什麼是Lazy Evaluation (惰性求值)?

簡單點說就是, 按需索取, 能不多做事, 絕對不做.

好處, 提高Performance: Code Execution, Memory Usage

例子

比如我們生成100個值

var oneHunderItems = function(end){
     var results = [];
     for(var i=0; i< end; ++i){
          results.push(i);
     }
     return results;
}

雖然生成了100個, 但是我們實際就需要前10個. 怎麼辦? 造成了Memory Usage的浪費.
所以, Lazy Evaluation 就來了幫忙了.

我們實際想要的是如下結果:

oneHunderItems.take(10).each(doWork);

var threshold = 10;
for (var i = 0; i < 100; i++) {
  if (i >= threshold) {
      break;
  }
  else {
    var item = items[i];
    // do work on item
     doWork(item)
  }
}

更誇張點, 來個 Infinity. 懂了原理的我們也是不怕的 :)

當然了, 這個寫法很... Not reusable. 所以推薦大家看看Lazy.js

Reference:

1. Optimizing your JavaScript with Functional Programming
2. Simple JavaScript implementation of cons list with lazy evaluation of map, filter etc.
   Raw
3. Into the Land of Functional Programming with JavaScript