.catch()

The catch() method is used to handle errors during the execution of an asynchronous operation.

The method takes one argument:

• onReject — a callback function that will be called when the promise transitions to the "error" state rejected. The function has one parameter that receives information about the error.

It returns a promise.

        
          
          // getPasswords() — an asynchronous function that returns a promisegetPasswords()  .then(function (result) {    // will execute if the operation is successful  })  .catch(function (err) {    // the callback will execute if getPassword finishes with an error    alert(err.message)  })
          // getPasswords() — an asynchronous function that returns a promise
getPasswords()
  .then(function (result) {
    // will execute if the operation is successful
  })
  .catch(function (err) {
    // the callback will execute if getPassword finishes with an error
    alert(err.message)
  })

        
        
          
            
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catch() executes the provided callback when the asynchronous operation:

• calls the reject() function inside the promise.

        
          
          const rejectInSecond = new Promise(function (resolve, reject) {  setTimeout(function () {    reject(new Error('time is up'))  }, 1000)})rejectInSecond.catch(function (err) {  console.error(err.message)  // 'time is up'})
          const rejectInSecond = new Promise(function (resolve, reject) {
  setTimeout(function () {
    reject(new Error('time is up'))
  }, 1000)
})

rejectInSecond.catch(function (err) {
  console.error(err.message)
  // 'time is up'
})

        
        
          
            
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• throws an error using throw.

        
          
          const throwInSecond = new Promise(function (resolve, reject) {  setTimeout(function () {    throw new Error('time is up')  }, 1000)})throwInSecond.catch(function (err) {  console.error(err.message)  // 'time is up'})
          const throwInSecond = new Promise(function (resolve, reject) {
  setTimeout(function () {
    throw new Error('time is up')
  }, 1000)
})

throwInSecond.catch(function (err) {
  console.error(err.message)
  // 'time is up'
})

        
        
          
            
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🔧 Technical detail

Under the hood, catch() contains a call to then(), where the first callback is set to undefined: catch(onReject) → then(undefined, onReject).

In practice

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Advice 1

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🛠 Always complete your promise chains by calling the catch() method. If an error occurs in one of the operations in the chain and it is not handled, JavaScript will output the message Uncaught (in promise) Error in the developer console and will stop working on the entire page.