cache in js call

Mar 5, 2020

Caching in js

Recently, when I was communicating with someone on the Internet, he asked me what a function was. I looked at it for a long time, but I couldn’t find the secret. Let’s have a look:

function _path() {
  const data = require("path");

  _path = function _path() {
    return data;
  };

  return data;
}

What does this function do? Look at 5 more min.

Here is the anwser: https://stackoverflow.com/a/55430181 (Object to stackoverflow programming)， After reading and understanding it, just want to say: fucking great, by redefining the function to cache the function call,

Say a little more about the stackoverflow example:

function f() {
  console.log("outer");
  f = function f() {
    console.log("inner");
  }
}
f(); //outer
f(); //inner
f(); //inner

The first time I call f(), outer is printed, and then the function itself becomes a new f(). When I call it again later, outer will not be called, only the new function will be called to print inner. What I know is that the cache is usually done by the method of key on the Object. And this method can be used inside js I think because js is an interpreted language, the code will be called when its excuted.

I went through loadash source codes to see how it do cache?

// lodash
function memoize(func, resolver) {
    if (typeof func != 'function' || (resolver != null && typeof resolver != 'function')) {
    throw new TypeError(FUNC_ERROR_TEXT);
    }
    var memoized = function() {
    var args = arguments,
        key = resolver ? resolver.apply(this, args) : args[0],
        cache = memoized.cache;

    if (cache.has(key)) {
        return cache.get(key);
    }
    var result = func.apply(this, args);
    memoized.cache = cache.set(key, result) || cache;
    return result;
    };
    memoized.cache = new (memoize.Cache || MapCache);
    return memoized;
}

// Expose `MapCache`.
memoize.Cache = MapCache;

Familiar taste, loadash is more perfect, after all, so many people are using it, I think this is a common practice

There’s a similar concept in react: useCallback，

function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

function updateCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;
  if (prevState !== null) {
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      // note!!!!!
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        return prevState[0];
      }
    }
  }
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

I didn’t take a close look at the source code, but it is roughly divided into two parts, mountCallback and mountCallback are called when the component is mounted and updated . Then use areHookInputsEqual(nextDeps, prevDeps) to compare parameters and determine whether the cache is returned.

To sum up, it seems that everyone is doing the same, but there are differences in writing, according to the situation of the code to do some adjustion.

referrence: