JavaScript cost.
WHY SCRIPTS COST MORE THAN THEIR SIZE
They must be downloaded, parsed, compiled and executed.
WHAT THAT MEANS
A script costs more than an image of the same size, particularly on modest devices.
WHAT TO DO
Ship less Split code, loading only what a page needs Defer anything not required immediately Remove unused dependencies
WHAT TO AUDIT
What is actually included in what you ship.
WHAT YOU WILL FIND
Libraries included for one function Duplicate dependencies Code for features long removed
WHAT TO QUESTION
Every dependency.
WHY
Each is downloaded, parsed and executed by every visitor.
WHAT TO PREFER
Platform features over libraries, where they suffice.
WHAT TO DEFER
Anything not needed for the initial view: analytics, chat widgets, secondary features.
WHAT TO BE CAREFUL WITH
Third-party scripts, which you do not control.
WHY
They can be slow, can fail, and can change without notice.
WHAT TO MEASURE
Time spent executing scripts, on a modest device.
