README.md

Promise APIs

1. Promise.all()
2. Promise.allSettled()
3. Promise.any()
4. Promise.race()
5. Promise.resolve()
6. Promise.reject()

• Promise.all():
  • It executes multiple promises and return a promise.
  • It will wait for all the input promises to get resolved.
  • If any of the input promises gets rejected, Promise.all also gets rejected immediately.
  • The execution time of Promise.all() is depends on the maximum time taken by an input promise.

Example

const red = new Promise((resolve, reject) => {
   setTimeout(() => resolve("red), 1000);
});

const green = new Promise((resolve, reject) => {
   setTimeout(() => resolve("green"), 3000);
});

const blue = new Promise((resolve, reject) => {
   setTimeout(() => resolve("blue"), 5000);
});

//using .then()

const allPromises = Promise.all([red, green, blue]);
allPromises.then((value) => console.log(value));
allPromises;

//using async and await

const testAll = async () => {
   const colors = await Promise.all([red, green, blue]);
   console.log(colors);
};
testAll();

Output:
["red", "green", "blue"]

• Promise.allSettled:

  • The only difference between Promise.all() and Promise.allSettled(), With Promise.allSettled(), if any of the input promise gets rejected, it rejects only that promise and executes rest all other promise.
  • Output will contain the status and value for resolved promises and for rejected promises status and reason of rejection.

  Example:

  const red = new Promise((resolve, reject) => {
     setTimeout(() => resolve("red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => reject("Error:green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = Promise.allSettled([red, green, blue]);
     colors.map((color) => console.log(color));
  };
  
  testAll();
  
  Output:
  {status: 'fulfilled', value: 'red'}
  {status: 'rejected', reason: 'Error:green'}
  {status: 'fulfilled', value: 'blue'}
  
  

  • Promise.any():
    • It returns a promise if any one of the input promise gets resolved.
    • If all the input promises are gets rejected , Promise.any() will throw an AggregateError: All promises were rejected

  Example:

  Scenario 1:
  
  const red = new Promise((resovle,reject) => {
     setTimeout(() => resolve("red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => resolve("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.any([red, green, blue]);
     console.log(colors)
  };
  
  testAll();
  
  Output:
  red - As it is one to get resolved first.
  
  Scenario 2:
  
  const red = new Promise((resovle,reject) => {
     setTimeout(() => reject("Error:red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => resolve("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.any([red, green, blue]);
     console.log(colors)
  };
  
  testAll();
  
  Output:
  green - As this is the next one to get resolve first.
  
  Scenario 3:
  
  const red = new Promise((resovle,reject) => {
     setTimeout(() => reject("Error:red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => reject("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => reject("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.any([red, green, blue]);
     console.log(colors)
  };
  
  testAll();
  
  Output:
  AggregateError: All promises were rejected - As all the input promises gets rejected.
  
  

• Promise.race():

  This will execute the fastest input promise and return a promise. if it gets rsolved it will show the output. if te fastest promise gets rejected, Promise.race() will be rejected immediately.

  Example:

  const red = new Promise((resolve, reject) => {
     setTimeout(() => resolve("red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => resolve("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.race([red, green, blue]);
     console.log(colors);
  };
  
  testAll();
  
  Output:
  red as this is the promise will take less time than any other promise.
  
  Note: if red promise gets rejected, Promise.race() will get rejected immediately.
  
  

  • Promise.resolve():

  • This will reolve only one of the input promise whichever we are going to resolve.

  Example:

  const red = new Promise((resolve, reject) => {
     setTimeout(() => resolve("red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => resolve("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.resolve(red);
     console.log(colors);
  };
  testAll();
  
  Output:
  red - as we are resolving the same.
  
  

  • Promise.reject():

  • This will reject promise and preints the error stack trace.

  Example:

  const red = new Promise((resolve, reject) => {
     setTimeout(() => resolve("red"), 1000);
  });
  
  const green = new Promise((resolve, reject) => {
     setTimeout(() => reject("green"), 3000);
  });
  
  const blue = new Promise((resolve, reject) => {
     setTimeout(() => resolve("blue"), 5000);
  });
  
  const testAll = async () => {
     const colors = await Promise.reject(green);
     console.log(colors);
  };
  testAll();
  
  Output:
  UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "#<Promise>