JavaScript Fundamentals

Values and their types

A value is a piece of data that JavaScript can work with. The language has several primitive value types: undefined, null, Boolean, Number, BigInt, String, and Symbol. A primitive is a value that is not an object. At an introductory level, the most visible examples are numbers such as 12, strings such as "hello", Booleans true and false, and the special values null and undefined. These labels are not interchangeable. A number is suitable for arithmetic; a string represents text; a Boolean represents one of two logical states. null is an explicit primitive value often used to represent an intentional absence, whereas undefined is its own primitive value and commonly appears when no value has been supplied. Later lessons explain how names, collections, and objects hold or organize values; this lesson concentrates on the values themselves.

Type matters because an operator's behavior depends on its operands. 2 + 3 is numeric addition and produces the Number value 5. The comparison 2 < 3 produces the Boolean value true. A careful reader therefore asks both questions: what values are present, and what type of result should this operation produce? That habit is more reliable than reading punctuation as decoration. JavaScript can perform conversions in some situations, but relying on an unexamined conversion makes code harder to reason about. When a result surprises you, inspect the values and their types first.

Expressions evaluate

An expression is code that resolves to a value. A literal such as "Ada" is an expression: evaluating it yields that string. An arithmetic expression such as 6 * 7 evaluates to 42. Expressions can be combined. In 2 + 3 * 4, multiplication is evaluated before addition, so the resulting Number value is 14, not 20. Parentheses can deliberately change that grouping: (2 + 3) * 4 evaluates to 20. Executing these examples in a JavaScript runtime confirms the results.

Operators are symbols or keywords that participate in expressions. Arithmetic operators include +, -, *, and /. Comparison operators, including ===, <, and >, evaluate whether a relationship is true and yield a Boolean result. 5 === 5 evaluates to true; 5 === 6 evaluates to false. The strict equality operator compares without first converting the operands to a common type, which makes its result easier to predict than a loose equality check in many introductory cases. This lesson does not attempt to catalog every operator. The central model is that operators receive operands and an expression evaluates to one resulting value.

A result need not be displayed merely because an expression has a value. Interactive developer consoles often show the result of an entered expression as a convenience. That display is a feature of the host environment, not an implication that every expression automatically appears on a web page. Separating JavaScript language evaluation from browser output prevents a common early misunderstanding.

Statements organize actions

A statement is a complete instruction in a program. ECMAScript source text contains statements, and an expression statement is one familiar form: it evaluates an expression as an instruction. In many JavaScript examples, a semicolon marks the end of a statement. JavaScript has automatic semicolon insertion rules, but beginners should not treat omitted punctuation as a separate feature to depend on; clear formatting makes statement boundaries easier for humans to see. A program generally contains a sequence of statements, with each statement contributing to the program's behavior.

The expression/statement distinction is about role, not about which one is more advanced. 3 * 5 is an expression because it yields 15. Written as a standalone expression statement, it instructs the engine to evaluate that expression, though the completion value is not thereby made useful to a person. By contrast, a comment is not an instruction for the engine at all. JavaScript comments—// to the end of a line and /* ... */ across text—are ignored by the engine. They can record intent for future readers, but a comment cannot alter an evaluated result.

This vocabulary helps with debugging. If an expected result is wrong, inspect the expression: its operands, operator, precedence, and types. If intended work never occurs, inspect whether the necessary statement exists and is reached; control flow is a later topic. If only a comment describes an intended action, the engine will not carry it out. These distinctions remain useful as code becomes more complex, even though later lessons add variables, functions, arrays, objects, and web APIs.