Great question! Let's break down the key differences between Array.prototype.reduce() and Array.prototype.join(), when to pick each, and how they stack up performance-wise.

Core Functional Differences #

First, let's get the basics straight:

• join(): This is a purpose-built method for turning array elements into a single string. It takes an optional separator argument (defaults to a comma), and handles all the heavy lifting of concatenation under the hood. For your example with ["H","e","l","l","o"], arr.join('') directly spits out "Hello"—no extra logic needed. It's single-responsibility, clean, and immediately readable.
• reduce(): This is a general-purpose accumulator. It can do way more than just string concatenation (think summing numbers, merging objects, filtering data on the fly). To merge strings with reduce, you have to write a custom callback:

  arr.reduce((accumulator, currentChar) => accumulator + currentChar, '')
  

  It's flexible, but it's not optimized specifically for string joining.

When to Choose Which Method? #

Go with join() if: #

• Your only goal is to concatenate array elements into a string, with no extra processing per element.
• Readability matters most—any developer glancing at arr.join('') will instantly know what you're trying to do, whereas a reduce callback takes a split second to parse.
• You're working with a large array (more on performance below).

Go with reduce() if: #

• You need to modify or filter elements while concatenating. For example:
  • Skip empty strings or unwanted characters:

    // Remove spaces before joining
    ["H", " ", "e", "l", "l", "o"].reduce((acc, curr) => curr !== ' ' ? acc + curr : acc, '')
    

  • Transform elements on the fly (e.g., uppercase certain characters):

    // Capitalize the first letter, lowercase others
    ["h", "E", "L", "L", "O"].reduce((acc, curr, idx) => idx === 0 ? acc + curr.toUpperCase() : acc + curr.toLowerCase(), '')
    

• You're already using reduce for other logic in the same code block, and it makes sense to chain or combine operations.

Performance Differences #

Here's where the gap gets noticeable:

• join() is drastically faster for pure string concatenation, especially with large arrays. Why? Browser engines have optimized join() to pre-calculate the total length of the final string, allocate memory once, and copy all elements into that single buffer. This avoids the overhead of immutable string operations.
• reduce() with acc + curr is slow because JavaScript strings are immutable—every + operation creates a brand new string in memory. For an array with 1 million elements, that's 1 million new string allocations, which eats up memory and slows down execution.
• Pro tip: If you must use reduce for string work (e.g., with processing), you can optimize it by accumulating into an array instead of a string, then joining at the end:

  arr.reduce((acc, curr) => { acc.push(curr); return acc; }, []).join('')
  

  This brings performance close to join(), but it's still redundant if you don't need the intermediate processing.

内容的提问来源于stack exchange，提问作者Ahmad F