One can think of an object as an associative array (a.k.a. map, dictionary, hash, lookup table). The keys in this array are the names of the object's properties.

It's typical when speaking of an object's properties to make a distinction between properties and methods. However, the property/method distinction is little more than a convention. A method is a property that can be called (for example, if it has a reference to a Function instance as its value).

There are two ways to access properties: dot notation and bracket notation.

In the object.property syntax, the property must be a valid JavaScript identifier. (In the ECMAScript standard, the names of properties are technically "IdentifierNames", not "Identifiers", so reserved words can be used but are not recommended). For example, object.$1 is valid, while object.1 is not.

const variable = object.propertyName;
object.propertyName = value;

const object = {};
object.$1 = 'foo';
console.log(object.$1);  // 'foo'

const object = {};
object.1 = 'bar';        // SyntaxError
console.log(object.1);   // SyntaxError

Here, the method named createElement is retrieved from document and is called.

document.createElement('pre')

If you use a method for a numeric literal, and the numeric literal has no exponent and no decimal point, you should leave white-space(s) before the dot preceding the method call, so that the dot is not interpreted as a decimal point.

77 .toExponential()
// or
77
.toExponential()
// or
;(77).toExponential()
// or
77..toExponential()
// or
77.0.toExponential()
// because 77. === 77.0, no ambiguity

In the object[propertyName] syntax, the propertyName is just a string or Symbol. So, it can be any string, including '1foo', '!bar!', or even ' ' (a space).

const variable = object[propertyName];
object[propertyName] = value;

This does the exact same thing as the previous example.

document['createElement']('pre')

A space before bracket notation is allowed.

document ['createElement']('pre')

Property names are string or Symbol. Any other value, including a number, is coerced to a string. This outputs 'value', since 1 is coerced into '1'.

const object = {};
object['1'] = 'value';
console.log(object[1]);

This also outputs 'value', since both foo and bar are converted to the same string.

const foo = { uniqueProp: 1 };
const bar = { uniqueProp: 2 };
const object = {};
object[foo] = 'value';
console.log(object[bar]);

A method is not bound to the object that it is a method of. Specifically, this is not fixed in a method. Put another way, this does not necessarily refer to the object containing a method. Instead, this is "passed" by the function call. See method binding.

JavaScript novices often make the mistake of using eval() where the bracket notation can be used instead.

For example, the following syntax is often seen in many scripts.

x = eval(`document.forms.form_name.elements.${strFormControl}.value`);

eval() is slow and should be avoided whenever possible. Also, strFormControl would have to hold an identifier, which is not required for names and ids of form controls. It is better to use bracket notation instead:

x = document.forms['form_name'].elements[strFormControl].value;

• Object
• Object.defineProperty()
• Optional chaining