What does the JSLint error 'body of a for in should be wrapped in an if statement' mean?

javascript

for-in-loops

javascript-best-practices

prototype-inheritance

byAnton Shumikhin·Mar 10, 2025

The JSLint error "body of a for in should be wrapped in an if statement" appears when you use a for...in loop without a corresponding hasOwnProperty check. Here's how to avoid it:

for (var key in obj) {
  if (obj.hasOwnProperty(key)) {
    // Safe to proceed, cap'n!
  }
}

In a nutshell, the hasOwnProperty function includes only the object's own properties, excluding inherited ones.

Going beyond surface: for...in loops

JavaScript's for...in loop traverses over an object's properties. It, however, includes inherited properties due to JavaScript's prototype-based inheritance, a phenomenon that can lead to some spooky, unexpected results. 👻

Array vs Objects: Choose your weapon

for...in might be tempted to iterate over an array, but resist. Arrays and traditional for loops are a perfect duo. For key-value structures, for...in with a hasOwnProperty filter is your go-to choice:

for (var key in myObject) {
    if (myObject.hasOwnProperty(key)) {
        // Go wild here, but don't wake up regretting anything! 😜
    }
}

Defensive programming: Play it safe, kid

Your best defense against potential horror movie material includes:

Prototype Invasion: Safety first! Keep an eye out for unexpected properties sneaking in while using libraries or frameworks that extend prototypes.
Inheritance Inspections: Engage hasOwnProperty to sift out inherited properties, keeping focus on the object's personal belongings.
Strict Comparison: JavaScript is a loose cannon. Ensure to use === to avoid type coercion blindsides.

jQuery for the save

If jQuery is a part of your toolkit, enjoy a clean ride with $.each():

$.each(myObject, function(key, value) {
  // Enjoy the smooth ride, BUT keep your seatbelts fastened ✌️
});

However, JavaScript isn't the wild west anymore. Stannis-approved official functions like Object.keys, Object.entries, and array iteration functions offer safer alternatives.

ES6: Future is here

With ES6, the Object.keys() and Object.entries() methods allow safe traversal over an object's properties, almost rendering for...in loops obsolete.

Object.keys(myObject).forEach((key) => {
   // Get in! We're going iterating.
});

For a solid, Crockford-approved set of practices, check out Douglas Crockford's video series. Also, the Airbnb JS Style Guide has some pretty nifty tips for typists of all kinds.

explain-codes / Javascript / What does the JSLint error 'body of a for in should be wrapped in an if statement' mean?

Linked

Iterate through object properties

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Getting a list of associative array keys

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How to loop through a plain JavaScript object with the objects as members

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Javascript for...in vs for

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Checking if a key exists in a JavaScript object?

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