Summary

Compare with === and !==, never == and !=, which convert types first. In a condition, false, 0, an empty string, null, undefined and NaN count as false, and everything else as true. A cheat sheet of C# and its JavaScript equivalents.

Apprentice · JavaScript · Updated

After this lesson you can

  • Compare values with === and !==, and predict which values are truthy and falsy

Key takeaways

  • Compare with === and !==, which work like C#'s == and !=. == converts types first, so 5 == '5' is true.
  • false, 0, '', null, undefined and NaN are falsy. Everything else is truthy, including '0', 'false' and [].
  • When 0 is a real answer, compare with === instead of checking whether the value is truthy.
  • undefined means nothing was ever given; null is a value that's deliberately empty, like C#'s null.
  • name ?? 'Guest' gives 'Guest' when name is null or undefined, as in C#.

Two more differences catch out almost every C# developer. Then a cheat sheet to keep beside you.

=== not ==

JavaScript has two equality operators. == converts both values to the same type before comparing, which gives some strange answers. ===, strict equality, compares without converting: different types are never equal.

console.log(5 == '5');
console.log(5 === '5');
console.log(0 == '');
console.log(0 === '');
console.log(null == undefined);
console.log(null === undefined);

true
false
true
false
true
false

Always use === and !==. They behave like C#'s == and !=: 5 and '5' are different things. Forget == and != exist.

Truthy and falsy

In C#, an if needs a bool. In JavaScript, any value can go in a condition, and it counts as true or false. These values are falsy, counted as false (plus rarities such as -0):

Falsy values
false
0
'' (an empty string)
null
undefined
NaN

Everything else is truthy, counted as true, including '0', 'false', empty arrays [] and empty objects {}.

const values = [0, 1, '', 'hello', null, undefined, [], 'false'];

for (const value of values) {
    console.log(`${JSON.stringify(value)} is ${value ? 'truthy' : 'falsy'}`);
}

0 is falsy
1 is truthy
"" is falsy
"hello" is truthy
null is falsy
undefined is falsy
[] is truthy
"false" is truthy

It makes short checks easy, such as if (name) for "name has some text". But it's also why a count of 0 can be mistaken for "no count", so when 0 is a real answer, compare with ===.

null and undefined

JavaScript has two "no value" values:

  • undefined: nothing was ever given. A variable with no value, a missing argument, a property that isn't there, a function with no return.
  • null: a value that's deliberately empty, set on purpose, like C#'s null.

Both are falsy, and ?? works like C#'s: name ?? 'Guest' gives 'Guest' when name is null or undefined.

Cheat sheet

C# JavaScript
int x = 5; let x = 5; (or const)
string s = quot;Hi {name}"; const s = `Hi ${name}`;
Console.WriteLine(x); console.log(x);
int Add(int a, int b) { ... } function add(a, b) { ... }
x => x * 2 x => x * 2
List<int> list = new List<int>(); const list = [];
list.Add(7); / list.Count list.push(7); / list.length
list.Where(...).Select(...) list.filter(...).map(...)
new Person { Name = "Ada" } { name: 'Ada' }
a == b a === b
null null, and undefined
foreach (var item in list) for (const item of list)
try { } catch (Exception ex) { } try { } catch (error) { }

Learning more

The best reference for JavaScript is MDN, the Mozilla Developer Network: the JavaScript Guide for learning, and the JavaScript reference for looking things up. When you search the web for a JavaScript question, add mdn to the search.

Check yourself

1 question to try on the lesson page.

So what

Treat JavaScript as a new language that happens to look familiar. Compare with ===, know the falsy values, keep the cheat sheet handy, and the C# you know will carry you most of the way.

Key terms

Cheat sheets