js.array

let a = ['a', 'b', 'c'];

a.map(string => "c'est "+string) 
// output
// 0: "c'est a"
// 1: "c'est b"
// 2: "c'est c"

let a = ['a', 'b', 'c'];

a.map((string, i) => string+", c'est le "+i) 
// output
// 0: "a, c'est le 0"
// 1: "b, c'est le 1"
// 2: "c, c'est le 2"

let arr1 = ["Hello", "my", "dear"]
let reducer1 = (acc, cur) => acc+" "+cur // la fonction accueillie dans reduce
console.log(arr1.reduce(reducer1)) // output: "hello my dear" (String) 

let arr2 = [1, 2, 3]
let reducer2 = (acc, cur) => acc+cur
console.log(arr2.reduce(reducer2)) // output: 6 (Number)
console.log(arr2.reduce(reducer1)) // output: "1 2 3" (String)