# Contract

The Contract class allows you to create objects whose properties can only be updated through its methods. This enables the development of smart contracts in JavaScript. For details, please see here.

If a class C extends from Contract and if c is an instance of C then an error is thrown if

a property of c is created, updated or deleted outside of a method of c
a property _id, _rev or _root of c is created, updated, or deleted.

In order to provide the two guarantees above we also need to forbid assigning to this inside of constructors. Instead, an object can be passed into super to initialize an object.

# Examples

# Updating Properties Outside of Methods

class C extends Contract {
  constructor() {
    super({ n: 0 })
  }

  set(n: number) {
    this.n = n
  }
}

const c = new C()

// Assigning n through a method works
c.set(1)

expect(() => {
  // Assigning to property outside of a method throws an error
  c.n = 2
}).to.throw("Cannot set property 'n' directly")

# Updating `_id`, `_rev` or `_root`

class C extends Contract {
  set(rev) {
    this._rev = rev
  }
}

const c = new C()

expect(() => {
  // Assigning a provenance property throws an error, even inside a method
  c.set('rev')
}).to.throw('Cannot set _rev')

# Using the Initialization Object

class C extends Contract {
  n: number

  constructor() {
    // Use the initialization object
    super({ n: 0 })
  }
}

const c = new C()
expect(c.n).eq(0)

# Contract

# Examples

# Updating Properties Outside of Methods

# Updating _id, _rev or _root

# Using the Initialization Object

# Updating `_id`, `_rev` or `_root`