Static Methods

If a method is closely associated with a constructor or class but should not be used on individual instances, where is the appropriate location for this method? For example, consider an Animal constructor with common utility methods stored in its prototype. In this case, a function that finds the animal with the most energy from an array of animals would be more appropriately placed as a static method on the Animal class.

Static Method with Prototypal Instantiation

function Animal(name, energy) {
  this.name = name
  this.energy = energy
}
 
Animal.prototype.eat = function(amount) {
  console.log(`${this.name} is eating.`)
  this.energy += amount
}
 
// Create a static method in the Animal constructor
Animal.mostEnergetic = function(animals) {
  const sortedByMostEnergy = animals.sort((a,b) => {
    return b.energy - a.energy
  })
  return sortedByMostEnergy[0].name
}
 
const leo = new Animal('Leo', 7)
const snoop = new Animal('Snoop', 10)
 
// Use the static method directly from the constructor
console.log(Animal.mostEnergetic([leo, snoop])) // Snoop

When dealing with a method specific to a class, not meant for individual instance use, declare it as a static method of the class.

Static method with ES6 Classes

class Animal {
  constructor(name, energy) {
    this.name = name
    this.energy = energy
  }
  eat(amount) {
    console.log(`${this.name} is eating.`)
    this.energy += amount
  }
  
  // Create a static method in the Animal class
  static mostEnergetic(animals) {
    const sortedByMostEnergy = animals.sort((a,b) => {
	  return b.energy - a.energy
	})
	return sortedByMostEnergy[0].name
  }
}
 
const leo = new Animal('Leo', 7)
const snoop = new Animal('Snoop', 10)
 
// Use the static method directly from the class
console.log(Animal.mostEnergetic([leo, snoop])) // Snoop

References

• A Beginner’s Guide to JavaScript’s Prototype