syntax-and-methods.md

Syntax

semi-colons;

In Javascript, an expression is a line of code that returns a value. A statement is any line of code. Every expression needs a semi-colon at the end. Statements that aren't expressions don't generally require semi-colons, and may cause syntax errors.

For example,

// variable assignments are expressions
let x = 5;

// function declarations are statements
function() {}

if (x === 5) { // `if` blocks are statements
  console.log("hello"); // console.log's are expressions
}  

{ curly braces }

Curly braces are used to delineate code blocks such as in function definitions, loops, and if blocks. Curly braces can also be used to define JavaScript objects.

// `if/else` block
if (boolean) {
  // code block...
} else {
  // another code block...
}

// loop
while (condition) {
  // code block...
}

// function definition
function foo() {
  // code block...
}

// JS object
let obj = { key: "value" };

Missing curly braces is a very common mistake! Build good habits early.

Looping

Use blocks to delineate code within loops.

for Loops

for (let i = 0; i < 10; i++) {
  // code block
}

while loops

while (condition) {
 // code block
}

Looping Keywords

continue

Skips the current iteration.

let result = [];
for (let i = 1; i < 10; i++) {
  if (i % 3 === 0) {
    continue;
  }
  result.push(i);
}

console.log(result); // [1, 2, 4, 5, 7, 8]

break

Exits the loop.

let result = [];
for (let i = 1; i < 10; i++) {
  if (i % 3 === 0) {
    break;
  }
  result.push(i);
}

console.log(result); // [1, 2]

switch Statements

switch (expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

Useful Methods

You'll notice that many of these operators are very similar to Ruby! Give them a try in Node.

console.log

This is similar to puts in Ruby.

> console.log('hello')
hello

Comments in JS

// this is a javascript in-line comment

/*
  this is
  a javascript
  comment block
*/

Mathematical Operators

• +
• -
• *
• /
• %

Comparison Operators

• >
• <
• >=
• <=
• ===
• !==

NB: Three equals signs? What's that about? There is a double equals operator (==) in JS as well, but we'll mostly be sticking with ===. The double equals sign does some type conversion which can lead to confusing results. Learn more about it in Effective Javascript.

Logical Operators

• &&: and
• ||: or
• !: not

String Methods

• String.prototype.toLowerCase
• String.prototype.toUpperCase
• String.prototype.indexOf
• +: concatenation

Array Properties

• Array.prototype.length

Array Methods

• Array.prototype.pop
• Array.prototype.push
• Array.prototype.unshift
• Array.prototype.shift
• Array.prototype.indexOf
  • Similar to Array#index in Ruby, but returns -1 when it does not find an item.
• []
  • Index using bracket notation (similar to Ruby).
• Array.prototype.slice([start, [end]])
  • Makes a copy of an array from the start index up to but not including the end index. Both arguments are optional (the first and last elements are used by default).
• Array.includes (ES6+)
  • Identical to Array#include? in Ruby
• Array.isArray(ES6+):
  • Class method that accepts one argument and returns a boolean representing whether it is an Array.