"Source code interpretation" famous programmer TJ's only library

Hello everyone , I'm brother watermelon . I recently tried to open a series of source code interpretation , Interpret some simple or complex source code from time to time .

Today I want to interpret npm Third party library source code ：only, Warehouse address is ：https://github.com/tj/node-only

This library is tj Tools and methods written by great God .tj God is a programmer boss , Mainly in the nodejs The field has made many open source contributions , yes Express、Koa、node-canvas And the creators and contributors of a series of well-known open source projects .

only What can the method do ？

only Its function is very simple ： Return the whitelist attribute of an object . To be specific , It is to extract some attributes of the specified object and put them into the new object , And return this new object .

const only = require('only');
​
var obj = {
    
  name: 'tobi',
  last: 'holowaychuk',
  email: '[email protected]',
  _id: '12345'
};
​
only(obj, 'name last email');
/* { name: 'tobi', last: 'holowaychuk', email: '[email protected]' } */

only Method requires an object to be passed in , And a white list , This list can be an array of attribute names , It can also be a string represented by a space spaced attribute name .

The second parameter supports string format , Because it is more convenient to use , Can write a lot less quotation marks . Elegance in use , It's our design API It needs to be considered .

only The implementation of the library

module.exports = function(obj, keys){
    
  obj = obj || {
    };
  if ('string' == typeof keys) keys = keys.split(/ +/);
  return keys.reduce(function(ret, key){
    
    if (null == obj[key]) return ret;
    ret[key] = obj[key];
    return ret;
  }, {
    });
};

Code reading

First, for the first parameter obj Simple fault tolerance . If obj Is a value of undefined or null False values like , We set it as an empty object , Prevent throwing errors .

Then it is possible to convert the white list in the form of string into the format of array .

The method used is ：keys.split(/ +/), It's used here split Regular expression parameter writing method ,/ +/ This regular expression represents a number greater than or equal to 1 Multiple space characters of . This is actually a very common way to split strings and get arrays .

Finally, use what Lao Niao likes very much Array.prototype.reduce() Writing , Come on keys Iterating through arrays .

For each key, As long as the original array is not null or undefined, Just copy it to the new object .

reduce() The return value of the method is the new object we want , Return it directly to .

Let's talk about why some senior front-end like to use reduce() Method . Because reduce This method is compared with forEach、map These methods should be more flexible , It can return any type of value without losing brevity . forEach no return value , and map Only arrays of the same size of array type can be returned , Although it conforms to the principle of single responsibility , But it also loses flexibility .

I once wrote an article about reduce The article , If you are interested, you can have a look ： Why do we say array instances reduce The method is flexible ？

only The method is interpreted .