Errors and async · Lesson 34 of 45

Handling errors: try and catch

Keep the program alive when something goes wrong.

8 minutes3 games

When an error happens JavaScript stops the code. try { … } catch (err) { … } lets you “catch” it and handle it instead of crashing. finally runs either way. You can raise your own with throw new Error("…").

Step by step1 / 5

1function divide(a, b) {
2 if (b === 0) throw new Error("Cannot divide by zero")
3 return a / b
4}
5try {
▶ console.log(divide(10, 2))
7 console.log(divide(1, 0))
8 console.log("never printed")
9} catch (err) {
10 console.log("Error:", err.message)
11} finally {
12 console.log("done")
13}

Variables

—

Output

The first call works.

Type	Happens when
`ReferenceError`	you use a variable that doesn't exist
`TypeError`	an operation doesn't fit the type (`undefined.name`)
`SyntaxError`	the code is malformed (or `JSON.parse` on bad text)
`RangeError`	a number is out of range

What's the output?

try {
  null.length
} catch (e) {
  console.log(e.name)
}

What will this code print?

Complete the code

Raise your own error when the age is negative.

if (age < 0)  new Error("Invalid age")

Exercise

Try JSON.parse on the bad text; if it errors, print Invalid JSON.

Expected output: Invalid JSON

JavaScript

const text = "{oops"

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JavaScript

Errors and async · Lesson 34 of 45

Handling errors: try and catch

Keep the program alive when something goes wrong.

8 minutes3 games

Step by step1 / 5

1function divide(a, b) {
2 if (b === 0) throw new Error("Cannot divide by zero")
3 return a / b
4}
5try {
▶ console.log(divide(10, 2))
7 console.log(divide(1, 0))
8 console.log("never printed")
9} catch (err) {
10 console.log("Error:", err.message)
11} finally {
12 console.log("done")
13}

Variables

—

Output

The first call works.

Type	Happens when
`ReferenceError`	you use a variable that doesn't exist
`TypeError`	an operation doesn't fit the type (`undefined.name`)
`SyntaxError`	the code is malformed (or `JSON.parse` on bad text)
`RangeError`	a number is out of range

What's the output?

try {
  null.length
} catch (e) {
  console.log(e.name)
}

What will this code print?

Complete the code

Raise your own error when the age is negative.

if (age < 0)  new Error("Invalid age")

Exercise

Try JSON.parse on the bad text; if it errors, print Invalid JSON.

Expected output: Invalid JSON

JavaScript

const text = "{oops"

Learn more from

Played 0 of 3 games