var obj = {...};
for (var key in obj) {
    var value = obj[key];

}

php语法简直就是糖。

为…他会为你工作的。

for( var key in obj ) {
  var value = obj[key];
}

在现代JavaScript中，你也可以这样做:

for ( const [key,value] of Object.entries( obj ) ) {

}

for (var k in target){
    if (target.hasOwnProperty(k)) {
         alert("Key is " + k + ", value is " + target[k]);
    }
}

hasOwnProperty用于检查目标是否真的具有该属性，而不是从其原型继承该属性。更简单的说法是:

for (var k in target){
    if (typeof target[k] !== 'function') {
         alert("Key is " + k + ", value is" + target[k]);
    }
}

它只是检查k是不是一个方法(如果目标是数组，你会得到很多方法的警告，例如indexOf, push, pop等)。

for (var key in myMap) {
    if (myMap.hasOwnProperty(key)) {
        console.log("key =" + key);
        console.log("value =" + myMap[key]);
    }
}

在javascript中，每个对象都有一堆内置的键值对，其中包含元信息。当你循环遍历一个对象的所有键值对时，你也是在循环遍历它们。使用hasOwnProperty()可以过滤掉这些。

var global = (function() {
   return this;
})();

// Pair object, similar to Python

function Pair(key, value) {
    this.key = key;
    this.value = value;

    this.toString = function() {
       return "(" + key + ", " + value + ")";
    };
}

/**
 * as function
 * @param {String} dataName A String holding the name of your pairs list.
 * @return {Array:Pair} The data list filled
 *    with all pair objects.
 */
Object.prototype.as = function(dataName) {
    var value, key, data;
    global[dataName] = data = [];

    for (key in this) {
       if (this.hasOwnProperty(key)) {
          value = this[key];

          (function() {
             var k = key,
                 v = value;

            data.push(new Pair(k, v));
          })();
       }
    }

    return data;
};

var d = {
   'one': 1,
   'two': 2
};

// Loop on your (key, list) pairs in this way
for (var i = 0, max = d.as("data").length; i < max; i += 1) {
   key = data[i].key;
   value = data[i].value;

   console.log("key: " + key + ", value: " + value);
}

// delete data when u've finished with it.
delete data;

为什么不简单地说

var donuts = [
{ type: "Jelly", cost: 1.22 },
{ type: "Chocolate", cost: 2.45 },
{ type: "Cider", cost: 1.59 },
{ type: "Boston Cream", cost: 5.99 }];

donuts.forEach(v => {console.log(v["type"]+ " donuts cost $"+v["cost"]+" each")});