Mastering JavaScript Functional Programming

Mutator functions

A common source of unexpected problems comes from the fact that several JS methods actually modify the underlying object. In this case, by merely using them, you will be causing a side-effect, which you may even not recognize. Arrays are the basic source of problems and the list of troublesome methods is not short. (See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#Mutator_methods for more on each method.)

• .copyWithin() lets you copy elements within the array
• .fill() fills an array with a given value
• .push() and .pop() let you add or delete elements at the end of an array
• .shift() and .unshift() work the same way, but at the beginning of the array
• .splice() lets you add or delete ...