Value-Semantics

The JavaScript/TypeScript Value Semantics Toolkit

All the functions you need to program as if objects in JavaScript had value semantics, including comprehensive and highly customisible deep cloning and equality functions

• Value-Semantics
  • What is value-semantics?
  • Installation
  • Usage
    • Deep Cloning
    • Value-Equality
    • Customizing clone and equals Implementations
      • Customizing clone Implementations
        • Constructor Parameter Decorator
        • Property Inclusion/Exclusion Decorators
      • Customizing equals Implementations
        • Property Inclusion/Exclusion Decorators
      • Customizing clone and equals Implementations Simultaneously
        • Property Inclusion/Exclusion Decorators
  • License

What is value-semantics?

value-semantics, the JavaScript/TypeScript value semantics toolkit, is a TypeScript utility library which allows (almost) every JavaScript object to be treated as if it had value semantics, including user-defined classes and builtin exotic objects. The toolkit's clone function allows objects to be copy-assigned like primitive values, rather than alias-assigned like objects typically are in JavaScript, while the toolkit's equals function allows objects to be compared by value-equality, rather than reference-equality. Easy customization of these functions for user-defined classes, such as excluding properties from cloning/comparison and setting arguments for cloning constructors, is possible using decorators. Of course, these functions are not limited to mimicking value semantics, but can be used anywhere deep cloning or equality comparisons are desired.

// Compare objects by value-equality
const lincolnBirthDate = new Date(1809, 1, 12);
const darwinBirthDate = new Date(1809, 1, 12);
console.assert(equals(lincolnBirthDate, darwinBirthDate));

// Deep clone objects to avoid unwanted aliasing and to pass function parameters 
//   by value
type Vector = { x: number, y: number };
function scale(vector: Vector, scale: number): Vector {
  vector.x *= scale;
  vector.y *= scale;
  return vector;
}

const vector1 = { x: 2, y: 3 };
const vector2 = scale(clone(vector1), 2);
console.assert(equals(vector2, { x: 4, y: 6 }));
console.assert(equals(vector1, { x: 2, y: 3 }));

// Customize `equals` and `clone` implementations on user-defined classes using 
//   decorators
@customize.value({ runConstructor: true }) // Customize `equals` and `clone` implementations
//   simultaneously using the `@customize.value` decorator
class Rectangle {
  @clone.constructorParam private height: number; // Specify which properties 
  //   should be used as parameters for the cloning constructor
  @clone.constructorParam private width: number;
  @equals.exclude private orientation: number; // Exclude properties from cloning 
  //   and/or equality comparison

  constructor(height: number, width: number) {
    this.height = height;
    this.width = width;
    this.orientation = 0;
  }
}

const rect1 = new Rectangle(10, 20);
const rect2 = clone(rect1);
console.assert(rect1 !== rect2);
console.assert(equals(rect1, rect2));
rect2.orientation = 90;
console.assert(equals(rect1, rect2));
console.assert(rect1.orientation === 0);

Installation

Run npm install value-semantics to install this library.

Usage

Deep Cloning

clone<T>(source: T): T

Use the clone function to create a deep clone of a JavaScript value. Roughly, the clone of an object will have the same prototype and the same (enumerable, own) property keys as the original, and the respective values for those keys are clones of the values of the original keys. Clones are independent of their originals, in the sense that any changes to a clone will not propagate to the original object, and vice versa.

const obj = { a: 1, b: [2, 3], c: new Date(2000, 0, 1), d: { e: 4 } };
const objcopy = clone(obj);
console.assert(objcopy.d.e === 4);
console.assert(objcopy.b[1] === 3);
obj.d.e = 5;
objcopy.b[1] = 6;
console.assert(objcopy.d.e === 4);
console.assert(obj.b[1] === 3);

Value-Equality

equals(lhs: unknown, rhs: unknown): boolean 

Use the equals function to compare two JavaScript values for value-equality. Broadly speaking, this function considers two objects equal when they both have the same prototype, same (enumerable, own) property keys and the respective values for those keys are value-equal. This function can be customized for user-created classes, as discussed below.

const obj1 = { a: 1, b: [2, 3], c: new Date(2000, 0, 1), d: { e: 4 } };
const obj2 = { d: { e: 4 }, c: new Date(2000, 0, 1), b: [2, 3], a: 1 };
console.assert(equals(obj1, obj2));

Customizing clone and equals Implementations

By a clone or equals implementation for a class, I mean the algorithm used to determine the results of calling clone or equals on an instance of that class. User-defined classes automatically have default implementations for clone and equals, where equals will compare an instance of a class equal to another value if and only if the other object is of the same class and has the same property values, and clone will return a new instance of the class with cloned property values. However, clone and equals implementations can be customized for user-defined classes using the decorators included in this toolkit.

