JavaScript: Run a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI
JavaScript: Exercise-151 with Solution
Write a JavaScript program to run a function in a separate thread by using a Web Worker, allowing long running functions to not block the UI.
Note: Create a new Worker using a Blob object URL, the contents of which should be the stringified version of the supplied function. Immediately post the return value of calling the function back. Return a promise, listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.
NOTE: Since the function is running in a different context, closures are not supported. The function supplied to `runAsync` gets stringified, so everything becomes literal. All variables and functions must be defined inside.
- Create a new Worker() using a Blob object URL, the contents of which should be the stringified version of the supplied function.
- Immediately post the return value of calling the function back.
- Return a new Promise(), listening for onmessage and onerror events and resolving the data posted back from the worker, or throwing an error.
Sample Solution:
JavaScript Code:
//#Source https://bit.ly/2neWfJ2
const runAsync = fn => {
const worker = new Worker(
URL.createObjectURL(new Blob([`postMessage((${fn})());`]), {
type: 'application/javascript; charset=utf-8'
})
);
return new Promise((res, rej) => {
worker.onmessage = ({ data }) => {
res(data), worker.terminate();
};
worker.onerror = err => {
rej(err), worker.terminate();
};
});
};
const longRunningFunction = () => {
let result = 0;
for (let i = 0; i < 1000; i++) {
for (let j = 0; j < 700; j++) {
for (let k = 0; k < 300; k++) {
result = result + i + j + k;
}
}
}
return result;
};
/*
*/
runAsync(longRunningFunction).then(console.log); // 209685000000
runAsync(() => 10 ** 3).then(console.log); // 1000
let outsideVariable = 50;
runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'
Sample Output:
1000 undefined
Flowchart:

Live Demo:
See the Pen javascript-basic-exercise-151-1 by w3resource (@w3resource) on CodePen.
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JavaScript: Tips of the Day
Checking if a key exists in a JavaScript object?
Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?
var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists!
You should instead use the in operator:
"key" in obj // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:
!("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
obj.hasOwnProperty("key") // true
Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined? var obj = { key: undefined }; obj["key"] !== undefined // false, but the key exists! You should instead use the in operator: "key" in obj // true, regardless of the actual value If you want to check if a key doesn't exist, remember to use parenthesis: !("key" in obj) // true if "key" doesn't exist in object !"key" in obj // ERROR! Equivalent to "false in obj" Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty: obj.hasOwnProperty("key") // true For performance comparison between the methods that are in, hasOwnProperty and key is undefined.
Ref: https://bit.ly/2CFNp1X
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