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Java Script

Basics

  • end with semicolon - not necessary

Numbers, string and operators

  • only one number type - ieee 754 double
    • 1, 1.111, 1e1
  • and have strings, no character, character are string with length 1
    • "hi" 'hi'
  • also a boolean type - true false

Operations

  • addition +
  • multiplication *
  • ...

  • left shift <<

  • not !

  • not equal !==
  • equal ===
  • less than <
  • greater than >
  • <=

  • >=

  • == type coercion is performed

    • "5" == 5 true
    • null == undefined true

  • three special not-a-real-number

    • Infinity 1/0
    • -Infinity -1/0
    • NaN - not a number 0/0

Stings

s = "This is a string

  • s.charAt(0) - 'T'
  • s.substring(0, 5) - "This "
  • s.length 16

null undefined

  • null - indicate a deliberate non value
  • undefined - value not set, but it is a value itself

what is falsy

  • false
  • null
  • undefined
  • NaN
  • 0

other are true, even "0" is true

Variable

  • var
    • scope global or within the function
    • can be redeclare
    • hoisted to top and set to undefined
  • let
    • block scoped
    • can be update but not re-declared
    • hoisted to top but not initialized, so if you use them without defining you will get Reference Error
  • const
    • block scoped
    • can't be updated and re-declared
    • hoisted to top but not initialized

Arrays

var array = ["hi", "a", true]
array[1] = 45
  • .push() - add element to last
  • .pop() - remove element from last and return it
  • .shift() - remove first element and return it
  • .unshift() - add a element to front

.join()

  • join elements with some separator
arr = [1,2,3];
arr.join(';'); // "1;2;3"

.slice()

Get some part form array, arr.slice(start, end+1).

arr.splice(1,3);

Objects

  • equivalent to map, dictionaries in other languages
var myobj = {mykey: "myvalue", "my other key": 4}

// how to access values
myobj['my other key']; // 4
myobj.mykey; // "myvalue"
  • objects are mutable
// add another attribute
myobj.color = "red"

Conditionals and Iterations

if-else

var count = 1;
if (count == 3){
    // evaluated if count is 3
} else if (count == 4){
    // evaluated if count is 4
} else {
    // evaluated if it's not either 3 or 4
}

while

// As does `while`.
while (true){
    // An infinite loop!
}

do while

var input;
do {
    input = getInput();
} while (!isValid(input));

for

for (var i = 0; i < 5; i++){
    // will run 5 times
}

// Breaking out of labeled loops is similar to Java
outer:
for (var i = 0; i < 10; i++) {
    for (var j = 0; j < 10; j++) {
        if (i == 5 && j ==5) {
            break outer;
            // breaks out of outer loop instead of only the inner one
        }
    }
}

for in - objects

// The for/in statement allows iteration over properties of an object.
var description = "";
var person = {fname:"Paul", lname:"Ken", age:18};
for (var x in person){
    description += person[x] + " ";
} // description = 'Paul Ken 18 '

for of - iterables

// The for/of statement allows iteration over iterable objects (including the built-in String, 
// Array, e.g. the Array-like arguments or NodeList objects, TypedArray, Map and Set, 
// and user-defined iterables).
var myPets = "";
var pets = ["cat", "dog", "hamster", "hedgehog"];
for (var pet of pets){
    myPets += pet + " ";
} // myPets = 'cat dog hamster hedgehog '

case

grade = 'B';
switch (grade) {
  case 'A':
    console.log("Great job");
    break;
  case 'B':
    console.log("OK job");
    break;
  case 'C':
    console.log("You can do better");
    break;
  default:
    console.log("Oy vey");
    break;
}

Functions

function myFunction(thing){
    return thing.toUpperCase();
}
myFunction("foo"); // = "FOO"