Array API: array.findIndex()

Array.prototype.findIndex makes finding items in arrays easier

takes a compare function, returns the index where it returned true

const foundAt = [false, true].findIndex(item); assert.equal(foundAt, 1);

returns the first position it was found at

const foundAt = [0, 1, 1, 1].findIndex(item => item = 1); assert.equal(foundAt, 1);

returns -1 when nothing was found

const foundAt = [1, 2, 3].findIndex(item => item > 1); assert.equal(foundAt, -1);

the findIndex callback gets the item, index and array as arguments

const three = 3; const containsThree = arr => arr.indexOf(three) > -1; function theSecondThree(index, arr) { const preceedingItems = arr.slice(0, index); return containsThree(preceedingItems); } const foundAt = [1, 1, 2, 3, 3, 3].findIndex(theSecondThree); assert.equal(foundAt, 4);

combined with destructuring complex compares become short

const bob = {name: 'Bob'}; const alice = {name: 'Alice'}; const foundAt = [bob, alice].findIndex(({name:{length:l}}) => length > 3); assert.equal(foundAt, 1);

Required Knowledge

• `const` declaration (Block scope)

Related Katas

Array API

• array.sort() basics
• array.sort() can take a compare function
• array.shift() (as introduced in ES3)
• array.push() (as introduced in ES3)
• Array.from()
• Array.of()
• array.fill()
• array.find()
• array.findIndex()
• array.entries()
• array.keys()
• array.values()
• array.includes()
• array.toReversed()

Difficulty Level

TBD

First Published

4 May 2015

Stats

5 tests to solve