JavaScript Enumerability

JavaScript object properties have an enumerable property that controls whether that property shows up in for/in loops and the Object.keys() function. If you create a POJO using {}, all the POJO's properties will be enumerable by default.

const obj = {
  name: 'Jean-Luc Picard',
  age: 59
};

Object.keys(obj); // ['name', 'age']

However, you can also define a property on an object using the Object.defineProperty() function. Set enumerable to false and that property won't show up in Object.keys().

Object.defineProperty(obj, 'hidden', {
  enumerable: false,
  value: 42
});

obj.hidden; // 42
Object.keys(obj); // ['name', 'age'], no 'hidden'!

You can check if a property if enumerable using the propertyIsEnumerable() function which returns a Boolean.

const obj = { name: 'first' };
obj.propertyIsEnumerable('name'); // true

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