dynamic keys for object literals in Javascript [duplicate]

Computed property names are supported in ECMAScript2015:

var name = 'key';
var value = 'value';
var o = {
  [name]: value
};
alert("o as json : " + JSON.stringify(o));

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer

Prior to ECMAScript 2015 (ed 6), an object literal (ECMAScript calls it an "object initializer") key must be one of:

1. IdentifierName
2. StringLiteral
3. NumericLiteral

So you couldn't use an expression as the key in an initialiser. This was changed as of ECMAScript 2015 (see below). You could use an expression with square bracket notation to access a property, so to set the properties with an expression you had to do:

var required = { directories : {}};
required.directories[this.applicationPath] = "Application " + this.application + " does not exists";
required.directories[this.applicationPath + "/configs"] = "Application config folder does not exists";
...

and so on. Since this.applicationPath is reused a lot, better to store a reference to help with performance and cut down the amount of code:

var a = this.applicationPath;
var required = { directories : {}};
var rd = required.directories;
rd[a] = "Application " + this.application + " does not exists";
rd[a + "/configs"] = "Application config folder does not exists";
...

Edit

As of ECMAScript 2015 (ed 6), object initializers can have computed keys using:

[expression]: value

There is also shorthand syntax for property and method names.

See MDN: Object Initializer or ECMAScript Object Initializer.

You can set dynamic keys is with bracket notation:

required.directories[this.applicationPath + "/configs"] = "Application config folder does not exists";

(of course wherever you do this definition, this.applicationPath must exist)

But do you need this.applicationPath in the keys? How do you access theses values? Maybe you can just remove this.applicationPath from whatever value you use to access the properties.

But in case you need it:

You could use an array to initialize the keys if you want to avoid repeating a lot of code:

var dirs = ['configs', 'controllers', ...];
var files = ['init.js', 'controllers/index.js', ...];

var required = { directories: {}, files: {} };
required.directories[this.applicationPath] = "Application " + this.application + " does not exists";

for(var i = dirs.length; i--;) {
    required.directories[this.applicationPath + '/' + dirs[i]] = "Application " + dirs[i] + " folder does not exists";
}

for(var i = files.length; i--;) {
    // same here
}

Inspired by how babel coverts the new ES6 syntax ({[expression]: value}) to old Javascript, I learned that you can do it with a one liner:

var obj = (_obj = {}, _obj[expression] = value, _obj);

Example:

var dynamic_key = "hello";
var value = "world";
var obj = (_obj = {}, _obj[dynamic_key] = value, _obj);

console.log(obj);
// Object {hello: "world"}

(Tested on latest Chrome)