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What is JavaScript syntax?
Syntax describes how tokens such as names, keywords, punctuation, and operators can be arranged into valid code. It is not the same as a program’s behavior: syntax determines whether the code can be parsed, while execution determines what it does.
These categories often appear together:
const price = 12;
if (price > 10) {
console.log("over 10");
}const price = 12;is a declaration that creates a binding namedprice. The value12is an expression.price > 10is an expression. It evaluates to the Boolean valuetrue.- The
ifconstruct is a statement that controls whether its block runs. console.log("over 10")is an expression statement here; it calls a function and produces a side effect.
A line is not necessarily a statement: one statement can span several lines, and multiple statements can share a line. Declarations and statements are distinct grammar categories, even though beginner explanations sometimes use “statement” loosely for both. See MDN’s statements and declarations reference.
How do bindings and values work?
A binding associates a name with a value. JavaScript is case-sensitive, so score and Score are different names. You do not ordinarily declare a variable’s type; the value has a type, and a binding can refer to values of different types over time.
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Choose between const and let
Use const when you will not assign a different value to the binding. Use let when reassignment is part of the logic. Both are block-scoped: their names are available within the enclosing pair of braces, not outside it.
const siteName = "Example";
let visits = 0;
visits = visits + 1;const does not make an object immutable; it prevents reassignment of the binding. An object referenced by a const binding can still be changed:
const settings = { theme: "light" };
settings.theme = "dark"; // allowed
// settings = {}; // TypeError: cannot reassign a const bindingRecognize var in older code
var is not block-scoped like let and const. Its scope is tied to a function, or to the global context when declared outside a function. You will encounter it in older code; for new bindings, let and const usually make scope and reassignment intent clearer. MDN’s language overview explains these declaration forms.
Know the common value types
Begin by recognizing strings, numbers, Booleans, null, and undefined. These are primitive values. Objects are a broader category that includes ordinary objects, arrays, and functions; arrays and functions are not primitive types.
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const count = 3; // number
const ready = false; // Boolean
const missing = null; // intentional absence of a value
let notAssigned; // undefinednull and undefined are not interchangeable by meaning: null is commonly used to represent an intentional absence, while an uninitialized binding declared with let has the value undefined. For syntax, types, and declarations, see MDN’s grammar and types guide.
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How do operators combine expressions?
Operators take values or expressions and produce a result. = assigns; it does not test equality. Use === for a strict equality comparison in typical beginner code.
let total = 4; // assignment
const isFour = total === 4; // comparison; evaluates to trueArithmetic operators include +, -, *, and /. The plus operator can also concatenate strings, and JavaScript can convert operands implicitly in some operations:
const sum = 2 + 3; // 5
const label = "Item " + 3; // "Item 3"
const surprising = "2" + 3; // "23"Logical operators such as && (and), || (or), and ! (not) are commonly used in conditions. Operator precedence controls grouping; use parentheses when they make the intended order clearer:
const result = (2 + 3) * 4; // 20Parentheses are especially helpful when a line combines several operators, or when implicit conversion could make the result less obvious. MDN’s expressions and operators guide covers precedence and conversion behavior.
How do conditions, loops, and errors shape control flow?
Control-flow statements determine which code runs and how often. Braces mark a block and help keep the boundary of conditional and loop bodies clear.
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Choose a branch
const temperature = 18;
if (temperature >= 25) {
console.log("warm");
} else {
console.log("cooler");
}Use switch when one value is compared with several cases:
switch (day) {
case "Saturday":
console.log("weekend");
break;
default:
console.log("weekday or another value");
}Repeat work with a loop
A for loop is useful when its setup, continuation condition, and update fit together. A while loop repeats while its condition remains true.
for (let i = 0; i < 3; i++) {
console.log(i);
}This logs 0, 1, then 2. The loop variable uses let because it changes on each iteration.
Handle or raise an error
try and catch let code respond to an exception. throw raises one:
try {
throw new Error("Invalid input");
} catch (error) {
console.log(error.message);
}How do functions, objects, and classes fit together?
Functions accept inputs and can return values
A function declaration names reusable behavior. Parameters receive inputs, local declarations hold temporary values, and return sends a result back to the caller.
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function double(number) {
const result = number * 2;
return result;
}
const answer = double(4); // 8An arrow function is an expression, so it can be assigned to a binding or passed to another function:
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const double = (number) => number * 2;
const answer = double(4); // 8Objects group related values
An object literal uses braces and key-value pairs. Properties can be read with dot notation when the property name is a valid identifier:
const user = {
name: "Rae",
active: true
};
const userName = user.name; // "Rae"Classes define a form of object construction
Class syntax provides a structured way to define constructors and methods. It is part of JavaScript, not a separate runtime API.
class Counter {
constructor(value = 0) {
this.value = value;
}
increment() {
this.value += 1;
}
}
const counter = new Counter();What do modules and asynchronous syntax do?
Modules divide code into files and make selected bindings available to other modules. The export and import keywords are language syntax; how a project loads modules depends on its host and configuration.
// counters.js
export const start = 0;
// app.js
import { start } from "./counters.js";Asynchronous language constructs include async functions and await. They help express work that completes later, such as retrieving data. The syntax is part of JavaScript, while the mechanism that performs a particular task may be supplied by the environment.
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async function loadData() {
const response = await fetch("/data.json");
return response.json();
}Here, fetch is not a JavaScript syntax feature: it is provided by some runtimes, including modern browsers and Node.js. Availability and behavior depend on the host environment.
Are browser APIs part of JavaScript syntax?
No. ECMAScript defines JavaScript’s core language, including its syntax, values, operators, functions, classes, and modules. A host environment supplies APIs. Browsers add interfaces such as the DOM; Node.js supplies a different set of runtime capabilities. For example, document.querySelector and window are browser interfaces, not grammar features. MDN separates these areas in its JavaScript technologies overview.
Do you need semicolons in JavaScript?
Not after every closing brace or in every position, but semicolons are part of JavaScript’s statement-termination rules. Ecma International’s specification says, “Most ECMAScript statements and declarations must be terminated with a semicolon.” The same specification defines cases where a semicolon may be inserted automatically, so that wording does not mean you must type one after every construct. The exact rules are described in the current Automatic Semicolon Insertion section.
Automatic semicolon insertion (ASI) is conditional, not a rule that ends every statement at every newline. In certain parsing situations, JavaScript inserts a semicolon, including when a line terminator occurs before a token that cannot continue the preceding statement. A line break can also be harmless inside an expression. For example, this assignment continues on the next line:
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const total = 2
+ 3;A particularly important restricted case is return. If the returned expression begins on the next line, the function returns undefined instead:
function getValue() {
return
42;
}
// The function returns undefined.Keep the expression on the same line as return when it is the return value:
function getValue() {
return 42;
}Teams may choose explicit semicolons or a consistent omission style. Whichever style you follow, keep statement boundaries clear and do not rely on subtle line-break behavior.
What should you learn next?
A useful progression is bindings and values, operators, conditionals and loops, functions, objects, then classes, asynchronous constructs, and modules. MDN’s JavaScript Guide and JavaScript fundamentals curriculum organize these topics for continued learning.
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