var ABRecord = Record({a:1, b:2})
var myRecord = new ABRecord({b:3})

Records always have a value for the keys they define. removeing a key from a record simply resets it to the default value for that key.

myRecord.size // 2
myRecord.get('a') // 1
myRecord.get('b') // 3
myRecordWithoutB = myRecord.remove('b')
myRecordWithoutB.get('b') // 2
myRecordWithoutB.size // 2

Values provided to the constructor not found in the Record type will be ignored. For example, in this case, ABRecord is provided a key "x" even though only "a" and "b" have been defined. The value for "x" will be ignored for this record.

var myRecord = new ABRecord({b:3, x:10})
myRecord.get('x') // undefined

Because Records have a known set of string keys, property get access works as expected, however property sets will throw an Error.

Note: IE8 does not support property access. Only use get() when supporting IE8.

myRecord.b // 3
myRecord.b = 5 // throws Error

Record Classes can be extended as well, allowing for custom methods on your Record. This is not a common pattern in functional environments, but is in many JS programs.

Note: TypeScript does not support this type of subclassing.

class ABRecord extends Record({a:1,b:2}) {
  getAB() {
    return this.a + this.b;
  }
}

var myRecord = new ABRecord({b: 3})
myRecord.getAB() // 4

Immutable.js

Record()