jQuery jQuery.isPlainObject() Method

What You’ll Learn

The jQuery.isPlainObject() utility returns true when a value is a plain {}-style object — safe for $.extend() and option maps. This tutorial covers syntax, five worked examples from the official API, comparisons with $.type(), and guards before merging plugin settings.

Introduction

In JavaScript, many values are technically “objects” — arrays, DOM nodes, dates, and custom class instances all report typeof x === "object". jQuery plugins, however, usually expect configuration as a plain object: a simple key/value map created with {} or new Object().

jQuery provides jQuery.isPlainObject() (also written $.isPlainObject()) to distinguish those simple maps from everything else. Use it before $.extend(), before iterating option keys, and anywhere user input must be a plain settings object — not an array, DOM node, or class instance.

Understanding the jQuery.isPlainObject() Method

jQuery.isPlainObject(obj) returns true when obj is an object whose internal prototype is Object.prototype or null — the shapes produced by object literals and Object.create(null).

It returns false for null, arrays, functions, DOM elements, window, document, and instances of custom constructors whose prototype chain does not lead to plain Object.

📝 Syntax

General form of jQuery.isPlainObject:

jQuery.isPlainObject( obj )
// or
$.isPlainObject( obj )

Parameters

• obj — any JavaScript value to test.

Return value

• true if obj is a plain object.
• false for all other types and non-plain objects.

Minimal workflow

if ($.isPlainObject(userOptions)) {
  settings = $.extend({}, defaults, userOptions);
}

Examples Gallery

Each example uses $.isPlainObject(). Open DevTools or use the Try-it links to run them interactively.

console.log($.isPlainObject({}));           // true
console.log($.isPlainObject({ a: 1 }));     // true
console.log($.isPlainObject(new Object())); // true

true
true
true

console.log($.isPlainObject([]));       // false
console.log($.isPlainObject(null));     // false
console.log($.isPlainObject(window));    // false
console.log($.isPlainObject(document));  // false

false (×4)

var defaults = { theme: "light", lang: "en" };

function resolveSettings(input) {
  if (!$.isPlainObject(input)) {
    return $.extend({}, defaults);
  }
  return $.extend({}, defaults, input);
}

console.log(resolveSettings({ theme: "dark" }).theme); // dark
console.log(resolveSettings([]).theme);                // light

dark
light

function Config() {
  this.speed = 300;
}

console.log($.isPlainObject(new Config())); // false
console.log($.isPlainObject({ speed: 300 })); // true

false
true

var defaults = { size: "md" };

function applyOptions(opts) {
  if ($.isPlainObject(opts) && !$.isEmptyObject(opts)) {
    return $.extend({}, defaults, opts);
  }
  return $.extend({}, defaults);
}

console.log(applyOptions({}).size);           // md
console.log(applyOptions({ size: "lg" }).size); // lg
console.log(applyOptions(null).size);          // md

md
lg
md

🚀 Common Use Cases

• Plugin options — validate settings before $.extend.
• AJAX config — ensure payload maps are plain objects.
• Deep extend guards — only recurse into plain nested objects.
• JSON-like data — confirm parsed data is a key/value map.
• Reject DOM nodes — block accidental element merges.
• Pair with isEmptyObject — shape check plus emptiness check.

📝 Notes

• Available since jQuery 1.2 — core to $.extend internals.
• $.type(obj) === "object" is broader — always use isPlainObject for extend targets.
• Object.create(null) is considered plain — no inherited keys.
• Cross-frame plain objects are plain if their prototype chain qualifies.
• Does not validate nested structure — only top-level plainness.
• Related: $.extend(), $.isEmptyObject(), $.type().

💡 Best Practices