实现 memo 函数 about fe-questions HOT 1 OPEN

/**
 * @param {Function} func
 * @param {(args:[]) => string }  [resolver] - cache key generator
 */
/*
function memo(func, resolver) {
  // your code here
  const cache = new Map();

  // If my cache key is in my cache, use that value
  // Why use function() over () => {} ?
  // Because function() uses the context from the caller.
  // Arrow functions use lex scoping, so it'll use the context from the memo function.
  return function() {
    const cacheKey = resolver ?  resolver(...arguments) : Array.from(arguments).join(',');

    if (cache.has(cacheKey)) {
      console.log('cached');
      return cache.get(cacheKey);
    }
    // Otherwise invoke the function and add it to my cache
    const val = func.apply(this, arguments);
    cache.set(cacheKey, val);
    return val;
  }
}
*/

// With this context in mind, check test cases below.
function memo(func, resolver) {
    // your code here
    const cache = new Map();
  
    // Map<cacheKey, Map<context, value>>
    return function() {
      const cacheKey = resolver ?  resolver(...arguments) : Array.from(arguments).join(',');
  
      const contextMap = cache.get(cacheKey);
      // If there is a corresponding context map to cachekey
      // Check if context is in the map, if so, return value.
      // Else if no corresponding add contextMap, add new entry to the context map
      if (!contextMap) {
        const value = func.apply(this, arguments);
        cache.set(cacheKey, new Map([[ this, value ]]));
        return value;
      }
  
      if (contextMap.has(this)) {
        return contextMap.get(this);
      } 
      // If context not in the map, calculate and add to context map.
      const value = func.apply(this, arguments);
      contextMap.set(this, value);
      return value;
    }
  }
  
  function testThis(a) {
    return `${this.val}_${a}`;
  }
  
  const memoFunc = memo(testThis)
  
  const testSubject = {
    val: 1,
    memo: memoFunc,
  }
  
  const testSubject2 = {
    val: 2,
    memo: memoFunc,
  }
  
  // 1_1
  console.log(testSubject.memo(1));
  // Expected no caching and output is 2_1
  console.log(testSubject2.memo(1));
  // Expected to cache
  console.log(testSubject2.memo(1));