JavaScript Expressions and Operators

Basic

An expression is JavaScript syntax that is evaluated to produce a value. A statement is a larger syntactic unit that performs an action or controls execution. The distinction matters because expressions can appear inside many statements, including assignments, conditions, function calls, and returns.

1. Expressions produce values

A literal such as 42 is an expression. So is a variable reference such as score and an operation such as score + 10. A statement can contain an expression, but the two concepts are not interchangeable.

const total = ?;

2. Assignment is an expression too

An assignment such as score = 20 evaluates to the value that was assigned. That is why assignment can be nested in another expression. Prefer straightforward assignments when nesting would make the program harder to read.

const result = (count = ? );

3. Arithmetic and comparison are expressions

Arithmetic operators calculate numeric values. Comparison operators such as === and < produce Boolean values. Keep the result in mind: an expression does not merely “do something”; it evaluates to a value.

const result = 12 < 20;
console.log(result);
?

4. Logical operators can stop before evaluating the right side

&& and || combine Boolean-oriented conditions, but they also return one of their operands. They evaluate left to right and may short-circuit: with && a falsy left operand prevents the right operand from being evaluated; with || a truthy left operand does the same.

const result = false && ?check();

5. Nullish coalescing answers a different question

?? uses the right operand only when the left operand is null or undefined. It is different from || because || also treats other falsy values, such as 0 and "", as a reason to use its right operand.

const result = 0 ?? ?10;

6. Conditional and logical assignment operators express small decisions

The conditional operator condition ? whenTrue : whenFalse chooses one of two expressions. Logical assignment operators such as &&=, ||= and ??= combine a logical test with assignment. Use them when the compact form remains clear.

let name = undefined;
name ? "Guest";

7. Grouping and precedence change how expressions are evaluated

Operators have defined precedence and associativity. Parentheses explicitly group an expression and are often the clearest way to communicate intent. Do not rely on memorized precedence when grouping makes the intended calculation clearer.

const result = ?;

8. Other operators are part of the language, not magic syntax

JavaScript also defines operators and operator-like expressions such as typeof, delete, void, in and instanceof. Bitwise operators work on integer-like bit patterns. Increment and decrement change a variable and produce a value; use them carefully when embedding side effects inside larger expressions.

const type = typeof ?42;

Practice what you learned

Use the expressions and operators code-completion exercise and then take the expressions and operators quiz.

Authoring rules to remember

• Distinguish expressions that produce values from statements that organize execution.

• Use parentheses when they make grouping or intent clearer.

• Remember that &&, ||, and ?? return operands and can short-circuit.

• Use ?? when only null and undefined should trigger a fallback.

• Keep side effects out of dense expressions when they reduce readability.

Next step

Next, deepen the model of JavaScript values, types, coercion, and equality.