function to find string enumerations but i didn `t succeed to repeat it in a async way, i'm not getting any result:

const stringEnumerationsAsync = async (str) => {
    let strChars = str.split("");
    let enumerations = new Set();

    const getEnumerations = (chars, enumertion) => {
        return new Promise((resolve) => {
            if (chars.length === 0) {
                resolve(enumertion);
            } else {
                for (let i = 0; i < chars.length; i++) {
                    let newChars = [...chars];
                    let char = newChars.splice(i, 1);
                    let newEnumeration = enumertion + char;
                    getEnumerations(newChars, newEnumeration).then((finalEnumeration) => {
                        enumerations.add(finalEnumeration);
                    });
                }
            }
        });
    };

    await getEnumerations(strChars, "");

    return [...enumerations];
};

Answer 1

There are a few issues:

• with getEnumerations(newChars, newEnumeration).then((finalEnumeration)... you seem to expect one enumeration from the recursive call, but you should expect one or more enumerations.

• In fact, you are mixing up two patterns: one where you collect the result from (resolved) return values, and one where you collect them in a non-local variable. So if you go for the latter, you should not expect anything useful from the resolved value you get from the recursive call. Instead add enumerations to the non-local variable in the base case, and only there.

• In the else block you don't call resolve , meaning that the new Promise never gets resolved.

• You should actually not need to create a new Promise in the else case, as you will get promises from the recursive calls, and it is an anti-pattern to wrap that inside a promise constructor call back. So the if...else should be moved outside of new Promise

• And then in the if case you would get an immediately resolving promise, because you call resolve immediately. In that case you might as well call Promise.resolve instead of new Promise .

• Even more simple is to make getEnumerations an async function so that you only need to return the resolution value, and since we don't care about the resolution value,... well, we just need to return ;-)

So that brings us to this code:

 const stringEnumerationsAsync = async (str) => { let strChars = str.split(""); let enumerations = new Set(); const getEnumerations = async (chars, enumertion) => { if (chars.length === 0) { enumerations.add(enumertion); } else { for (let i = 0; i < chars.length; i++) { let newChars = [...chars]; let char = newChars.splice(i, 1); let newEnumeration = enumertion + char; await getEnumerations(newChars, newEnumeration); } } }; await getEnumerations(strChars, ""); return [...enumerations]; }; // demo stringEnumerationsAsync("abc").then(console.log); console.log("this is synchronous");

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