What is array besides string/arrays?

Question

Trying to learn javascript by reading underscore sourcecode and came across the below code:

var shallowProperty = function(key) {
 return function(obj) {
 return obj == null ? void 0 : obj[key];
 };
};
var getLength = shallowProperty('length');
console.log(getLength('123'))//3
console.log('123'['length'])//3
console.log(getLength(['1','2']))//2
console.log(['1','2']['length'])//2

My question is, what is [length] besides ['1','2']? Any technical terms to call it? Where can we get the full list of keys/attributes available other than length?

• Ibrahim Tareq: I can't seem to find console.log(['1','2']['length']) in the page you hyperlinked. Can you send me a link to the actual page?
• Nir Alfasi: Possible duplicate of Get array of object's keys
• zer00ne: Why learn JavaScript by studying a one off version of the language? Syntax, methods, properties, etc. might be slightly different or not even exist in JavaScript. You'll start getting confused and ask questions like: "Any technical terms to call it?"
• Isaac: @zer00ne: Having comments beside every sections of code can really help a total newbie like me very much. Or there is any other better recommendations?
• zer00ne: Yes there is, sir. https://javascript.info is current and designed for n00bs. I glanced at what you are studying and I got dizzy o_O.

Answer 1

An array is a JavaScript object. An object can have properties. You can access them in a few, equivalent ways:

myObject.property
myObject['property']

See this MDN documentation.

To show all the properties of an object:

Object.getOwnPropertyNames(myObject);

You might like to refer to this question about listing the properties of an object.

• Isaac: Maybe I was just overcomplicated when trying to understand the syntax..
• Matt: JavaScript is confusing sometimes. :)

Answer 2

Suppose you have the following object:

let person = {
 name: 'foo',
 age: 23
}
// Then, there are two possible ways to get the properties of "person" object
console.log(person.name); // logs 'foo'
console.log(person['name']); // logs 'foo'