Some Definitions
We learned how to produce values from using operators on other values. We created expressions!.
- expression - a fragment of code that produces a value
- not a statement by itself
- every literal value is an expression - 27, "YES!"
- statement - a full instruction/action for the computer
- in JavaScript most statements end in a semicolon (;)
- the simplest statement is an expression with a semicolon at the end - 27;, "YES!";
- program - a sequence of statements that specify to a computer actions to perform
- function - a named sequence of code that can be called by name
- built-in function - a function available in the global namespace that is part of the core language
A Quick Note on Functions
If you declare a function using this (function declaration) syntax: →
// function declaration, a single statement
function f() {
// do stuff
}
… you have a single statement (note, no semicolon at the end).
However, any statement that:
- involves functions
- but doesn't start with the actual keyword, function
- will have a function expression (that is, an expression that evaluates to a function) in it:
const foo = function bar(bax) {}; // <-- function expression
const qux = function(corge) {}; // <-- function expression
Soooo… most statements end with a semicolon.
Variables
Variables are symbolic names for values; you can use a variable's name wherever you want to use that value.
To create a variable, start with one of the following keywords:
- const or let (ES6) … or var
- followed by a variable name
- optionally, assign a value to the variable by using = and a value or expression
- of course, you can add a semicolon (or not)
- notice that type does not have to be specified!
const s = 'hello';
let i = 21 * 2;
console.log(s);
console.log(++i);
Block Scope w/ const or let (ES6)
Wait, so it looks like there are three ways to declare variables. What's the difference? Let's look at const and let first →
- the scope of variables declared with const and let is the block that they're contained in
- a block is simply the area between opening and closing curly braces
{
const s = 'in'; // just need two curly braces to make a block!
}
for(let i = 0; i < 10; i++) {
const s = 'also in'; // for loop body is a block!
}
function f() {
const s = 'in too'; // function body is a block!
}
Block Scope Continued
Accessing a variable when it's out of scope produces a run-time error (specifically a ReferenceError)!
console.log('out there');
{
const s = 'in here';
}
console.log(s);
(heeeey… sort of just like Java)
Variables in an outer scope are visible from an inner scope:
{
const first = 'out there';
{
const full = `${first} in here`;
console.log(full);
}
}
const vs let
OK… so what's the difference between const and let then? →
- a variable declared with const can't be assigned a different value after it had been declared
- const reassignment will result in a run-time error (TypeError)const dontChangeMe = "I told you so";
- dontChangeMe = "why not?";
- note, however, that this does not mean the variable is immutable
- in fact, if a const declared variable is mutable, it can still be changed without errorconst arr = [1, 2, 3];
- arr[0] = 'wat? this is ok!?';
- console.log(arr);
- on the other hand, let declared variables can be reassignedlet i = 0;
- i = i + 2;
- console.log(i);
const vs let Continued
Again, a value cannot be assigned to a const declared variable after it's been declared. This implies that… →
- from mdn: "An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared"
- or, simply put, you must assign a value immediately when declaring a variable with const
- otherwise, you'll get a syntax error:const foo;
- foo = 'bar'
- // SyntaxError: Missing initializer in const declaration
Default Initial Value
When you declare a variable without assigning a value to it, it will be initialized to undefined (this is really only valid for let and var, of course). →
For example, the output of this…
let a;
console.log(a);
is
undefined
Redeclaring Variables with let and const
If a variable has already been declared (with let, const, or var)… →
- redeclaring a variable with the same identifier (name) with let and const will result in a SyntaxError
- for example:let i = 0;
- let i = 1;
A Note About Loops
let and const behavior in loops: →
- if the loop variable is incremented/decremented, it must be declared as 'let':for(let i = 0; i < 10; i++) { console.log(i); }
- repeatedly creating a variable with let or const in a loop body does not count as redeclaration (works fine; it's another scope!)for(let i = 0; i < 10; i++) {
- const j = i * 2;
- console.log(j);
- }
Always use const or let (or var, I guess) when declaring variable names!
If you don't declare variables with these keywords:
- you get global variables!
- This is particularly important when dealing with variable declarations in functions
And Speaking of a Look of Disapproval
(Variable Names)
What are the rules for a valid identifier (variable name) again? →
- start with a letter, underscore ( _ ), or dollar ( $ )
- following characters can be any of above, and/or digits (0-9)
- can't use reserved words / keywords
Don't Do This, But…
BTW, Unicode characters are allowed in variable names!!! →
const ಠ_ಠ = "disapproval";
console.log(ಠ_ಠ);
// totally works (O_o !?)
Thanks Stackoverflow! Also, here's more about that look.
Oh, and this is a site that let's you check if a variable name is valid or not.
Another Note on Variables
Because JavaScript is dynamically typed… variable reassignment (for let and var, even of different types, is ok →
let x = 25;
x = "foo";
Lastly, a quick aside…
- foo, bar, and baz are metasyntactic variables
- apparently there are different metasyntactic variables for different natural languages!