generatePairs and Future

future.js

class Future {
   #value = null;
   #error = null;
   #successCallbacks = [];
   #failureCallback = () => {}
   #finalCallback = () => {}

   constructor(fn) {
       const onSuccess = (value) => {
           this.#value = value;
           this.state = 'succeeded';
           this.#successCallbacks.forEach(cb => cb(this.#value));
           this.#finalCallback({state: this.state, value: this.#value, error: this.#error});
       }
       const onFailure = (error) => {
           this.#error = error;
           this.state = 'failed';
           this.#failureCallback(this.#error);
           this.#finalCallback({state: this.state, value: this.#value, error: this.#error});
       }
       this.state = 'pending';
       this.timeout = 1;
       fn(onSuccess, onFailure);

   }

   then(fnSuccess) {
       if(this.state === 'pending') {
           this.#successCallbacks.push(fnSuccess);
           return this;
       }
       fnSuccess(this.#value);
       return this;
   }
   catch(fnError) {
       if(this.state === 'pending') {
           this.#failureCallback = fnError;
           return this;
       }

       fnError(this.#error);
       return this;
   }
   finally(fnFinally) {
       if(this.state === 'pending') {
           this.#finalCallback = fnFinally;
           return;
       }
       fnFinally(this.#value);
   }
}


function doesSomethingAsync(a, b) {
   return new Future(function fn(success, failure) {
       setTimeout(() => {
           if (b === 0) {
               return failure("Cannot divide by 0");
           }
           return success(a / b);
       }, 2 * 1000);
   });
}

doesSomethingAsync(10, 20)
   .then(console.log)                          // 0.5
   .then(value => console.info("info", value)) // info 0.5
   .catch(console.error)
   .finally(console.log);                      // { state: 'succeeded', value: 0.5, error: null }

doesSomethingAsync(10, 0)
   .then(console.log)                          //
   .then(value => console.info("info", value)) //
   .catch(console.error)                       // Cannot divide by 0
   .finally(console.log);                      // { state: 'failed', value: null, error: 'Cannot divide by 0' }

const x = doesSomethingAsync(22, 7);
console.log(x, typeof x); // Future { state: 'pending', timeout: 1 } object

setTimeout(() => {}, 3000);
x.then(console.log); // 3.142857142857143

generatePairs.js

function generatePairs(n) {
    if(n < 0) {
        return [];
    }
    const pairs = [];
    for(let diff = 1; diff <= n; diff++) {
        for(let i = 0; i < n; i++) {
			if(i + diff > n) { break; }
			pairs.push([i, i+diff]);
		}
    }
    return pairs;
}