How to list all methods of an object in JavaScript

We can use the Object.getOwnPropertyNames() function to get all the property names linked to an object.

Then we can filter the resulting array, to only include that property name if it’s a function.

We determine if it’s a function by using typeof on it.

For example here is how we might create a utility function to do what we need:

getMethods = (obj) => Object.getOwnPropertyNames(obj).filter(item => typeof obj[item] === 'function')

This lists only the methods defined on that specific object, not any method defined in its prototype chain.

To do that we must take a slightly different route. We must first iterate the prototype chain and we list all the properties in an array. Then we check if each single property is a function.

An easy way to make sure we don’t duplicate methods as we navigate the prototype chain (like constructor which is always present), we use a Set data structure that makes sure values are unique:

const getMethods = (obj) => {
  let properties = new Set()
  let currentObj = obj
  do {
    Object.getOwnPropertyNames(currentObj).map(item => properties.add(item))
  } while ((currentObj = Object.getPrototypeOf(currentObj)))
  return [...properties.keys()].filter(item => typeof obj[item] === 'function')
}

Example usage:

getMethods("")
getMethods(new String('test'))
getMethods({})
getMethods(Date.prototype)

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Published Feb 04, 2019