Customizing clone Implementations

@customize.clone(
  semantics?: CloneSemantics = 'deep',
  options?: CustomizeCloneOptions = {}
)

The @customize.clone class decorator can be used to customize the clone implementation for a class. When called with no arguments, or (one or both) default arguments, the decorated class will have the default implementation (i.e. @customize.clone will be a no-op). Otherwise, the clone implementation can be customized in the following ways:

type CloneSemantics = 'deep' | 'returnOriginal' | 'errorOnClone';

The semantics parameter can be used to customize the semantics of the class' clone implementation. There are 3 kinds of semantics that a clone implementation can have:

• 'deep': This is the default semantics, where clone returns a deep clone of the class instance.
• 'returnOriginal': With this semantics, clone will return the original class instance without any cloning being performed.
• 'errorOnClone': With this semantics, clone will throw a ValueSemanticsError at runtime when applied to a instance of this class.

type CustomizeCloneOptions = {
  runConstructor?: boolean = false,
  propDefault?: 'include' | 'exclude' = 'include'
}

If a class' clone implementation has 'deep' semantics, then it can be further customized using the options parameter. The options parameter cannot be passed for classes with 'returnOriginal' or 'errorOnClone' semantics, as those semantics have no additional customization options. There are two properties which can be specified using the options parameter:

runConstructor: By default, and when this property is false, clones of a class instance are created by Object.create(), before the instance's properties are copied over. When this property is true, clones of a class instance are created by the class' constructor. Arguments for the constructor call can be specified using the @clone.constructorParam decorator described below, otherwise the constructor will run without any arguments.

For example:

@customize.clone({ runConstructor: true }) 
class Graph {
  public nodes: Nodes[];
  public edges: Edge[];

  constructor() {
    this.nodes = [];
    this.edges = [];
  }
}

const originalGraph = new Graph();
const clonedGraph = clone(originalGraph); 
  // calls Graph.prototype.constructor()

propDefault: By default, and when this property is 'include', every (own, enumerable) property of an instance of the class will be copied over to its clones, unless otherwise specified using the @clone.exclude (or @value.exclude) decorator described below. In contrast, when this property is 'exclude', no properties of an instance of the class will be copied over to its clones, unless otherwise specified using the @clone.include (or @value.include) decorator described below.

Constructor Parameter Decorator

@clone.constructorParam

On a class with 'deep' clone semantics and runConstructor: true, labelling a class field with @clone.constructorParam means that the value of that field will be provided to the constructor as an argument when cloning an instance of the class. On any other class, @clone.constructorParam has no effect. If multiple fields are labelled with @clone.constructorParam, they will be provided to the constructor in order, top to bottom.

For example:

@customize.clone({ runConstructor: true }) 
class Person {
  @clone.constructorParam private height: number;
  @clone.constructorParam private age: number;

  constructor(height: number, age: number) {
    this.height = height;
    this.age = age;
  }
}

const originalPerson = new Person(178, 36);
const clonedPerson = clone(originalPerson); 
  // calls Person.prototype.constructor(178, 36)

On a class which also has propDefault: include, labelling a class field with @clone.constructorParam will cause that field to be excluded from the clone implementation by default, unless the field is also labelled with @clone.include (or @value.include).

Property Inclusion/Exclusion Decorators

@clone.exclude
@clone.include

On a class with 'deep' clone semantics and propDefault: include, decorating a class field with @clone.exclude will override the default and exclude that field when cloning an instance of the class. On any class with 'deep' clone semantics, decorating a field with both @clone.exclude and @clone.include (or @value.include) will throw a ValueSemanticsError at runtime on class definition. Otherwise, @clone.exclude has no effect.

On a class with 'deep' clone semantics and propDefault: exclude, decorating a class field with @clone.include will override the default and include that field when cloning an instance of the class. On a class with 'deep' clone semantics, propDefault: include and runConstructor: true, decorating a @clone.constructorParam field with @clone.include in addition will override the default and include that field when cloning an instance of the class (in addition to it being a constructor parameter). On any class with 'deep' clone semantics, decorating a field with both @clone.include and @clone.exclude (or @value.exclude) will throw a ValueSemanticsError at runtime on class definition. Otherwise, @clone.include has no effect.

Customizing equals Implementations

@customize.equals(
  semantics?: EqualsSemantics = 'value',
  options?: CustomizeEqualsOptions = {}
)

The @customize.equals class decorator can be used to customize the equals implementation for a class. When called with no arguments, or (one or both) default arguments, the decorated class will have the default implementation (i.e. @customize.equals will be a no-op). Otherwise, the equals implementation can be customized in the following ways:

type EqualsSemantics = 'value' | 'ref';

The semantics parameter can be used to customize the semantics of the class' equals implementation. There are 2 kinds of semantics that an equals implementation can have:

• 'value': This is the default semantics, where equal compares two instances of the class as equal if and only if they are value-equals (roughly, when they have the same property values).
• 'ref': With this semantics, equal compares two instances of the class as equal if and only if they refer to the same instance. In other words, on this semantics equals is the same as ===.

type CustomizeEqualsOptions = {
  propDefault?: 'include' | 'exclude' = 'include'
}

If a class' clone implementation has 'value' semantics, then it can be further customized using the options parameter. The options parameter cannot be passed for classes with 'ref' semantics, as it has no additional customization options. There is one property which can be specified using the options parameter:

propDefault: By default, and when this property is 'include', every (own, enumerable) property of an instance of the class will used to compare instances for equality, unless otherwise specified using the @equals.exclude (or @value.exclude) decorator described below. In constrast, when this property is 'exclude', no properties of an instance of the class will be used to compare instances for equality (meaning all instances of the class compare as equal), unless otherwise specified using the @equals.include (or @value.include) decorator described below.

