How to Exit, Stop, or Break an Array#forEach Loop in JavaScript or Node.js

Leaving a loop early is a common practice in programming. Stopping or breaking out of an Array#forEach iteration in JavaScript is only possible by throwing an exception.

Also, the Mozilla Developer Network states “when you need to stop forEach, it’s the wrong tool.

This tutorial points you to alternatives that exit early when meeting a condition.

Node.js Series Overview

Exit a forEach Loop Early

When searching MDN for the Array#forEach method, you’ll find the following paragraph:

There is no way to stop or break a forEach() loop other than by throwing an exception. If you need such behavior, the forEach() method is the wrong tool.

Fair enough. What are alternatives?

The good news: you have a handful of other ways early exiting a loop:

In one of our projects, we decided to replace our Array#forEach statement using a for…of loop. This way, we are able to exit the loop early using a break statement.

Here are Two Examples

Imagine two virtualization hosts each running 15 virtual machines. The task is to consolidate as many VMs as possible on the second host. If the second host is missing any free capacity for more VMs, leave them on the first host.

Here are simplified sample code snippets using Array#forEach, Array#some, and a for…of loop. Each loop moves all VMs not running MySQL from the first host to the second one. The loop can stop as soon as the first host doesn’t run any VM with MySQL.

Using Array#some

Using Array#some allows you to iterate through the list of items until a condition evaluates to true. Related to the example above you can move as many VMs onto the second host until it’s full. Array#some stops iterating as soon as the condition secondHost.isFull() returns true:

firstHost.vms().some(vm => {  
  vm.moveTo(secondHost)

  return secondHost.isFull() // exits the loop if the second host is full
})

Using a for…of Loop

You may use a for…of loop instead of Array#forEach. You can exit a for…of loop using the break keyword. Then, JavaScript will break out of the loop:

for (vm of firstHost.vms()) {  
  vm.moveTo(secondHost)

  if (secondHost.isFull()) {
    break
  }
}

Find more details about exiting a for loop in JavaScript in a related tutorial here on Future Studio.

Sweet!


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