JavaScript property access: dot notation vs. brackets?

ghz 1years ago ⋅ 9016 views

Question

Other than the obvious fact that the first form could use a variable and not just a string literal, is there any reason to use one over the other, and if so under which cases?

In code:

// Given:
var foo = {'bar': 'baz'};

// Then
var x = foo['bar'];

// vs. 
var x = foo.bar;

Context: I've written a code generator which produces these expressions and I'm wondering which is preferable.

Answer

(Sourced from here.)

Square bracket notation allows the use of characters that can't be used with dot notation:

var foo = myForm.foo[]; // incorrect syntax
var foo = myForm["foo[]"]; // correct syntax

including non-ASCII (UTF-8) characters, as in myForm["ダ"] (more examples).

Secondly, square bracket notation is useful when dealing with property names which vary in a predictable way:

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

Roundup:

  • 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

Another example of characters that can't be used with dot notation is property names that themselves contain a dot.

For example a json response could contain a property called bar.Baz.

var foo = myResponse.bar.Baz; // incorrect syntax
var foo = myResponse["bar.Baz"]; // correct syntax
#javascript