Expressions and Statements in JavaScript

Here’s a rough mapping of those concepts from English to JavaScript:

  • Word => Token: In English, words are the smallest units that have standalone meaning. In programming, the smallest units are called tokens. These could be variables, operators, keywords, etc. For example, in the JavaScript line of code let x = 5;, let, x, =, 5, and ; are all tokens.

  • Phrase => Expression: In English, phrases are groups of words that express a single idea but may not necessarily form a full sentence. In programming, expressions are pieces of code that resolve to a value. They might be simple (like 2 + 2) or complex (like calling a function with arguments, e.g., findMax(1, 2, 3)), but they always return a value.

  • Sentence => Statement: Sentences in English are complete thoughts, usually containing a subject and a verb. Similarly, in JavaScript, a statement is a standalone unit of execution, usually performing some action. Examples include declaring a variable (let x = 5;), making a function call (myFunction();), or using a control structure like an if statement.

  • Paragraphs => Blocks of Code: In English, a paragraph is a set of related sentences that form a cohesive whole. In programming, especially in JavaScript, we have code blocks, usually defined by being enclosed within {} braces. Code blocks group together multiple statements that are related and should be executed sequentially. These blocks can represent the body of a function, the code to execute within a loop, the branches of a conditional (like if/else), and so on.

  • Essay/Story => Program: Finally, just as a complete essay or story in English is a carefully arranged series of paragraphs that convey a larger narrative, a program in JavaScript (or any language) is a set of interconnected code blocks, functions, and modules that work together to perform a complex task.

Remember, this is a simplification and analogy to aid understanding; the exact correspondences can vary, especially when considering the many different structures and paradigms within programming. But this should provide a good foundation!

In JavaScript, as in many programming languages, code is comprised of a series of statements and expressions. Although the terms are sometimes used interchangeably, there are important distinctions between them.

Expressions:

An expression in JavaScript is a piece of code that evaluates to a value. It doesn’t necessarily have to manipulate any data - the main point is that it results in a value. Expressions can be combined together with operators to form larger expressions.

Examples of expressions:

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5          // literal value, evaluates to 5
a          // variable, evaluates to the value of a
2 * 3      // arithmetic operation, evaluates to 6
foo()      // function call, evaluates to the return value of the function

Statements:

A statement in JavaScript is a piece of code that performs an action. Statements are executed for their behavior, not for their resulting value. In many cases, a statement does something and doesn’t return anything. Statements can include expressions, but not all expressions can be used as statements.

Examples of statements:

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let a = 5;    // variable declaration and assignment statement
if(a > 0) {}  // conditional statement
for(let i = 0; i < 10; i++) {}  // loop statement

Similarities:

  • Both are fundamental parts of JavaScript code, and a typical JavaScript program will include both expressions and statements.
  • Expressions can be included in statements. For example, the a > 0 in the if statement above is an expression.

Differences:

  • An expression results in a value, which can be used as part of larger expressions, statements, or function arguments. A statement performs an action and doesn’t necessarily return a value.
  • Not all expressions can be used as statements. For instance, a literal or a function call on its own is valid as an expression but not as a statement (unless it’s part of a larger statement, like an assignment or if statement).
  • Statements typically define the overall flow or control structure of the code (like loops or conditionals), whereas expressions define the individual operations within that structure (like arithmetic operations or function calls).

In short, you can think of expressions as the building blocks that are combined to make up statements, which in turn make up the larger structure of your JavaScript code.