how to break the each function in underscorejs

The _.each function in Underscore.js is a fundamental utility for iterating over collections, be they arrays or objects. It provides a clean, functional way to process each element, abstracting away the complexities of traditional for loops. However, a common challenge developers face is the need to break the _.each function in Underscore.js prematurely when a certain condition is met. Unlike native JavaScript loops like for or while, which support break and continue statements for controlling iteration, _.each does not inherently provide a direct mechanism for early termination. This can lead to inefficient code if you process an entire collection when only a subset of operations is necessary. Understanding its design and available workarounds is crucial for writing optimized and maintainable JavaScript.

Understanding the Design of _.each

Underscore.js’s _.each (also aliased as _.forEach) is designed to iterate through all elements in a list or properties in an object, invoking a provided iteratee callback function for each one. The core philosophy behind _.each is to ensure every element gets processed. It’s built on the concept of functional programming, where a function processes data without causing side effects or altering external state unnecessarily, and typically runs to completion across the entire input.

When you call _.each(collection, iteratee), the iteratee function is executed for every item. Because this iteratee is a simple function call within the internal loop of _.each, standard JavaScript control flow statements like break, continue, or return (from the iteratee itself) only affect the scope of that single iteratee call, not the encompassing _.each loop. A return statement inside the iteratee simply exits that specific callback invocation and moves to the next element, rather than stopping the entire iteration. This behavior is by design, promoting a pattern where all elements are considered.

This distinction from traditional imperative loops is vital. For instance, in a for loop, break immediately halts the loop’s execution. In contrast, _.each prioritizes uniformity and completion. This design choice, while elegant for simple iterations, necessitates alternative strategies when an early exit is genuinely required for performance or logical reasons. Developers often reach for _.each out of habit, but knowing its limitations helps in choosing the right tool for the job.

Strategies for Early Exit in Underscore.js

While _.each doesn’t support a direct break, there are several effective strategies to achieve an early exit from an iteration. The choice depends on the desired behavior and the context of your application.

1. Throwing an Exception

One common, albeit somewhat controversial, method to halt an _.each loop is to throw an exception from within the iteratee. This works because an unhandled exception will naturally propagate up the call stack, effectively stopping the execution of the _.each function itself. To prevent the program from crashing, you must wrap the _.each call in a try...catch block.

Consider a scenario where you’re searching for the first element that meets a specific criterion:

let foundItem = null; const myCollection = [10, 20, 30, 40, 50]; try { _.each(myCollection, (item) => { if (item > 25) { foundItem = item; throw new Error('FoundIt'); // Custom error to signal early exit } console.log(Processing item: ${item}); // This will not execute after 20 }); } catch (e) { if (e.message !== 'FoundIt') { throw e; // Re-throw unexpected errors } } console.log(First item greater than 25: ${foundItem}); // Output: 30 

This method allows you to control flow by forcing an abrupt stop. However, it’s generally not considered idiomatic JavaScript for loop control, as exceptions are typically reserved for exceptional error conditions rather than regular control flow. Misusing exceptions can make code harder to debug and understand, potentially impacting performance optimization due to the overhead of exception handling.

2. Utilizing _.some or _.every for Conditional Iteration

For scenarios where you need to stop iteration based on a condition, Underscore.js provides more semantic and cleaner alternatives than throwing exceptions: _.some (aliased as _.any) and _.every (aliased as _.all). These functions are specifically designed for conditional iteration and naturally support an early exit.

To effectively break the _.each function in Underscore.js when a condition is met, the most idiomatic and recommended approach is to use _.some. The _.some function iterates over a collection, calling a predicate function for each element. If the predicate returns a truthy value for any element, _.some immediately stops iterating and returns true. Otherwise, if no element satisfies the predicate, it iterates through all elements and returns false. This perfectly aligns with the requirement of finding the first matching element and stopping.

Here’s how to achieve the same “find first” logic using _.some:

let foundItemSome = null; const myCollection = [10, 20, 30, 40, 50]; _.some(myCollection, (item) => { if (item > 25) { foundItemSome = item; return true; // Return true to stop iteration } console.log(Processing item with _.some: ${item}); // This will not execute after 20 return false; // Continue iteration }); console.log(First item greater than 25 (using _.some): ${foundItemSome}); // Output: 30 

Similarly, _.every iterates until the predicate returns a falsy value, then stops and returns false. If all elements satisfy the predicate, it returns true. These functions are ideal for conditional iteration and are the preferred way to achieve an early exit in Underscore.js when your goal is to check for existence or universal satisfaction.

3. Modifying the Collection (Discouraged)

While technically possible, modifying the collection itself Question & Answer :

I’m looking for a way to stop iterations of underscore.js _.each() method, but can’t find the solution. jQuery .each() can break if you do return false.

Is there a way to stop underscore each()?

_([1,2,3]).each(function(v){ if (v==2) return /*what?*/; }) 

You can’t break from the each method—it emulates the native forEach method’s behavior, and the native forEach doesn’t provide to escape the loop (other than throwing an exception).

However, all hope is not lost! You can use the Array.every method. :)

From that link:

every executes the provided callback function once for each element present in the array until it finds one where callback returns a false value. If such an element is found, the every method immediately returns false.

In other words, you could do something convoluted like this (link to JSFiddle):

[1, 2, 3, 4].every(function(n) { alert(n); return n !== 3; }); 

This will alert 1 through 3, and then “break” out of the loop.

You’re using underscore.js, so you’ll be pleased to learn that it does provide an every method—they call it every, but as that link mentions, they also provide an alias called all.