Monday November 10

Daily JS

DOM Manipulation

The following code is from an app where the speed of a car is accelerated using the accelerate function. But, right now the code is broken, if we call accelerate() without passing in any arguments, speed will be set to NaN. Try to explain why this is and rewrite the accelerate function to fix this error.

var speed = 0;

/* Original */
// function accelerate(amount) {
//   if (amount === undefined) {
//     amount = 0;
//   }
//
//   speed += amount;
// }

function accelerate(amount) {
  if (amount === undefined) {
    amount = 0;
  }

  speed += amount;
}

accelerate(5); // speed is 5
accelerate(); // speed is NaN

When no argument is sent to the accelerate function then amount will be undefined. undefined plus a number gives a result of NaN.

Examples

Using reduce only to perform a filter then map. These two statements would return the same result.

characters.filter((item) => {
  return item.age > 12;
}).map((item) => {
  return item.name;
});

characters.reduce((carry, item) => {
  if (item.age > 12) {
    carry.push(item.name);
  }

  return carry;
}, []);

Topics

• Objects
• Constructors
• Prototypes

Code

https://github.com/TIY-LR-FEE-2015-Fall/lesson-04-01

Homework

https://github.com/TIY-LR-FEE-2015-Fall/assignments/tree/master/13-constructors