resolve-once/dist/esm/index.js

Resolves a promise only once and memoizes the result

const UNRESOLVED = 0;
const RESOLVING = 1;
const RESOLVED_SUCCESS = 2;
const RESOLVED_ERROR = 3;
export default function resolveOnce(fn) {
    let state = UNRESOLVED;
    let result;
    const waiting = [];
    function resolveResult() {
        if (state === RESOLVING) return;
        state = RESOLVING;
        Promise.resolve(fn()).then((value)=>{
            state = RESOLVED_SUCCESS;
            result = value;
            while(waiting.length)waiting.pop().resolve(result);
        }).catch((err)=>{
            state = RESOLVED_ERROR;
            result = err;
            while(waiting.length)waiting.pop().reject(result);
        });
    }
    return ()=>{
        if (state === RESOLVED_SUCCESS) return Promise.resolve(result);
        if (state === RESOLVED_ERROR) return Promise.reject(result);
        const promise = new Promise((resolve, reject)=>{
            waiting.push({
                resolve: resolve,
                reject: reject
            });
        });
        resolveResult();
        return promise;
    };
}