JavaScript Promises - reject vs. throw
There is no advantage of using one vs the other, but, there is a specific case where throw
won't work. However, those cases can be fixed.
Any time you are inside of a promise callback, you can use throw
. However, if you're in any other asynchronous callback, you must use reject
.
For example, this won't trigger the catch:
new Promise(function() {
setTimeout(function() {
throw 'or nah';
// return Promise.reject('or nah'); also won't work
}, 1000);
}).catch(function(e) {
console.log(e); // doesn't happen
});
Instead you're left with an unresolved promise and an uncaught exception. That is a case where you would want to instead use reject
. However, you could fix this in two ways.
- by using the original Promise's reject function inside the timeout:
new Promise(function(resolve, reject) {
setTimeout(function() {
reject('or nah');
}, 1000);
}).catch(function(e) {
console.log(e); // works!
});
- by promisifying the timeout:
function timeout(duration) { // Thanks joews
return new Promise(function(resolve) {
setTimeout(resolve, duration);
});
}
timeout(1000).then(function() {
throw 'worky!';
// return Promise.reject('worky'); also works
}).catch(function(e) {
console.log(e); // 'worky!'
});
Another important fact is that reject()
DOES NOT terminate control flow like a return
statement does. In contrast throw
does terminate control flow.
Example:
new Promise((resolve, reject) => {
throw "err";
console.log("NEVER REACHED");
})
.then(() => console.log("RESOLVED"))
.catch(() => console.log("REJECTED"));
vs
new Promise((resolve, reject) => {
reject(); // resolve() behaves similarly
console.log("ALWAYS REACHED"); // "REJECTED" will print AFTER this
})
.then(() => console.log("RESOLVED"))
.catch(() => console.log("REJECTED"));