Section 3: Types and Operators

Conceptual Aside: Types and Javascript

Dynamic Typing: with JS, you don't have to tell JS what type of data a variable holds, JS will figure that out while your code is running.

Primitive Types

Primitive type: a type of data that represents a single value. That is, not an object. There are 6 of the them:

1. undefined: there's a bucket, but nothing in it.
2. null: there's not even a bucket.
3. boolean: true or false
4. number: this includes floating numbers.
5. string: a sequence of character enclosed in single or double quotes.
6. symbol: new for ES6.

Conceptual Aside: Operators

Operator: Generally, operators take two parameters and return one result. Using the operators invokes a special function; operators are functions.

+ Addition

- Subtraction

* Multiplication

/ Division

% Modulus

++ Increment

-- Decrement

Operator Precedence and Associativity

Operator precedence: determines which operator function gets called first. Functions are called in order of precedence.

Associativity: if we have two or more operators of equal precedence, then the equation may move left or right. Here's handy table. For example:

var a = 10 / 2 * 100;

Is a 500 or .05? Because associativity for multiplication and division has us moving from left-to-right, a is 500.

Conceptual Aside: Coercion

Coercion: Converting a value from one type to another (sometimes when you don't want it to). For example: var a = 1 + '2'; would return a as 12.

To avoid coercion, use "==" equality, or "===" strict equality. Use strict by default, to keep JS from coercing anything.

Here's a comparison table using double or triple equal signs.

Comparison Operators

Comparison operators move from left-to-right. The following code would equal true.

console.log(1 < 2 < 3);

But what about this code?

console.log(3 < 2 < 1);

It also returns true. Huh? Let's walk through the code. Since we're going from left-to-right, JS runs this first: 3 < 2 which is false. Still moving from left-to-right, JS now sees the code as:

console.log(false < 1);

In programming, false is converted to 0 through coercion, so 0 is the less than 1!

If you try to pass undefined as a number, you'll get NaN.
null will return 0.

Existence and Booleans

Coercion also occurs with booleans. You can use to check on values. That's pretty much it.

Default Values

Regarding the or || operation, if you pass two value to coerced to true and false, it will always return the true value. Example:

function greet(name) {
  name = name || '$Your name here$';
  console.log('Hello ' + name);
}

greet();

Since name was never defined in the parameter, the '$Your name here$' is returned instead. Console.log would read,Hello $Your name here$.

Framework Aside: Default Values

What happens when libraries or plugins contain the same variables? The last variable loaded in the global execution context will take precedence. To check if a variable name is already in use, you can use a code like this:

window.libraryName = window.libraryName || "Lib 2";

This will provide a stop gap. Wouldn't it make more sense to use an && inside of ||?