Control Structures

CSCI-UA.0480-008

Control Structures

as you might expect, JavaScript programs are generally executed line-by-line from top to bottom
but there are ways to control that flow of execution (of course)! →
- conditionals
- repetition
- creating a function (to jump to the code in the function definition)
- (we'll see others as well)

Conditionals and Loops

This stuff should look really familiar …

we'll go through the following material quickly because…
you know all of these concepts already
and it looks a lot like Java/C/C++

Again With the Definitions

boolean expression - an expression that results in a boolean value
- 423 === 423 →
  - JavaScript ❤s coercing values for you; a value will be coaxed into boolean when the need arises
- !!5 →
  - logical not, logical not, number 5 … ¯\_(ツ)_/¯
block - a sequence of grouped statements bound by curly braces - {'s and }'s
iteration - repeated execution of a set of programming statements
loop - the construct that allows allows us to repeatedly execute a statement or a group of statements until a terminating condition is satisfied

If and Else

Conditionally execute a block of code (should look familiar):

keyword if followed by a boolean expression enclosed in parentheses
block of code to be executed if expression is true
optional additional else if's to chain conditionals, with corresponding block of code to execute
optional else, with corresponding block of code to execute


if (some_boolean_expression) {
	// do stuff here if expression is true
}

if (some_boolean_expression) {
	// do stuff 
} else if (another_boolean_expression) {
	// do other stuff
} else {
	// do other other stuff
}

A For Loop

Again, should look familiar… repeatedly execute a block of code:


//    initialization
//    |        condition
//    |        |       afterthought/increment
//    |        |       |
for(let i = 0; i <= 5; i = i + 1) {
	console.log(i);
}

// (i++ works too, of course)

Speaking JavaScript calls the three parts: initialization, check and update.

Hey - notice that let in front of the loop variable declaration? Do that.

A While Loop

Conditionally repeat a block of code:


while (boolean_expression) {
	// repeat this stuff as long as boolean expression is true
}

Conditionally repeat a block of code and ensure that code is executed at least once:


do {
	// repeat this stuff at least once
} while (boolean_expression)

Break and Continue

The keyword break immediately stops the execution of a loop:


for (let num = 1; num < 30; num++) { 
	if (num % 7 == 0 && num % 3 == 0)
		break; 
	console.log(num);
}

The keyword continue stops the current iteration and immediately skips to the next one:


for(let num = 1; num < 30; num++) { 
	if (num % 7 == 0 && num % 3 == 0)
		continue; 
	console.log(num);
}

What is the output of the above programs? →

1 through 20 and 1 through 29 skipping 21 respectively

Switch-Case

Execute code based on value of switch.


var day = "thu";
switch (day) { 
	case "fri":
		console.log("Friday");
		break;
	case "thu":
		console.log("Thursday");
		break;
	case "wed":
		console.log("Wednesday");
		break;
}

What is the output of the above programs? →

Thursday

Lastly, a Preview of Functions

One way to define a function in JavaScript is to create a variable, and set it equal to a function:


var f = function(x) {
	return x
}

Control Structures

CSCI-UA.0480-008