memoize-decorator.ts

/**
 * Caches the return value of get accessors and methods.
 *
 * Notes:
 * - Doesn't really make sense to put this on a method with parameters.
 * - Creates an obscure non-enumerable property on the instance to store the memoized value.
 * - Could use a WeakMap, but this way has support in old environments.
 */
export function Memoize(target: any, propertyName: string, descriptor: TypedPropertyDescriptor<any>) {
    if (descriptor.value != null) {
        descriptor.value = getNewFunction(descriptor.value);
    }
    else if (descriptor.get != null) {
        descriptor.get = getNewFunction(descriptor.get);
    }
    else {
        throw "Only put a Memoize decorator on a method or get accessor.";
    }
}

let counter = 0;
function getNewFunction(originalFunction: () => void) {
    const identifier = ++counter;

    return function (this: any, ...args: any[]) {
        const propName = `__memoized_value_${identifier}`;
        let returnedValue: any;

        if (this.hasOwnProperty(propName)) {
            returnedValue = this[propName];
        }
        else {
            returnedValue = originalFunction.apply(this, args);
            Object.defineProperty(this, propName, {
                configurable: false,
                enumerable: false,
                writable: false,
                value: returnedValue
            });
        }

        return returnedValue;
    };
}


// ------------ ES6 VERSION ----------
export function Memoize(target: any, propertyName: string, descriptor: TypedPropertyDescriptor<any>) {
    if (descriptor.value != null)
        descriptor.value = getNewFunction(descriptor.value);
    else if (descriptor.get != null)
        descriptor.get = getNewFunction(descriptor.get);
    else
        throw new Error("Only put a Memoize decorator on a method or get accessor.");
}

const weakMap = new WeakMap<object, Map<string, unknown>>();

let counter = 0;
function getNewFunction(originalFunction: (...args: any[]) => void) {
    const identifier = counter++;
    function decorator(this: any, ...args: any[]) {
        let propertyValues = weakMap.get(this);
        if (propertyValues == null) {
            propertyValues = new Map<string, unknown>();
            weakMap.set(this, propertyValues);
        }
        let propName = `__memoized_value_${identifier}`;
        if (arguments.length > 0)
            propName += "_" + JSON.stringify(args);
        let returnedValue: any;

        if (propertyValues.has(propName))
            returnedValue = propertyValues.get(propName);
        else {
            returnedValue = originalFunction.apply(this, args);
            propertyValues.set(propName, returnedValue);
        }

        return returnedValue;
    }

    return decorator;
}

Just wondering, how is this different from a static property

@SteveStrong in certain cases where you want to use the prototype this is useful.

type hashCode = "hashCode";

/**
 * Indicates that this hashcode function should be memoized (calculated only once)
 * ***This should only be used on READONLY composite data types for predictable results***
 */
function memoized(target: ValueType, propertyKey: hashCode, descriptor: TypedPropertyDescriptor<() => number>) {

  const original = descriptor.get ? descriptor.get() : descriptor.value;

  Object.defineProperty(target.constructor.prototype, 'hashCode', {
    value: function hashCode() {
      if (!this.__hashCode__) {
        this.__hashCode__ = original.call(this);
      }
      return this.__hashCode__;
    }
  });

  descriptor.value = target.constructor.prototype.hashCode;

  return descriptor;
}

I found this to work - no WeakMap needed.

This might not work as well if you want to dynamically memoize things, but I am using it to memoize hashcode computations for immutable ValueTypes with Immutable.js

Edit: I am not sure how well this would work with sub-classes overriding a memoized function who also decorates the overridden function to memoize the calculation, it might loop infinitely?

Here is a WeakMap based version of this decorator, that respects single argument (by a reference for non-primitives)