For In

The for...in loop is used to iterate over the properties of a JS object.

Syntactically, the for...in loop is constructed as for (aliasForProperty in object) {}. Below is an example of how it can be used.

let dog = {
  breed : "labrador",
  name : "scout",
  age : 4
};

for (let p in dog) {
  console.log(p + " : " + dog[p]);
}

// breed : labrador
// name : scout
// age : 4

The p in the example above is a placeholder for the property being iterated over. p will be a string representation of the property name. To access the value for that property from the object, you must use subscript notation.

Example

At first, this syntax may seem strange, but it is very similar to Java's HashMap<>(). Instead of using put() and get() to read and write from the JS object literal, we simply use the name of the property.

Here is an example in Java:

Map<String,String> data = new HashMap<>();
data.put("name","Bob Dobbs");
data.put("email","bdobbs@gmail.com");

System.out.println(data.get("name")); // Bob Dobbs
System.out.println(data.get("email")); // bdobss@gmail.com

Here is the JavaScript equivalent:

let data = {};
data.name = "Bob Dobbs";
data.email = "bdobbs@gmail.com";

console.log(data.name); // Bob Dobbs
console.log(data.email); // bdobbs@gmail.com

Skill Drill

1. Create an array called log.

2. Create an object called day1 and assign it properties date (String representation of the date), didWorkout (boolean), foodConsumed (an array of strings),

3. Push the day1 object into the log array.

4. Create a few other day objects and add them to the log array as well.

5. Iterate over the log array, and with each iteration perform a second loop (a for...in) and print out the property as well as its corresponding value.

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