为何mongoose代码触发ESLint的Unexpected function expression报错？

Hey there! Let's break down this ESLint error you're hitting with your Mongoose code.

问题根源 #

The Unexpected function expression. (prefer-arrow-callback) error comes directly from ESLint's prefer-arrow-callback rule. This rule is designed to encourage using arrow functions instead of traditional function expressions for callbacks, and here's why:

• Arrow functions don't bind their own this value — they inherit the this from the surrounding scope, which avoids common bugs where this points to something unexpected in callbacks.
• They have a more concise syntax that makes your code cleaner and easier to read.

In your code, you're using a standard function() expression as the callback for kitty.save(), which violates this rule.

修复方案 #

The fix is straightforward: replace the traditional function with an arrow function. Here's how your code can be adjusted:

原代码 #

kitty.save(function (err) { 
  if (err) { 
    console.log('a'); 
  } 
});

修改后的代码 #

kitty.save((err) => { 
  if (err) { 
    console.log('a'); 
  } 
});

If you want to make it even more concise (since the callback only has one parameter), you can omit the parentheses around err:

kitty.save(err => { 
  if (err) { 
    console.log('a'); 
  } 
});

临时禁用规则（不推荐） #

If for some reason you need to keep the traditional function (e.g., if you rely on the callback's own this), you can disable the rule for that specific line with a comment:

// eslint-disable-next-line prefer-arrow-callback
kitty.save(function (err) { 
  if (err) { 
    console.log('a'); 
  } 
});

额外说明 #

Using arrow functions for Mongoose callbacks is generally a good practice because it keeps the this context consistent with your outer code (like if you're working inside a class method). You won't have to do tricks like const that = this or .bind(this) to access outer scope variables.

内容的提问来源于stack exchange，提问作者Saeed Heidarizarei