JavaScript is a dynamically-typed language. These means that types are checked at runtime when you try to do something with a variable, rather than by a compiler. For example, the following is valid JavaScript code:

var myVariable = 0; 
console.log(typeof myVariable); // Prints "number"
myVariable = "1";
console.log(typeof myVariable); // Prints "string"

Although variables do have a type, this may change throughout the lifetime of the variable.

JavaScript also tries to implicitly convert types where possible, for example, using the equality operator:

console.log(2 == "2"); // Prints "true"

Although this might make sense for frontend JavaScript (for example comparing against the value of a form input), in general, it is more likely to be a source of errors or confusion. For this reason, it is recommended to always use the strict equality and inequality operators:

console.log(2 === "2"); // Prints "false"
console.log(2 !== "2"); // Prints "true"