What does enumerable mean?

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An enumerable property is one that can be included in and visited during for..in loops (or a similar iteration of properties, like Object.keys()).

If a property isn’t identified as enumerable, the loop will ignore that it’s within the object.

var obj = { key: 'val' };

console.log('toString' in obj); // true
console.log(typeof obj.toString); // "function"

for (var key in obj)
    console.log(key); // "key"

A property is identified as enumerable or not by its own [[Enumerable]] attribute. You can view this as part of the property’s descriptor:

var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');

console.log(descriptor.enumerable); // true
console.log(descriptor.value);      // 1

console.log(descriptor);
// { value: 1, writable: true, enumerable: true, configurable: true }

A for..in loop then iterates through the object’s property names.

var foo = { bar: 1, baz: 2};

for (var prop in foo)
    console.log(prop); // outputs 'bar' and 'baz'

But, only evaluates its statement – console.log(prop); in this case – for those properties whose [[Enumerable]] attribute is true.

This condition is in place because objects have many more properties, especially from inheritance:

console.log(Object.getOwnPropertyNames(Object.prototype));
// ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", /* etc. */]

Each of these properties still exists on the object:

console.log('constructor' in foo); // true
console.log('toString' in foo);    // true
// etc.

But, they’re skipped by the for..in loop because they aren’t enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'constructor');

console.log(descriptor.enumerable); // false

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