19. Equality

Equality

So far, you’ve seen how you can use == and != to compare numbers and strings for equality. However, if you use == and != in situations where the data you’re comparing is mixed, it can lead to some interesting results. For example,

"1" == 1

Returns: true

and

0 == false

Returns: true

both evaluate to true. Why is that?

Implicit type coercion

JavaScript is known as a loosely typed language.

Basically, this means that when you’re writing JavaScript code, you do not need to specify data types. Instead, when your code is interpreted by the JavaScript engine it will automatically be converted into the "appropriate" data type. This is called implicit type coercion and you’ve already seen examples like this before when you tried to concatenate strings with numbers.

"julia" + 1

Returns: "julia1"

In this example, JavaScript takes the string "julia" and adds the number 1 to it resulting in the string "julia1". In other programming languages, this code probably would have returned an error, but in JavaScript the number 1 is converted into the string "1" and then is concatenated to the string "julia".

It’s behavior like this which makes JavaScript unique from other programming languages, but it can lead to some quirky behavior when doing operations and comparisons on mixed data types.

DEFINITION: A strongly typed language is a programming language that is more likely to generate errors if data does not closely match an expected type. Because JavaScript is loosely typed, you don’t need to specify data types; however, this can lead to errors that are hard to diagnose due to implicit type coercion.

Example of strongly typed programming language code

int count = 1;
string name = "Julia";
double num = 1.2932;
float price = 2.99;

Equivalent code in JavaScript

// equivalent code in JavaScript
var count = 1; 
var name = "Julia";
var num = 1.2932;
var price = 2.99;

In the example below, JavaScript takes the string "1", converts it to true, and compares it to the boolean true.

"1" == true

Returns: true

When you use the == or != operators, JavaScript first converts each value to the same type (if they’re not already the same type); this is why it's called "type coercion"! This is often not the behavior you want, and it’s actually considered bad practice to use the == and != operators when comparing values for equality.

Strict equality

Instead, in JavaScript it’s better to use strict equality to see if numbers, strings, or booleans, etc. are identical in type and value without doing the type conversion first. To perform a strict comparison, simply add an additional equals sign = to the end of the == and != operators.

"1" === 1

Returns: false

This returns false because the string "1" is not the same type and value as the number 1.

0 === false

Returns: false

This returns false because the number 0 is not the same type and value as the boolean false.