Transform collections without surprises

The idea

Array methods communicate the shape of a transformation. Use map to produce one output per input, filter to retain selected inputs, and reduce to combine inputs into a single accumulated result. Give reduce an explicit initial value when empty arrays are allowed. A chain is useful when each step has a clear purpose, not simply because fewer lines appear elegant.

Creating a new array does not automatically create new objects inside it. Array spread and slice copy the outer collection, leaving nested objects shared. When an update must preserve its input, create a replacement object for the changed item as well. Also distinguish nonmutating transformations from sort, reverse, and splice, which change their receiver. Tests should inspect the original input after the operation.

const tasks = [{id: 1, done: false}, {id: 2, done: false}];
const updated = tasks.map(t => t.id === 2 ? {...t, done: true} : t);
console.log(tasks[1].done);
console.log(updated[1].done);

false
true

The second item is replaced with a new object. Unchanged items may remain shared because the code does not modify them.