How do I get the list() method to wait for the data to be loaded in the constructor before it resolves its promise back to the caller?
import fetch from 'node-fetch';
class Employees {
constructor() {
if (Employees._instance) {
return Employees._instance
}
Employees._instance = this;
this.employees = [];
this.dataLoaded = false;
this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';
(async () => {
const response = await fetch(this.url);
this.employees = await response.json();
this.dataLoaded = true;
console.log(`work done: got ${this.employees.length} employees`);
})();
}
list() {
return new Promise((resolve) => {
resolve(this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
});
}
}
const employees = new Employees();
(async () => {
console.log(await employees.list());
})();
I recommend having the constructor save the promise from the data loading onto this, and then list can await that promise:
class Employees() {
constructor() {
if (Employees._instance) {
return Employees._instance
}
Employees._instance = this;
this.employees = [];
this.dataLoaded = false;
this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';
this.initPromise = (async () => {
const response = await fetch(this.url);
this.employees = await response.json();
this.dataLoaded = true;
console.log(`work done: got ${this.employees.length} employees`);
})();
}
async list() {
await this.initPromise;
return this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
}
}
If the load hasn't finished yet, then the await will cause list to wait however long is necessary. If loading has finished, then initPromise is in a resolved state, and list will resume more or less immediately (when the microtask queue executes).
Use a Deferred object.
a deferred represents work that is not yet finished
// fetch mock with 3 seconds wait
fetch = () => {
return new Promise((resolve) => {
setTimeout(() => {
resolve({
json: () => new Promise((r) => r([
{firstName: 'Arthur', lastName: 'Pym', id: 1},
{firstName: 'August', lastName: 'Barnard', id: 2},
{firstName: 'M.', lastName: 'Poe', id: 3}
]))
});
}, 3000);
});
}
class MyDeferred {
resolve = null;
reject = null;
promise = null;
constructor() {
this.promise = new Promise((res, rej) => {
this.resolve = res;
this.reject = rej;
});
}
}
class Employees {
deferred = new MyDeferred();
constructor() {
if (Employees._instance) {
return Employees._instance
}
Employees._instance = this;
this.employees = [];
this.dataLoaded = false;
this.url = 'https://raw.githubusercontent.com/graphql-compose/graphql-compose-examples/master/examples/northwind/data/json/employees.json';
(async () => {
const response = await fetch(this.url);
this.employees = await response.json();
this.dataLoaded = true;
this.deferred.resolve();
console.log(`work done: got ${this.employees.length} employees`);
})();
}
list() {
return new Promise((resolve) => {
this.deferred.promise.then(() => {
resolve(this.employees.map(m => `${m.firstName} ${m.lastName} (${m.id})`));
});
});
}
}
const employees = new Employees();
(async () => {
console.log(await employees.list());
})();