Javascript Promise (Examples)

Tech:
Javascript
Since:
1 year ago
Views:
8

In Javascript promises are used to handle asynchronous operations. It is either resolved or rejected based on the result of the asynchronous operation.

Basic 1

The following example shows how to create a promise and how to resolve it.

let done = true;

new Promise((resolve, reject) => {
  if (done) {
    resolve("Job done");
  } else {
    reject("Still working");
  }
})
  .then(val => {
    console.log(val);
  })
  .catch(val => {
    console.log(val);
  });

Basic 2

The following example shows how to create a promise in a variable and how to listen to the promise using that variable.

let done = true;

const isJobDone = new Promise((resolve, reject) => {
  if (done) {
    resolve("Job done");
  } else {
    reject("Still working");
  }
});

isJobDone
  .then(val => {
    console.log(val);
  })
  .catch(val => {
    console.log(val);
  });

Basic 3

The following example shows how to create a promise in a variable and how to listen whenever we want.

let done = true;

const isJobDone = new Promise((resolve, reject) => {
  if (done) {
    resolve("Job done");
  } else {
    reject("Still working");
  }
});

const check = function() {
  isJobDone
    .then(val => {
      console.log(val);
    })
    .catch(val => {
      console.log(val);
    });
};

check();

Chaining 1

This example shows how to chain promises.

let done = true;

new Promise(function(resolve, reject) {
  if (done) {
    resolve("Job done");
  } else {
    reject("Still working");
  }
})
  .then(function(val) {
    return val + " - Verified";
  })
  .then(function(val) {
    console.log(val);
  })
  .catch(val => {
    console.log(val);
  });

Chaining 2

This example shows how to chain promises by returning a promise.

const check1 = function(response) {
  if (response == "Job done") {
    return Promise.resolve(response + "- verified");
    // or
    return response + "- verified";
  } else {
    return Promise.reject("Verification failed");
    // or
    return "Verification failed";
  }
};

const check2 = function(response) {
  console.log(response);
};

let done = true;

new Promise(function(resolve, reject) {
  if (done) {
    resolve("Job done");
  } else {
    reject("Still working");
  }
})
  .then(check1)
  .then(check2)
  .catch(val => {
    console.log(val);
  });
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