Case Sensitive

JavaScript is a case-sensitive language. That means JavaScript treats two words with same letters but different cases as two separate identifiers.

This applies to the language keywords, variables, function names and other identifiers.

Keywords

let is a keyword in JavaScript to declare a variable. Here is an example of its usage.

let a = 10;

Since keywords are case sensitive, all statements below are not valid.

Let a = 10;
LET a = 10;

Variables

Variables store values in JavaScript. Since variables are case sensitive, below code is creating 3 variables, not 1.

let age = 10;
let Age = 10;
let AGE = 10;

Functions

Functions are callable actions. We can group a set of statements under a function name. Here, we have two functions:

// First function
function sum() {
  // some action
}

// Second function
function Sum() {
  // some action
}

Even though two functions have same name, they are declared separately due to different case.

Switch Case

Here is another scenario where case sensitivity is demonstrated. switch keyword is used to pick a logic based on the input given to it.

const a = "Apple";
switch (a) {
  case "apple":
    console.log("First case");
    break;
  case "Apple":
    console.log("Second case");
    break;
  default:
    break;
}

Since the value of a is "Apple" with a capital A, above code prints "Second case" as output.

Identifiers

Other than above mentioned examples, case of an identifier is considered in using object properties, function arguments or class names.