_extend property in 'util' node module

Question

In one of the node examples , I came across below line

const extend = require('util')._extend;

Can anyone explain whats the purpose of _extend method of 'util' node module ?

Answer 1

Low-frills deep copy:

var obj2 = JSON.parse(JSON.stringify(obj1));

For a shallow copy, use Node's built-in util._extend() function.

var extend = require('util')._extend;

var obj1 = {x: 5, y:5};
var obj2 = extend({}, obj1);
obj2.x = 6;
console.log(obj1.x); // still logs 5

Source code of Node's _extend function is in here: https://github.com/joyent/node/blob/master/lib/util.js

exports._extend = function(origin, add) {
// Don't do anything if add isn't an object
if (!add || typeof add !== 'object') return origin;

  var keys = Object.keys(add);
var i = keys.length;
while (i--) {
origin[keys[i]] = add[keys[i]];
}
return origin;
};

For more information you can follow EXTEND

Hope this helps you.

Answer 2

In addition to Shallow copy as mentioned by Nitin above, it also combines the properties of the parameters passed

From the source code of util._extend

exports._extend = function(origin, add) {
// Don't do anything if add isn't an object
if (!add || typeof add !== 'object') return origin;

  var keys = Object.keys(add);
var i = keys.length;
while (i--) {
origin[keys[i]] = add[keys[i]];
}
return origin;
};

So it combines the properties of origin and add parameters from the above code snippet and returns the extended object.