yangsu

"37" - 7 // 30
"37" + 7 // "377"
"1.1" + "1.1" // "1.11.1"
(+"1.1") + (+"1.1") // 2.2   // Note: the parentheses are added for clarity, not required.

yangsu

typeof true // 'boolean'
typeof false // 'boolean'

yangsu

Boolean("") // false
Boolean(234) // true
Number("1e2") // 100
String(true) // 'true'

yangsu

true && true    // true
true && false   // false
true || false   // true
false || false  // false

yangsu

'0' == false  // true
'0' == 0      // true
!!'0'         // true
!!0 || !!'' || !!"" || !!null || !!undefined || !!false || !!NaN // false

yangsu

Number.prototype.times = function (fn) {
  var n = this;
  while (n > 0) {
    fn();
    n--;
  }
}

yangsu

var courseInfo = {
  title: 'Programming Languages',
  number: 'HOUSECS 59.18',
  instructors: [{
    name: 'Yang Su',
    major: 'ECE/CS'
  }, {
    name: 'Kevin Gao',
    major: 'ECE/CS'
  }]

yangsu

var a = { value: 1 };
var b = a;
a.value = 2;
b.value // 2

function subone(obj) {
  obj.value -= 1;
}
subone(a);
a.value // 1

yangsu

var a = [];
a.length // 0
a[100] // undefined
a = [1, 2, "three", "4", { value: 5}];
a[4].value // 5
a[10] = 1;
a.length // 11
a[7] // undefined

yangsu

dontDoThis = 'ever !!!';

var a;
if (a === undefined){
  console.log('a is ' + a); // 'a is undefined'
}
if (b) {} // this will generate an error "ReferenceError: b is not defined"

var n = null;
console.log(n * 32); // 0. Note: implicit type conversion