Performance of key lookup in JavaScript object

The V8 design docs imply lookups will be at least this fast, if not faster:

Most JavaScript engines use a dictionary-like data structure as storage for object properties - each property access requires a dynamic lookup to resolve the property's location in memory. This approach makes accessing properties in JavaScript typically much slower than accessing instance variables in programming languages like Java and Smalltalk. In these languages, instance variables are located at fixed offsets determined by the compiler due to the fixed object layout defined by the object's class. Access is simply a matter of a memory load or store, often requiring only a single instruction.

To reduce the time required to access JavaScript properties, V8 does not use dynamic lookup to access properties. Instead, V8 dynamically creates hidden classes behind the scenes. [...] In V8, an object changes its hidden class when a new property is added.

It sounds like adding a new key might be slightly slower, though, due to the hidden class creation.

Yes, you can assume that adding a key, and later using it for access are effectively constant time operations.

Under the hood the JS engine may apply some techniques to optimize subsequent lookups, but for the purposes of any algorithm, you can assume O(1).

Take a look at a similar question How does JavaScript VM implements Object property access? and the answer. Here I described the optimization technique used by JS engines and how it affects the key lookup performance. I hope these details help you write more efficient JS code.