Javascript CheatSheet

Based on this Tutorial.

Table of Contents

Types

Numbers

var num = 0.1;
var x = Math.sin(3.5);
var d = Math.PI * 3 * 2;

Convert string to int

parseInt("123", 10); // 123
Number("123"); // 123

Convert int to boolean

Boolean(234); // true

Strings

"hello".charAt(0); // "h"
"hello, world".replace("hello", "goodbye"); // "goodbye, world"
"hello".toUpperCase(); // "HELLO"

Undefined

3 cases where we will get undefined:

1) forgetting to assign a variable.

var x;    
console.log(x);

2) access a key in a hash that doesn't exist.

var foo = {};
foo.x;

3) forgetting to pass a variable to a function

function foo(x) { console.log(x); }
foo();

Boolean

Boolean("");  // false
Boolean(234); // true

Variables

var a;
var name = "simon";

Numeric Operators

+, -, *, / and % 

var y = 2;
var x = (y * 5) / 3;

Control structures

if, while, &&, ||, ?, switch 

var name = "kittens";

if (name === "puppies") {
  name += "!";
} else if (name === "kittens") {
  name += "!!";
} else {
  name = "!" + name;

}

// name = kittens!!
while (true) {
  // an infinite loop!
}

Check that o is truthy using &&

var name = o && o.getName();

Set default value using ||

var name = otherName || "default";

Use thhe ternary operator for conditional expressions:

var allowed = (age > 18) ? "yes" : "no";

Switch statement:

switch(action) {
  case 'draw':
    drawIt();
    break;
  case 'eat':
    eatIt();
    break;
  default:
    doNothing();
}

Objects

var foo = {};
var person = {name: 'josh', age: 23};

Arrays

// a is an array
var a = ["dog", "cat", "hen"];

// assigning a string into index 100 of our array
a[100] = "fox";
a.length; // 101

// add item to the end of the array
a.push('kitten');

// print each element
a.forEach(function(item){
  console.log(item);
});

A few functions of Arrays

Usage What it does
a.toString() Returns a string with the toString() of each element separated by commas.
a.toLocaleString() Returns a string with the toLocaleStringring() of each element separated by commas.
a.concat(item1[, item2[, …[, itemN]]]) Returns a new array with the items added on to it.
a.join(sep) Converts thate array to a string - values delimited by the sep param
a.pop() Removes and returns the last item.
a.push(item1, …, itemN) Push adds one or more items to the end.
a.reverse() Reverse the array.
a.shift() Removes and returns the first item.
a.slice(start, end) Returns a sub-array.
array.sort([cmpfn]) Takes an optional comparison function.
a.splice(start, delimitedelcount[, item1[, …[, itemN]]]) Lets you modify an array by deleting a section and replacing it with more items.
a.unshift([item]) Prepe

Functions

function add(x, y) {
  var total = x + y;
  return total;
}

var res = add(2,3);

Closures

function makeAdder(a) {
  return function(b) {
    return a + b;
  };
}

x = makeAdder(5);
y = makeAdder(20);
x(6); // 11
y(7); // 27

What happens when you return a function from a function?
Everytime you call a function, JavaScript keeps a 'scope' object with the parameters you passed.
In the first example we pass 5. so the scope object is {a: 5}.
Since we asigned a function into x variable, x keeps a reference to that object so it's not being garbage collected.

Custom objects

function Person(first, last) {
  this.first = first;
  this.last = last;
}

Person.prototype.fullName = function() {
  return this.first + ' ' + this.last;
};

Person.prototype.fullNameReversed = function() {
  return this.last + ', ' + this.first;
};

// create new instance of Person object
s = new Person("Simon", "Willison");
s.firstNameCaps(); // TypeError on line 1: s.firstNameCaps is not a function

Person.prototype.firstNameCaps = function() {
  return this.first.toUpperCase()
};
s.firstNameCaps(); // "SIMON"

Person.prototype is an object shared by all instances of Person. It forms part of a lookup chain (that has a special name, "prototype chain"): any time you attempt to access a property of Person that isn't set, JavaScript will check Person.prototype to see if that property exists there instead. As a result, anything assigned to Person.prototype becomes available to all instances of that constructor via the this object.