CODEWITHSUNDEEP
javascriptsyntax
user2561505
user2561505

Reputation:

What does ._ in javascript?

I have come across this while going through underscore js source code.

var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this ||
            {};

  // Save the previous value of the `_` variable.
  var previousUnderscore = root._;

What is that root._ does here?

Upvotes: 1

Views: 4257

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370819

In general, that block of code will result in previousUnderscore refering to the _ property on self, or on global, or on this (whichever object root evaluated to), and said property value may not exist. It's no different from any other property name.

For example, if the first condition passes:

typeof self == 'object' && self.self === self && self

then if self has a _ property which contains 'foo', then previousUnderscore will be assigned that same string, 'foo'.

const self = {
  _: 'foo'
};
self.self = self;

var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this ||
            {};

// Save the previous value of the `_` variable.
var previousUnderscore = root._;
console.log(previousUnderscore, typeof previousUnderscore);

If self did not have the _ property, then previousUnderscore would be undefined.

The exact same logic applies to any other property name, for example:

const self = {
  somePropName: 'foo'
};
self.self = self;

var root = typeof self == 'object' && self.self === self && self ||
            typeof global == 'object' && global.global === global && global ||
            this ||
            {};

// Save the previous value of the `_` variable.
var previousUnderscore = root.somePropName;
console.log(previousUnderscore, typeof previousUnderscore);

In particular, for underscore.js, what this code is doing is it

(1) determines which object should be used for the root variable name

(2) saves the _ property in that object in the variable name previousUnderscore, so that when underscore.js reassigns root._, one can then call noConflict to

Give control of the _ variable back to its previous owner

by running the following line:

root._ = previousUnderscore;

For example, if you ran underscore.js and previously had _ on the global object (say, Lodash), _ would then refer to the Underscore object - but if you called noConflict, then _ would refer back to whatever it originally referred to (such as Lodash).

// Underscore runs after Lodash, so when the JS here runs initially,
// _ refers to Underscore, but Underscore has saved a reference to the previous _,
// which was Lodash,
// in the variable name previousUnderscore that your question is asking about:


// Underscore function - give _ back to Lodash:
_.noConflict();
// Lodash function:
_.noop()

console.log('done');
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.core.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js"></script>

Upvotes: 2

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