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ViewsESLint reports a promise misuse for code I consider ok
In a symbol table implementation I have this method:
public getAllSymbols(type?: typeof Symbol, localOnly = false): Promise<Set<Symbol>> {
const promise = super.getAllSymbols(type ?? Symbol, localOnly);
return new Promise(async (resolve, reject) => {
try {
let result = await promise;
if (!localOnly) {
this.dependencies.forEach(async (dependency) => {
result = new Set([...result, ...await dependency.getAllSymbols(type, localOnly)]);
});
}
resolve(result);
} catch (reason) {
reject(reason);
}
});
}
which works fine, however ESLint reports 2 promise misuses:
Promise returned in function argument where a void return was expected.
no-misused-promises
What's wrong with this code and how would I have to write it to get rid of the linter error?
1 answers
Answer question0
Problems in your code:
Executor function shouldn't be
async- it's an anti-patternAs
super.getAllSymbol(...)already returns a promise, so no need to wrap it in a promise constructor - it's another anti-pattern. Call thethen()method directly on the promise returned bysuper.getAllSymbol(...)Using
async-awaitwithforEach()won't give you the expected result because the callback function offorEach()won't wait for the awaited promise to settle - it will just continue to the next iteration.You can use
Promise.all()along with themap()method to get the expected output.You could also use
for-ofloop but usingPromise.all()is better if you don't want all the promises to settle in a sequential manner.
Your code could be re-written as (types removed for simplicity):
public getAllSymbols(type, localOnly = false) {
const promise = super.getAllSymbols(type ?? Symbol, localOnly);
return promise
.then(result => {
if (!localOnly) {
return Promise.all(this.dependencies.map(dep => (
dep.getAllSymbols(type, localOnly))
)))
.then(resultArr => {
return new Set([...result, ...resultArr]);
});
}
else {
return result;
}
});
}
or you could use the async-await syntax:
public async getAllSymbols(type, localOnly = false) {
const result = await super.getAllSymbols(type ?? Symbol, localOnly);
if (!localOnly) {
const resultArr = await Promise.all(this.dependencies.map(dep => (
dep.getAllSymbols(type, localOnly)
)));
return new Set([...result, ...resultArr]);
}
return result;
}
I have removed the catch block because, in my opinion, calling code should handle the errors, if any.
Above function can be called as:
getAllSymbols(...)
.then(result => { ... })
.catch(error => { ... });
or you can use async-await syntax:
try {
const result = await getAllSymbols(...);
...
} catch (error) {
// handle error
}