Review of Week 2 Day 3 Learning Objectives

1. Given multiple plausible reasons, identify why functions are called “First Class Objects” in JavaScript.

Here are some of the reasons:

• A function is an instance of the Object type
• A function can have properties and has a link back to its constructor method
• You can store the function in a variable
• You can pass the function as a parameter to another function
• You can return the function from a function

2. Given a code snippet containing an anonymous callback, a named callback, and multiple console.logs, predict what will be printed

function foo(callback) {
    console.log('grape');
    callback();
}

function bar() {
    console.log('banana');
}

const fruitBasket = function() {
    console.log('apple');
    bar();
    foo(bar);
    foo(function() {
        console.log('orange');
    });
    console.log('pear');
};

fruitBasket();

You should be able to predict what is going to be logged when we call fruitBasket.

3. Write a function that takes in a value and two callbacks. The function should return the result of the callback that is greater.

function greaterValue(value, cb1, cb2) {
    // compare cb1 invoked with value to cb2 invoked with value
    // return the greater result

    let res1 = cb1(value);
    let res2 = cb2(value);
    if (res1 > res2) {
        // if this is false, we move out of if statement
        return res1;
    }
    return res2;
}

let negate = function(num) {
    return num * -1;
};

let addOne = function(num) {
    return num + 1;
};

console.log(greaterValue(3, negate, addOne));
console.log(greaterValue(-2, negate, addOne));

Note: we do not invoke negate or addOne (by using () to call them), we are passing the function itself.

4. Write a function, myMap, that takes in an array and a callback as arguments. The function should mimic the behavior of Array#map.

function myMap(arr, callback) {
    // iterate through the array, perform the cb on each element
    // return a new array with those new values

    let mapped = [];

    for (let i = 0; i < arr.length; i++) {
        // remember that map passes three args with each element.
        let val = callback(arr[i], i, arr); 
        mapped.push(val);
    }

    return mapped;
}

let double = function(num) {
    return num * 2;
};

console.log(myMap([ 1, 2, 3 ], double));

5. Write a function, myFilter, that takes in an array and a callback as arguments. The function should mimic the behavior of Array#filter.

function myFilter(arr, callback) {
  let filtered = [];

  for(let i = 0; i < arr.length; i++) {
    let element = arr[i];

    if (callback(element, i, arr)) {
      filtered.push(element);
    }
  }

  return filtered;
}

6. Write a function, myEvery, that takes in an array and a callback as arguments. The function should mimic the behavior of Array#every.

function myEvery(arr, callback) {
  for(let i = 0; i < arr.length; i++) {
    let element = arr[i];

    if (callback(element, i , arr) === false) {
        return false;
    }
  }
  return true;
}