Getting started · Lesson 4 of 45

Type conversion

Number, String, Boolean, and the values that count as false.

8 minutes3 games

Sometimes a number arrives as text (like a value typed into a form) and you need to calculate with it. JavaScript converts types in two ways:

Explicit (you ask for it): Number("42"), String(42), Boolean(1).
Implicit (JavaScript does it for you): like "5" * 2 giving 10.

JavaScript

const input = "42"
console.log(input + 8)          // "428": joined
console.log(Number(input) + 8)  // 50
console.log(Number("12px"))     // NaN
console.log(parseInt("12px"))   // 12
console.log(String(99) + "!")   // "99!"

Truthy and falsy

In any condition JavaScript turns the value into a boolean. Only 6 values count as false (they're falsy); everything else is truthy:

falsy =

false[0]

0[1]

""[2]

null[3]

undefined[4]

NaN[5]

Even "0", "false", [] and {} are truthy.

JavaScript

console.log(Boolean(""))     // false
console.log(Boolean("0"))    // true
console.log(Boolean([]))     // true

const name = ""
if (!name) {
  console.log("Please enter your name")
}

What's the output?

console.log("10" * 2)
console.log("10" + 2)

What will this code print?

Quick question

Which value is truthy?

Exercise

Both values arrive as text. Print their sum as a number (30).

Expected output: 30

JavaScript

const a = "12"
const b = "18"

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JavaScript

Getting started · Lesson 4 of 45

Type conversion

Number, String, Boolean, and the values that count as false.

8 minutes3 games

Sometimes a number arrives as text (like a value typed into a form) and you need to calculate with it. JavaScript converts types in two ways:

Explicit (you ask for it): Number("42"), String(42), Boolean(1).
Implicit (JavaScript does it for you): like "5" * 2 giving 10.

JavaScript

const input = "42"
console.log(input + 8)          // "428": joined
console.log(Number(input) + 8)  // 50
console.log(Number("12px"))     // NaN
console.log(parseInt("12px"))   // 12
console.log(String(99) + "!")   // "99!"

Truthy and falsy

In any condition JavaScript turns the value into a boolean. Only 6 values count as false (they're falsy); everything else is truthy:

falsy =

false[0]

0[1]

""[2]

null[3]

undefined[4]

NaN[5]

Even "0", "false", [] and {} are truthy.

JavaScript

console.log(Boolean(""))     // false
console.log(Boolean("0"))    // true
console.log(Boolean([]))     // true

const name = ""
if (!name) {
  console.log("Please enter your name")
}

What's the output?

console.log("10" * 2)
console.log("10" + 2)

What will this code print?

Quick question

Which value is truthy?

Exercise

Both values arrive as text. Print their sum as a number (30).

Expected output: 30

JavaScript

const a = "12"
const b = "18"

Learn more from

Played 0 of 3 games