ES6의 for...of 루프와 반복 가능한 객체에 대한 자세한 설명

for(i=0;i<things.length;i++) {
    var thing = things[i]
    /* ... */
}

for(key in things) {
    if(!thing.hasOwnProperty(key)) { continue; }
    var thing = things[key]
    /* ... */
}

for(const thing of things) {
    /* ... */
}

const foo = [
'apples','oranges','pears'
]

for(const thing of foo) {
  console.log(thing)
}

apples
oranges
pears

const foo = [
'apples','oranges','pears'
]

for(const thing of foo.entries()) {
  console.log(thing)
}

[ 0, 'apples' ]
[ 1, 'oranges' ]
[ 2, 'pears' ]

const foo = [
    'apples','oranges','pears'
]

for(const [key, value] of foo.entries()) {
  console.log(key,':',value)
}

// 无法正常执行
const foo = {
  'apples':'oranges',
  'pears':'prunes'
}

for(const [key, value] of foo) {
  console.log(key,':',value)
}

$ node test.js
/path/to/test.js:6
for(const [key, value] of foo) {
TypeError: foo is not iterable

const foo = {
  'apples':'oranges',
  'pears':'prunes'
}

for(const [key, value] of Object.entries(foo)) {
  console.log(key,':',value)
}

$ node test.js
apples : oranges
pears : prunes

const foo = {
  [Symbol.iterator]: function() {
  }
}

for(const [key, value] of foo) {
  console.log(key, value)
}

for(const [key, value] of foo) {
                          ^
TypeError: Result of the Symbol.iterator method is not an object

const foo = {
  [Symbol.iterator]: function() {
    return {
      next: function() {
      }
    }
  }
}

for(const [key, value] of foo) {
  console.log(key, value)
}

for(const [key, value] of foo) {
                     ^
TypeError: Iterator result undefined is not an object