Property Inclusion/Exclusion Decorators

@equals.include
@equals.exclude

On a class with 'value' equals semantics and propDefault: exclude, decorating a class field with @equals.include will override the default and include that field when making equality comparisons. On any class with 'value' equals semantics, decorating a field with both @equals.include and @equals.exclude (or @value.exclude) will throw a ValueSemanticsError at runtime on class definition. Otherwise, @equals.include has no effect.

On a class with 'value' equals semantics and propDefault: include, decorating a class field with @equals.exclude will override the default and exclude that field when making equality comparisons. On any class with 'value' equals semantics, decorating a field with both @equals.exclude and @equals.include (or @value.include) will throw a ValueSemanticsError at runtime on class definition. Otherwise, @equals.exclude has no effect.

Customizing clone and equals Implementations Simultaneously

@customize.value(
  cloneSemantics?: CloneSemantics = 'deep' // cloneSemantics and equalsSemantics can be
  equalsSemantics?: EqualsSemantics = 'value' //    in either order
  options: CustomizeValueOptions = {}
)

It is possible to customize the implementations of both equals and clone on a class by decorating it with both @customize.equals and @customize.clone. However, to save space, this library also provides the @customize.value decorator, which customizes both of these functions at the same time.

When called with no arguments, or (one, two or all of the) default arguments, the decorated class will have the default implementations for equals and clone (i.e. @customize.value will be a no-op). Otherwise, the equals and clone implementations can be customized in the following ways:

The cloneSemantics parameter can be used to customize the semantics of the class' clone implementation. The values that this parameter can take, and their meanings, are the same as for the semantics parameter of the @customize.clone decorator.

The equalsSemantics parameter can be used to customize the semantics of the class' equals implementation. The values that this parameter can take, and their meanings, are the same as for the semantics parameter of the @customize.equals decorator.

Note that the cloneSemantics and equalsSemantics can be provided in either order, although if one or both of them are present they must come before the options parameter (if it is present).

type CustomizeValueOptions = {
  runConstructor?: boolean = false,
  propDefault?: 'include' | 'exclude' = 'include'
}

If a class' has 'deep' clone and/or 'value' equals semantics, then it can be further customized using the options parameter. Note that the options parameter cannot be passed for classes with 'ref' equals and 'returnOriginal' or 'errorOnClone' clone semantics, as those semantics have no additional customization options. There are two properties which can be specified using the options parameter:

runConstructor: The values that this property can take, and their meanings, are the same as for the runConstructor parameter of the @customize.clone decorator. As in the @customize.clone case, arguments for the constructor call can be specified using the @clone.constructorParam decorator described above, but note that there is no @value.constructorParam decorator. Note also that this property cannot be specified if the class has 'returnOriginal' or 'errorOnClone' clone semantics, as those semantics cannot involve running a constructor. This is true even if the class has 'value' equals semantics, and can therefore take an options argument.

propDefault: The values that this property can take, and their meanings, are the same as for the propDefault parameters of the @customize.clone and @customize.equals decorators. In other words, setting propDefault to 'include' ('exclude') is equivalent to setting propDefault to 'include' ('exclude') on both @customize.clone and @customize.equals. Note that @customize.value does not allow different values to be set for propDefault for clone and equals implementations. To do so, you would have to use seperate @customize.equals and @customize.clone decorators. Note also that, given a propDefault: 'include' value, decorating a class field with @clone.constructorParam will exclude that property from cloning but not equality comparisons. This can be overridden by either the @clone.include or @value.include decorators (but not @equals.include).

Property Inclusion/Exclusion Decorators

@value.include
@value.exclude

Decorating a class field with @value.include (@value.exclude) has the same effect as decorating that field with both @clone.include and @equals.include (@clone.exclude and @equals.exclude). This means that, for example, decorating a class field with @value.include on a class decorated only with @customize.equals({ propDefault: exclude }) will have the same effect has decorating that field with just @equals.include (i.e. the @clone.include aspect is a no-op). Any combination of field decorators which leads to a field being decorated with both @clone.include and @clone.exclude, and /or @equals.include and @equals.exclude, will throw a ValueSemanticsError at runtime on class definition. For example, decorating a field with @value.exclude and @clone.include will lead to such an error (even if the @clone.exclude aspect of @value.exclude is otherwise a no-op). On a class with 'deep' clone semantics, propDefault: include and runConstructor: true, decorating a @clone.constructorParam field with @value.include in addition will override the default and include that field when cloning an instance of the class (in addition to it being a constructor parameter).

License

MIT