JS: Compare for/each and for/in

compare_for_loops.js

// console.log("test");
var names = ["matt", "adam"];

// 1: use for/in, w/var
console.log("Starting 1 (for/in)...")
var idx = -9;
console.log("idx: before1", idx)
for (var idx in names) {
  console.log("idx: in1", idx);
}
console.log("idx: after1", idx)

console.log("******************")

// 1a: use for/in, w/o var
console.log("Starting 1a (w/o for/in)...")
var idx = -8;
console.log("idx: before1a", idx)
for (idx in names) {
  console.log("idx: in1a", idx);
}
console.log("idx: after1a", idx)

console.log("******************")

// 2: try it, initializing to 0
console.log("Starting 2 (for/loop w/init)...")
var idx = -7;
console.log("idx: before2", idx)
for (var idx=0; idx < names.length; idx++) {
  console.log("idx: in2", idx);
}
console.log("idx: after2", idx)

console.log("******************")

// 3: try it without initializing to 0
console.log("Starting 3 (for/loop w/o init)...")
var idx = -6;
console.log("idx: before3", idx)
for (var idx; idx < names.length; idx++) {
  console.log("idx: in3", idx);
}
console.log("idx: after3", idx)

Interested? Just copy and paste into your js console.