Difference between dot ‘.’ and Square Braces ‘[]’ notations

Accessing members with . is called dot notation. Accessing them with [] is called bracket notation.

The dot notation only works with property names which are valid identifier names [spec], so basically any name that would also be a valid variable name (a valid identifier, see also Valid Characters for JavaScript Variable Names) and any reserved keyword [spec].

Bracket notation expects an expression which evaluates to a string (or can be coerced to a string), so you can use any character sequence as property name. There are no limits to what a string can contain.

Examples:

obj.foo;  // valid
obj.else  // valid, reserved keywords are valid identifier names
obj.42    // invalid, identifier names cannot start with numbers
obj.3foo  // invalid,                ""
obj.foo-bar // invalid, `-` is not allowed in identifier names

obj[42]   // valid, 42 will be coerced to "42"
obj["--"] // valid, any character sequence is allowed
obj[bar]  // valid, will evaluate the variable `bar` and 
          // use its value as property name

Use bracket notation:

  • When the property name is contained in a variable, e.g. obj[foo].
  • The property name contains characters not permitted in identifiers, e.g. starts with a digit, or contains a space or dash (-), e.g. obj["my property"].

Use dot notation: In all other situations.

There is a caveat though regarding reserved keywords. While the specification permits to use them as property names and with the dot notation, not all browsers or tools respect this (notably older IE versions). So the best solution in my opinion is to avoid using reserved keywords for property names or use bracket notation if you cannot.

Square bracket notation allows use of characters that can't be used with dot notation: var foo = myForm.foo[]; // incorrect syntax var foo = myForm["foo[]"]; // correct syntax

The second advantage of square bracket notation is when dealing with variable property names.

for (var i = 0; i < 10; i++) {
  someFunction(myForm["myControlNumber" + i]);
}

  • Dot notation is faster to write and clearer to read.
  • Square bracket notation allows access to properties containing special characters and selection of properties using variables
  • The bracket notation allows you to access properties by name stored in a variable:
var obj = { "abc" : "hello" };
var x = "abc";
var y = obj[x];
console.log(y); //output - hello

obj.x would not work in this case.

 

You cannot use . to access array elements. However you can use both . and [] to access object properties.

When trying to access a non-existing object property with . you will get an error but if you try with []instead you will get undefined

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