Truthy and Falsy

When JavaScript encounters an element that is not true or false in a boolean context, it converts the element to either true or false.

if ("hey") {
  console.log("The string 'hey' is true in a boolean context, so this string is printed.");
}

The string "hey" is considered true in a boolean context. The empty string is considered false in a boolean context:

if ("") {
  console.log("This is not printed. An empty string is false in a boolean context.");
}

When an element is considered true in a boolean context, it is called truthy. When an element is considered false in a boolean context, it is called falsy.

The Boolean() function can be used to see which elements are truthy and falsy.

Boolean("hey"); // true
Boolean(""); // false
Boolean(42); // true
Boolean(false); // false
Boolean(undefined); // false

JavaScript has only six values that are falsy:

Boolean(undefined); // false
Boolean(NaN); // false
Boolean(null); // false
Boolean(0); // false
Boolean(""); // false
Boolean(false); // false

Every other value is truthy.

Here is a simple function that returns a string explaining if a value is truthy or falsy:

function booleanTruthiness (e) {
  var type = typeof(e);
  var bool = Boolean(e);
  return("The element " + e + " is a " + type + " and is " + bool + " in a boolean context" )
}

Here is an example of the booleanTruthiness() function in action.

booleanTruthiness("hi"); // "The element hi is a string and is true in a boolean context"
booleanTruthiness(false); // "The element false is a boolean and is false in a boolean context"

Here's a while loop that continues iterating until the value is falsy in a boolean context.

var lol = "haha";
while (lol) {
  console.log("so funny!");
  lol = undefined;
}

The while loop will iterate once because the lol variable is set to "haha", which is truthy. After the first iteration, the lol variable is reassigned to undefined, which is falsy. The loop exits before the second iteration.

if ([8, 8]) {
  console.log("I love 8!");
}

if (undefined) {
  console.log("here comes the sun");
}

var party = "dance";
while (party) {
  console.log("woo hoo!");
  party = "";
}